linux/fs/exportfs/expfs.c

611 lines
16 KiB

2025-03-19 14:01:32 +11:00
[PATCH] VFS: Make filldir_t and struct kstat deal in 64-bit inode numbers These patches make the kernel pass 64-bit inode numbers internally when communicating to userspace, even on a 32-bit system. They are required because some filesystems have intrinsic 64-bit inode numbers: NFS3+ and XFS for example. The 64-bit inode numbers are then propagated to userspace automatically where the arch supports it. Problems have been seen with userspace (eg: ld.so) using the 64-bit inode number returned by stat64() or getdents64() to differentiate files, and failing because the 64-bit inode number space was compressed to 32-bits, and so overlaps occur. This patch: Make filldir_t take a 64-bit inode number and struct kstat carry a 64-bit inode number so that 64-bit inode numbers can be passed back to userspace. The stat functions then returns the full 64-bit inode number where available and where possible. If it is not possible to represent the inode number supplied by the filesystem in the field provided by userspace, then error EOVERFLOW will be issued. Similarly, the getdents/readdir functions now pass the full 64-bit inode number to userspace where possible, returning EOVERFLOW instead when a directory entry is encountered that can't be properly represented. Note that this means that some inodes will not be stat'able on a 32-bit system with old libraries where they were before - but it does mean that there will be no ambiguity over what a 32-bit inode number refers to. Note similarly that directory scans may be cut short with an error on a 32-bit system with old libraries where the scan would work before for the same reasons. It is judged unlikely that this situation will occur because modern glibc uses 64-bit capable versions of stat and getdents class functions exclusively, and that older systems are unlikely to encounter unrepresentable inode numbers anyway. [akpm: alpha build fix] Signed-off-by: David Howells <dhowells@redhat.com> Cc: Trond Myklebust <trond.myklebust@fys.uio.no> Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-10-03 01:13:46 -07:00
statx: Add a system call to make enhanced file info available Add a system call to make extended file information available, including file creation and some attribute flags where available through the underlying filesystem. The getattr inode operation is altered to take two additional arguments: a u32 request_mask and an unsigned int flags that indicate the synchronisation mode. This change is propagated to the vfs_getattr*() function. Functions like vfs_stat() are now inline wrappers around new functions vfs_statx() and vfs_statx_fd() to reduce stack usage. ======== OVERVIEW ======== The idea was initially proposed as a set of xattrs that could be retrieved with getxattr(), but the general preference proved to be for a new syscall with an extended stat structure. A number of requests were gathered for features to be included. The following have been included: (1) Make the fields a consistent size on all arches and make them large. (2) Spare space, request flags and information flags are provided for future expansion. (3) Better support for the y2038 problem [Arnd Bergmann] (tv_sec is an __s64). (4) Creation time: The SMB protocol carries the creation time, which could be exported by Samba, which will in turn help CIFS make use of FS-Cache as that can be used for coherency data (stx_btime). This is also specified in NFSv4 as a recommended attribute and could be exported by NFSD [Steve French]. (5) Lightweight stat: Ask for just those details of interest, and allow a netfs (such as NFS) to approximate anything not of interest, possibly without going to the server [Trond Myklebust, Ulrich Drepper, Andreas Dilger] (AT_STATX_DONT_SYNC). (6) Heavyweight stat: Force a netfs to go to the server, even if it thinks its cached attributes are up to date [Trond Myklebust] (AT_STATX_FORCE_SYNC). And the following have been left out for future extension: (7) Data version number: Could be used by userspace NFS servers [Aneesh Kumar]. Can also be used to modify fill_post_wcc() in NFSD which retrieves i_version directly, but has just called