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Al Viro a61afd5482 fix a couple of races in MNT_TREE_BENEATH handling by do_move_mount() 
[ Upstream commit 0d039eac6e ]

Normally do_lock_mount(path, _) is locking a mountpoint pinned by
*path and at the time when matching unlock_mount() unlocks that
location it is still pinned by the same thing.

Unfortunately, for 'beneath' case it's no longer that simple -
the object being locked is not the one *path points to.  It's the
mountpoint of path->mnt.  The thing is, without sufficient locking
->mnt_parent may change under us and none of the locks are held
at that point.  The rules are
	* mount_lock stabilizes m->mnt_parent for any mount m.
	* namespace_sem stabilizes m->mnt_parent, provided that
m is mounted.
	* if either of the above holds and refcount of m is positive,
we are guaranteed the same for refcount of m->mnt_parent.

namespace_sem nests inside inode_lock(), so do_lock_mount() has
to take inode_lock() before grabbing namespace_sem.  It does
recheck that path->mnt is still mounted in the same place after
getting namespace_sem, and it does take care to pin the dentry.
It is needed, since otherwise we might end up with racing mount --move
(or umount) happening while we were getting locks; in that case
dentry would no longer be a mountpoint and could've been evicted
on memory pressure along with its inode - not something you want
when grabbing lock on that inode.

However, pinning a dentry is not enough - the matching mount is
also pinned only by the fact that path->mnt is mounted on top it
and at that point we are not holding any locks whatsoever, so
the same kind of races could end up with all references to
that mount gone just as we are about to enter inode_lock().
If that happens, we are left with filesystem being shut down while
we are holding a dentry reference on it; results are not pretty.

What we need to do is grab both dentry and mount at the same time;
that makes inode_lock() safe *and* avoids the problem with fs getting
shut down under us.  After taking namespace_sem we verify that
path->mnt is still mounted (which stabilizes its ->mnt_parent) and
check that it's still mounted at the same place.  From that point
on to the matching namespace_unlock() we are guaranteed that
mount/dentry pair we'd grabbed are also pinned by being the mountpoint
of path->mnt, so we can quietly drop both the dentry reference (as
the current code does) and mnt one - it's OK to do under namespace_sem,
since we are not dropping the final refs.

That solves the problem on do_lock_mount() side; unlock_mount()
also has one, since dentry is guaranteed to stay pinned only until
the namespace_unlock().  That's easy to fix - just have inode_unlock()
done earlier, while it's still pinned by mp->m_dentry.

Fixes: 6ac3928156 "fs: allow to mount beneath top mount" # v6.5+
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-05-02 07:59:02 +02:00
arch PCI/MSI: Convert pci_msi_ignore_mask to per MSI domain flag 2025-05-02 07:58:56 +02:00
block block: make sure ->nr_integrity_segments is cloned in blk_rq_prep_clone 2025-05-02 07:58:54 +02:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: api - Fix larval relookup type and mask 2025-04-10 14:39:20 +02:00
Documentation platform/x86: msi-wmi-platform: Workaround a ACPI firmware bug 2025-04-25 10:48:04 +02:00
drivers net: ethernet: mtk_eth_soc: net: revise NETSYSv3 hardware configuration 2025-05-02 07:59:02 +02:00
fs fix a couple of races in MNT_TREE_BENEATH handling by do_move_mount() 2025-05-02 07:59:02 +02:00
include drm/xe/bmg: Add one additional PCI ID 2025-05-02 07:58:58 +02:00
init ARM: 9443/1: Require linker to support KEEP within OVERLAY for DCE 2025-04-10 14:39:40 +02:00
io_uring block: add a rq_list type 2025-04-25 10:48:06 +02:00
ipc ipc: fix memleak if msg_init_ns failed in create_ipc_ns 2024-12-05 14:03:02 +01:00
kernel cgroup/cpuset-v1: Add missing support for cpuset_v2_mode 2025-05-02 07:59:00 +02:00
lib lib/Kconfig.ubsan: Remove 'default UBSAN' from UBSAN_INTEGER_WRAP 2025-05-02 07:58:58 +02:00
LICENSES LICENSES: add 0BSD license text 2024-09-01 20:43:24 -07:00
mm mm/vmscan: don't try to reclaim hwpoison folio 2025-05-02 07:58:52 +02:00
net tipc: fix NULL pointer dereference in tipc_mon_reinit_self() 2025-05-02 07:59:02 +02:00
rust rust: Fix enabling Rust and building with GCC for LoongArch 2025-04-10 14:39:36 +02:00
samples tracing: Verify event formats that have "%*p.." 2025-05-02 07:58:52 +02:00
scripts scripts: generate_rust_analyzer: Add ffi crate 2025-04-25 10:48:04 +02:00
security landlock: Prepare to add second errata 2025-04-20 10:15:56 +02:00
sound ASoC: qcom: Fix sc7280 lpass potential buffer overflow 2025-04-25 10:47:50 +02:00
tools selftests/bpf: extend changes_pkt_data with cases w/o subprograms 2025-04-25 10:48:08 +02:00
usr kbuild: hdrcheck: fix cross build with clang 2025-03-13 13:02:18 +01:00
virt KVM: Allow building irqbypass.ko as as module when kvm.ko is a module 2025-04-20 10:15:54 +02:00
.clang-format clang-format: Update with v6.11-rc1's for_each macro list 2024-08-02 13:20:31 +02:00
.clippy.toml rust: enable Clippy's check-private-items 2025-03-13 13:01:42 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore Add Jeff Kirsher to .get_maintainer.ignore 2024-03-08 11:36:54 +00:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore rust: introduce .clippy.toml 2025-03-13 13:01:42 +01:00
.mailmap mailmap: add entry for Thorsten Blum 2024-11-07 14:14:59 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Remove self from DSA entry 2024-11-03 12:52:38 -08:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS: add entry for the Rust alloc module 2025-03-13 13:01:47 +01:00
Makefile Linux 6.12.25 2025-04-25 10:48:08 +02:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.