vfs_getattr(). It could get it from the kstat struct if it used vfs_xgetattr() instead. (There's disagreement on the exact semantics of a single field, since not all filesystems do this the same way). (8) BSD stat compatibility: Including more fields from the BSD stat such as creation time (st_btime) and inode generation number (st_gen) [Jeremy Allison, Bernd Schubert]. (9) Inode generation number: Useful for FUSE and userspace NFS servers [Bernd Schubert]. (This was asked for but later deemed unnecessary with the open-by-handle capability available and caused disagreement as to whether it's a security hole or not). (10) Extra coherency data may be useful in making backups [Andreas Dilger]. (No particular data were offered, but things like last backup timestamp, the data version number and the DOS archive bit would come into this category). (11) Allow the filesystem to indicate what it can/cannot provide: A filesystem can now say it doesn't support a standard stat feature if that isn't available, so if, for instance, inode numbers or UIDs don't exist or are fabricated locally... (This requires a separate system call - I have an fsinfo() call idea for this). (12) Store a 16-byte volume ID in the superblock that can be returned in struct xstat [Steve French]. (Deferred to fsinfo). (13) Include granularity fields in the time data to indicate the granularity of each of the times (NFSv4 time_delta) [Steve French]. (Deferred to fsinfo). (14) FS_IOC_GETFLAGS value. These could be translated to BSD's st_flags. Note that the Linux IOC flags are a mess and filesystems such as Ext4 define flags that aren't in linux/fs.h, so translation in the kernel may be a necessity (or, possibly, we provide the filesystem type too). (Some attributes are made available in stx_attributes, but the general feeling was that the IOC flags were to ext[234]-specific and shouldn't be exposed through statx this way). (15) Mask of features available on file (eg: ACLs, seclabel) [Brad Boyer, Michael Kerrisk]. (Deferred, probably to fsinfo. Finding out if there's an ACL or seclabal might require extra filesystem operations). (16) Femtosecond-resolution timestamps [Dave Chinner]. (A __reserved field has been left in the statx_timestamp struct for this - if there proves to be a need). (17) A set multiple attributes syscall to go with this. =============== NEW SYSTEM CALL =============== The new system call is: int ret = statx(int dfd, const char *filename, unsigned int flags, unsigned int mask, struct statx *buffer); The dfd, filename and flags parameters indicate the file to query, in a similar way to fstatat(). There is no equivalent of lstat() as that can be emulated with statx() by passing AT_SYMLINK_NOFOLLOW in flags. There is also no equivalent of fstat() as that can be emulated by passing a NULL filename to statx() with the fd of interest in dfd. Whether or not statx() synchronises the attributes with the backing store can be controlled by OR'ing a value into the flags argument (this typically only affects network filesystems): (1) AT_STATX_SYNC_AS_STAT tells statx() to behave as stat() does in this respect. (2) AT_STATX_FORCE_SYNC will require a network filesystem to synchronise its attributes with the server - which might require data writeback to occur to get the timestamps correct. (3) AT_STATX_DONT_SYNC will suppress synchronisation with the server in a network filesystem. The resulting values should be considered approximate. mask is a bitmask indicating the fields in struct statx that are of interest to the caller. The user should set this to STATX_BASIC_STATS to get the basic set returned by stat(). It should be noted that asking for more information may entail extra I/O operations. buffer points to the destination for the data. This must be 256 bytes in size. ====================== MAIN ATTRIBUTES RECORD ====================== The following structures are defined in which to return the main attribute set: struct statx_timestamp { __s64 tv_sec; __s32 tv_nsec; __s32 __reserved; }; struct statx { __u32 stx_mask; __u32 stx_blksize; __u64 stx_attributes; __u32 stx_nlink; __u32 stx_uid; __u32 stx_gid; __u16 stx_mode; __u16 __spare0[1]; __u64 stx_ino; __u64 stx_size; __u64 stx_blocks; __u64 __spare1[1]; struct statx_timestamp stx_atime; struct statx_timestamp stx_btime; struct statx_timestamp stx_ctime; struct statx_timestamp stx_mtime; __u32 stx_rdev_major; __u32 stx_rdev_minor; __u32 stx_dev_major; __u32 stx_dev_minor; __u64 __spare2[14]; }; The defined bits in request_mask and stx_mask are: STATX_TYPE Want/got stx_mode & S_IFMT STATX_MODE Want/got stx_mode & ~S_IFMT STATX_NLINK Want/got stx_nlink STATX_UID Want/got stx_uid STATX_GID Want/got stx_gid STATX_ATIME Want/got stx_atime{,_ns} STATX_MTIME Want/got stx_mtime{,_ns} STATX_CTIME Want/got stx_ctime{,_ns} STATX_INO Want/got stx_ino STATX_SIZE Want/got stx_size STATX_BLOCKS Want/got stx_blocks STATX_BASIC_STATS [The stuff in the normal stat struct] STATX_BTIME Want/got stx_btime{,_ns} STATX_ALL [All currently available stuff] stx_btime is the file creation time, stx_mask is a bitmask indicating the data provided and __spares*[] are where as-yet undefined fields can be placed. Time fields are structures with separate seconds and nanoseconds fields plus a reserved field in case we want to add even finer resolution. Note that times will be negative if before 1970; in such a case, the nanosecond fields will also be negative if not zero. The bits defined in the stx_attributes field convey information about a file, how it is accessed, where it is and what it does. The following attributes map to FS_*_FL flags and are the same numerical value: STATX_ATTR_COMPRESSED File is compressed by the fs STATX_ATTR_IMMUTABLE File is marked immutable STATX_ATTR_APPEND File is append-only STATX_ATTR_NODUMP File is not to be dumped STATX_ATTR_ENCRYPTED File requires key to decrypt in fs Within the kernel, the supported flags are listed by: KSTAT_ATTR_FS_IOC_FLAGS [Are any other IOC flags of sufficient general interest to be exposed through this interface?] New flags include: STATX_ATTR_AUTOMOUNT Object is an automount trigger These are for the use of GUI tools that might want to mark files specially, depending on what they are. Fields in struct statx come in a number of classes: (0) stx_dev_*, stx_blksize. These are local system information and are always available. (1) stx_mode, stx_nlinks, stx_uid, stx_gid, stx_[amc]time, stx_ino, stx_size, stx_blocks. These will be returned whether the caller asks for them or not. The corresponding bits in stx_mask will be set to indicate whether they actually have valid values. If the caller didn't ask for them, then they may be approximated. For example, NFS won't waste any time updating them from the server, unless as a byproduct of updating something requested. If the values don't actually exist for the underlying object (such as UID or GID on a DOS file), then the bit won't be set in the stx_mask, even if the caller asked for the value. In such a case, the returned value will be a fabrication. Note that there are instances where the type might not be valid, for instance Windows reparse points. (2) stx_rdev_*. This will be set only if stx_mode indicates we're looking at a blockdev or a chardev, otherwise will be 0. (3) stx_btime. Similar to (1), except this will be set to 0 if it doesn't exist. ======= TESTING ======= The following test program can be used to test the statx system call: samples/statx/test-statx.c Just compile and run, passing it paths to the files you want to examine. The file is built automatically if CONFIG_SAMPLES is enabled. Here's some example output. Firstly, an NFS directory that crosses to another FSID. Note that the AUTOMOUNT attribute is set because transiting this directory will cause d_automount to be invoked by the VFS. [root@andromeda ~]# /tmp/test-statx -A /warthog/data statx(/warthog/data) = 0 results=7ff Size: 4096 Blocks: 8 IO Block: 1048576 directory Device: 00:26 Inode: 1703937 Links: 125 Access: (3777/drwxrwxrwx) Uid: 0 Gid: 4041 Access: 2016-11-24 09:02:12.219699527+0000 Modify: 2016-11-17 10:44:36.225653653+0000 Change: 2016-11-17 10:44:36.225653653+0000 Attributes: 0000000000001000 (-------- -------- -------- -------- -------- -------- ---m---- --------) Secondly, the result of automounting on that directory. [root@andromeda ~]# /tmp/test-statx /warthog/data statx(/warthog/data) = 0 results=7ff Size: 4096 Blocks: 8 IO Block: 1048576 directory Device: 00:27 Inode: 2 Links: 125 Access: (3777/drwxrwxrwx) Uid: 0 Gid: 4041 Access: 2016-11-24 09:02:12.219699527+0000 Modify: 2016-11-17 10:44:36.225653653+0000 Change: 2016-11-17 10:44:36.225653653+0000 Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
2017-01-31 16:46:22 +00:00
exportfs: support encoding non-decodeable file handles by default AT_HANDLE_FID was added as an API for name_to_handle_at() that request the encoding of a file id, which is not intended to be decoded. This file id is used by fanotify to describe objects in events. So far, overlayfs is the only filesystem that supports encoding non-decodeable file ids, by providing export_operations with an ->encode_fh() method and without a ->decode_fh() method. Add support for encoding non-decodeable file ids to all the filesystems that do not provide export_operations, by encoding a file id of type FILEID_INO64_GEN from { i_ino, i_generation }. A filesystem may that does not support NFS export, can opt-out of encoding non-decodeable file ids for fanotify by defining an empty export_operations struct (i.e. with a NULL ->encode_fh() method). This allows the use of fanotify events with file ids on filesystems like 9p which do not support NFS export to bring fanotify in feature parity with inotify on those filesystems. Note that fanotify also requires that the filesystems report a non-null fsid. Currently, many simple filesystems that have support for inotify (e.g. debugfs, tracefs, sysfs) report a null fsid, so can still not be used with fanotify in file id reporting mode. Reviewed-by: Jan Kara <jack@suse.cz> Reviewed-by: Jeff Layton <jlayton@kernel.org> Signed-off-by: Amir Goldstein <amir73il@gmail.com> Link: https://lore.kernel.org/r/20231023180801.2953446-5-amir73il@gmail.com Signed-off-by: Christian Brauner <brauner@kernel.org>
2023-10-23 21:08:01 +03:00
exportfs: support encoding non-decodeable file handles by default AT_HANDLE_FID was added as an API for name_to_handle_at() that request the encoding of a file id, which is not intended to be decoded. This file id is used by fanotify to describe objects in events. So far, overlayfs is the only filesystem that supports encoding non-decodeable file ids, by providing export_operations with an ->encode_fh() method and without a ->decode_fh() method. Add support for encoding non-decodeable file ids to all the filesystems that do not provide export_operations, by encoding a file id of type FILEID_INO64_GEN from { i_ino, i_generation }. A filesystem may that does not support NFS export, can opt-out of encoding non-decodeable file ids for fanotify by defining an empty export_operations struct (i.e. with a NULL ->encode_fh() method). This allows the use of fanotify events with file ids on filesystems like 9p which do not support NFS export to bring fanotify in feature parity with inotify on those filesystems. Note that fanotify also requires that the filesystems report a non-null fsid. Currently, many simple filesystems that have support for inotify (e.g. debugfs, tracefs, sysfs) report a null fsid, so can still not be used with fanotify in file id reporting mode. Reviewed-by: Jan Kara <jack@suse.cz> Reviewed-by: Jeff Layton <jlayton@kernel.org> Signed-off-by: Amir Goldstein <amir73il@gmail.com> Link: https://lore.kernel.org/r/20231023180801.2953446-5-amir73il@gmail.com Signed-off-by: Christian Brauner <brauner@kernel.org>
2023-10-23 21:08:01 +03:00
exportfs: add fid type This patchset is a medium scale rewrite of the export operations interface. The goal is to make the interface less complex, and easier to understand from the filesystem side, aswell as preparing generic support for exporting of 64bit inode numbers. This touches all nfs exporting filesystems, and I've done testing on all of the filesystems I have here locally (xfs, ext2, ext3, reiserfs, jfs) This patch: Add a structured fid type so that we don't have to pass an array of u32 values around everywhere. It's a union of possible layouts. As a start there's only the u32 array and the traditional 32bit inode format, but there will be more in one of my next patchset when I start to document the various filehandle formats we have in lowlevel filesystems better. Also add an enum that gives the various filehandle types human- readable names. Note: Some people might think the struct containing an anonymous union is ugly, but I didn't want to pass around a raw union type. Signed-off-by: Christoph Hellwig <hch@lst.de> Cc: Neil Brown <neilb@suse.de> Cc: "J. Bruce Fields" <bfields@fieldses.org> Cc: <linux-ext4@vger.kernel.org> Cc: Dave Kleikamp <shaggy@austin.ibm.com> Cc: Anton Altaparmakov <aia21@cantab.net> Cc: David Chinner <dgc@sgi.com> Cc: Timothy Shimmin <tes@sgi.com> Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> Cc: Hugh Dickins <hugh@veritas.com> Cc: Chris Mason <mason@suse.com> Cc: Jeff Mahoney <jeffm@suse.com> Cc: "Vladimir V. Saveliev" <vs@namesys.com> Cc: Steven Whitehouse <swhiteho@redhat.com> Cc: Mark Fasheh <mark.fasheh@oracle.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2007-10-21 16:42:03 -07:00
fhandle: relax open_by_handle_at() permission checks A current limitation of open_by_handle_at() is that it's currently not possible to use it from within containers at all because we require CAP_DAC_READ_SEARCH in the initial namespace. That's unfortunate because there are scenarios where using open_by_handle_at() from within containers. Two examples: (1) cgroupfs allows to encode cgroups to file handles and reopen them with open_by_handle_at(). (2) Fanotify allows placing filesystem watches they currently aren't usable in containers because the returned file handles cannot be used. Here's a proposal for relaxing the permission check for open_by_handle_at(). (1) Opening file handles when the caller has privileges over the filesystem (1.1) The caller has an unobstructed view of the filesystem. (1.2) The caller has permissions to follow a path to the file handle. This doesn't address the problem of opening a file handle when only a portion of a filesystem is exposed as is common in containers by e.g., bind-mounting a subtree. The proposal to solve this use-case is: (2) Opening file handles when the caller has privileges over a subtree (2.1) The caller is able to reach the file from the provided mount fd. (2.2) The caller has permissions to construct an unobstructed path to the file handle. (2.3) The caller has permissions to follow a path to the file handle. The relaxed permission checks are currently restricted to directory file handles which are what both cgroupfs and fanotify need. Handling disconnected non-directory file handles would lead to a potentially non-deterministic api. If a disconnected non-directory file handle is provided we may fail to decode a valid path that we could use for permission checking. That in itself isn't a problem as we would just return EACCES in that case. However, confusion may arise if a non-disconnected dentry ends up in the cache later and those opening the file handle would suddenly succeed. * It's potentially possible to use timing information (side-channel) to infer whether a given inode exists. I don't think that's particularly problematic. Thanks to Jann for bringing this to my attention. * An unrelated note (IOW, these are thoughts that apply to open_by_handle_at() generically and are unrelated to the changes here): Jann pointed out that we should verify whether deleted files could potentially be reopened through open_by_handle_at(). I don't think that's possible though. Another potential thing to check is whether open_by_handle_at() could be abused to open internal stuff like memfds or gpu stuff. I don't think so but I haven't had the time to completely verify this. This dates back to discussions Amir and I had quite some time ago and thanks to him for providing a lot of details around the export code and related patches! Link: https://lore.kernel.org/r/20240524-vfs-open_by_handle_at-v1-1-3d4b7d22736b@kernel.org Reviewed-by: Amir Goldstein <amir73il@gmail.com> Reviewed-by: Jeff Layton <jlayton@kernel.org> Signed-off-by: Christian Brauner <brauner@kernel.org>
2024-05-24 12:19:39 +02:00
fhandle: relax open_by_handle_at() permission checks A current limitation of open_by_handle_at() is that it's currently not possible to use it from within containers at all because we require CAP_DAC_READ_SEARCH in the initial namespace. That's unfortunate because there are scenarios where using open_by_handle_at() from within containers. Two examples: (1) cgroupfs allows to encode cgroups to file handles and reopen them with open_by_handle_at(). (2) Fanotify allows placing filesystem watches they currently aren't usable in containers because the returned file handles cannot be used. Here's a proposal for relaxing the permission check for open_by_handle_at(). (1) Opening file handles when the caller has privileges over the filesystem (1.1) The caller has an unobstructed view of the filesystem. (1.2) The caller has permissions to follow a path to the file handle. This doesn't address the problem of opening a file handle when only a portion of a filesystem is exposed as is common in containers by e.g., bind-mounting a subtree. The proposal to solve this use-case is: (2) Opening file handles when the caller has privileges over a subtree (2.1) The caller is able to reach the file from the provided mount fd. (2.2) The caller has permissions to construct an unobstructed path to the file handle. (2.3) The caller has permissions to follow a path to the file handle. The relaxed permission checks are currently restricted to directory file handles which are what both cgroupfs and fanotify need. Handling disconnected non-directory file handles would lead to a potentially non-deterministic api. If a disconnected non-directory file handle is provided we may fail to decode a valid path that we could use for permission checking. That in itself isn't a problem as we would just return EACCES in that case. However, confusion may arise if a non-disconnected dentry ends up in the cache later and those opening the file handle would suddenly succeed. * It's potentially possible to use timing information (side-channel) to infer whether a given inode exists. I don't think that's particularly problematic. Thanks to Jann for bringing this to my attention. * An unrelated note (IOW, these are thoughts that apply to open_by_handle_at() generically and are unrelated to the changes here): Jann pointed out that we should verify whether deleted files could potentially be reopened through open_by_handle_at(). I don't think that's possible though. Another potential thing to check is whether open_by_handle_at() could be abused to open internal stuff like memfds or gpu stuff. I don't think so but I haven't had the time to completely verify this. This dates back to discussions Amir and I had quite some time ago and thanks to him for providing a lot of details around the export code and related patches! Link: https://lore.kernel.org/r/20240524-vfs-open_by_handle_at-v1-1-3d4b7d22736b@kernel.org Reviewed-by: Amir Goldstein <amir73il@gmail.com> Reviewed-by: Jeff Layton <jlayton@kernel.org> Signed-off-by: Christian Brauner <brauner@kernel.org>
2024-05-24 12:19:39 +02:00
VFS: (Scripted) Convert S_ISLNK/DIR/REG(dentry->d_inode) to d_is_*(dentry) Convert the following where appropriate: (1) S_ISLNK(dentry->d_inode) to d_is_symlink(dentry). (2) S_ISREG(dentry->d_inode) to d_is_reg(dentry). (3) S_ISDIR(dentry->d_inode) to d_is_dir(dentry). This is actually more complicated than it appears as some calls should be converted to d_can_lookup() instead. The difference is whether the directory in question is a real dir with a ->lookup op or whether it's a fake dir with a ->d_automount op. In some circumstances, we can subsume checks for dentry->d_inode not being NULL into this, provided we the code isn't in a filesystem that expects d_inode to be NULL if the dirent really *is* negative (ie. if we're going to use d_inode() rather than d_backing_inode() to get the inode pointer). Note that the dentry type field may be set to something other than DCACHE_MISS_TYPE when d_inode is NULL in the case of unionmount, where the VFS manages the fall-through from a negative dentry to a lower layer. In such a case, the dentry type of the negative union dentry is set to the same as the type of the lower dentry. However, if you know d_inode is not NULL at the call site, then you can use the d_is_xxx() functions even in a filesystem. There is one further complication: a 0,0 chardev dentry may be labelled DCACHE_WHITEOUT_TYPE rather than DCACHE_SPECIAL_TYPE. Strictly, this was intended for special directory entry types that don't have attached inodes. The following perl+coccinelle script was used: use strict; my @callers; open($fd, 'git grep -l \'S_IS[A-Z].*->d_inode\' |') || die "Can't grep for S_ISDIR and co. callers"; @callers = <$fd>; close($fd); unless (@callers) { print "No matches\n"; exit(0); } my @cocci = ( '@@', 'expression E;', '@@', '', '- S_ISLNK(E->d_inode->i_mode)', '+ d_is_symlink(E)', '', '@@', 'expression E;', '@@', '', '- S_ISDIR(E->d_inode->i_mode)', '+ d_is_dir(E)', '', '@@', 'expression E;', '@@', '', '- S_ISREG(E->d_inode->i_mode)', '+ d_is_reg(E)' ); my $coccifile = "tmp.sp.cocci"; open($fd, ">$coccifile") || die $coccifile; print($fd "$_\n") || die $coccifile foreach (@cocci); close($fd); foreach my $file (@callers) { chomp $file; print "Processing ", $file, "\n"; system("spatch", "--sp-file", $coccifile, $file, "--in-place", "--no-show-diff") == 0 || die "spatch failed"; } [AV: overlayfs parts skipped] Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
2015-01-29 12:02:35 +00:00
fhandle: relax open_by_handle_at() permission checks A current limitation of open_by_handle_at() is that it's currently not possible to use it from within containers at all because we require CAP_DAC_READ_SEARCH in the initial namespace. That's unfortunate because there are scenarios where using open_by_handle_at() from within containers. Two examples: (1) cgroupfs allows to encode cgroups to file handles and reopen them with open_by_handle_at(). (2) Fanotify allows placing filesystem watches they currently aren't usable in containers because the returned file handles cannot be used. Here's a proposal for relaxing the permission check for open_by_handle_at(). (1) Opening file handles when the caller has privileges over the filesystem (1.1) The caller has an unobstructed view of the filesystem. (1.2) The caller has permissions to follow a path to the file handle. This doesn't address the problem of opening a file handle when only a portion of a filesystem is exposed as is common in containers by e.g., bind-mounting a subtree. The proposal to solve this use-case is: (2) Opening file handles when the caller has privileges over a subtree (2.1) The caller is able to reach the file from the provided mount fd. (2.2) The caller has permissions to construct an unobstructed path to the file handle. (2.3) The caller has permissions to follow a path to the file handle. The relaxed permission checks are currently restricted to directory file handles which are what both cgroupfs and fanotify need. Handling disconnected non-directory file handles would lead to a potentially non-deterministic api. If a disconnected non-directory file handle is provided we may fail to decode a valid path that we could use for permission checking. That in itself isn't a problem as we would just return EACCES in that case. However, confusion may arise if a non-disconnected dentry ends up in the cache later and those opening the file handle would suddenly succeed. * It's potentially possible to use timing information (side-channel) to infer whether a given inode exists. I don't think that's particularly problematic. Thanks to Jann for bringing this to my attention. * An unrelated note (IOW, these are thoughts that apply to open_by_handle_at() generically and are unrelated to the changes here): Jann pointed out that we should verify whether deleted files could potentially be reopened through open_by_handle_at(). I don't think that's possible though. Another potential thing to check is whether open_by_handle_at() could be abused to open internal stuff like memfds or gpu stuff. I don't think so but I haven't had the time to completely verify this. This dates back to discussions Amir and I had quite some time ago and thanks to him for providing a lot of details around the export code and related patches! Link: https://lore.kernel.org/r/20240524-vfs-open_by_handle_at-v1-1-3d4b7d22736b@kernel.org Reviewed-by: Amir Goldstein <amir73il@gmail.com> Reviewed-by: Jeff Layton <jlayton@kernel.org> Signed-off-by: Christian Brauner <brauner@kernel.org>
2024-05-24 12:19:39 +02:00