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Boris Burkov 742b90eaf3 btrfs: fix racy bitfield write in btrfs_clear_space_info_full() 
[ Upstream commit 38e818718c ]

From the memory-barriers.txt document regarding memory barrier ordering
guarantees:

 (*) These guarantees do not apply to bitfields, because compilers often
     generate code to modify these using non-atomic read-modify-write
     sequences.  Do not attempt to use bitfields to synchronize parallel
     algorithms.

 (*) Even in cases where bitfields are protected by locks, all fields
     in a given bitfield must be protected by one lock.  If two fields
     in a given bitfield are protected by different locks, the compiler's
     non-atomic read-modify-write sequences can cause an update to one
     field to corrupt the value of an adjacent field.

btrfs_space_info has a bitfield sharing an underlying word consisting of
the fields full, chunk_alloc, and flush:

struct btrfs_space_info {
        struct btrfs_fs_info *     fs_info;              /*     0     8 */
        struct btrfs_space_info *  parent;               /*     8     8 */
        ...
        int                        clamp;                /*   172     4 */
        unsigned int               full:1;               /*   176: 0  4 */
        unsigned int               chunk_alloc:1;        /*   176: 1  4 */
        unsigned int               flush:1;              /*   176: 2  4 */
        ...

Therefore, to be safe from parallel read-modify-writes losing a write to
one of the bitfield members protected by a lock, all writes to all the
bitfields must use the lock. They almost universally do, except for
btrfs_clear_space_info_full() which iterates over the space_infos and
writes out found->full = 0 without a lock.

Imagine that we have one thread completing a transaction in which we
finished deleting a block_group and are thus calling
btrfs_clear_space_info_full() while simultaneously the data reclaim
ticket infrastructure is running do_async_reclaim_data_space():

          T1                                             T2
btrfs_commit_transaction
  btrfs_clear_space_info_full
  data_sinfo->full = 0
  READ: full:0, chunk_alloc:0, flush:1
                                              do_async_reclaim_data_space(data_sinfo)
                                              spin_lock(&space_info->lock);
                                              if(list_empty(tickets))
                                                space_info->flush = 0;
                                                READ: full: 0, chunk_alloc:0, flush:1
                                                MOD/WRITE: full: 0, chunk_alloc:0, flush:0
                                                spin_unlock(&space_info->lock);
                                                return;
  MOD/WRITE: full:0, chunk_alloc:0, flush:1

and now data_sinfo->flush is 1 but the reclaim worker has exited. This
breaks the invariant that flush is 0 iff there is no work queued or
running. Once this invariant is violated, future allocations that go
into __reserve_bytes() will add tickets to space_info->tickets but will
see space_info->flush is set to 1 and not queue the work. After this,
they will block forever on the resulting ticket, as it is now impossible
to kick the worker again.

I also confirmed by looking at the assembly of the affected kernel that
it is doing RMW operations. For example, to set the flush (3rd) bit to 0,
the assembly is:
  andb    $0xfb,0x60(%rbx)
and similarly for setting the full (1st) bit to 0:
  andb    $0xfe,-0x20(%rax)

So I think this is really a bug on practical systems.  I have observed
a number of systems in this exact state, but am currently unable to
reproduce it.

Rather than leaving this footgun lying around for the future, take
advantage of the fact that there is room in the struct anyway, and that
it is already quite large and simply change the three bitfield members to
bools. This avoids writes to space_info->full having any effect on
writes to space_info->flush, regardless of locking.

Fixes: 957780eb27 ("Btrfs: introduce ticketed enospc infrastructure")
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-12-18 14:03:16 +01:00
arch RISC-V: KVM: Fix guest page fault within HLV* instructions 2025-12-18 14:03:12 +01:00
block block/blk-throttle: Fix throttle slice time for SSDs 2025-12-18 14:03:03 +01:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: ahash - Zero positive err value in ahash_update_finish 2025-12-18 14:03:12 +01:00
Documentation dt-bindings: PCI: amlogic: Fix the register name of the DBI region 2025-12-18 14:02:54 +01:00
drivers iommu/arm-smmu-v3: Fix error check in arm_smmu_alloc_cd_tables 2025-12-18 14:03:16 +01:00
fs btrfs: fix racy bitfield write in btrfs_clear_space_info_full() 2025-12-18 14:03:16 +01:00
include wifi: mt76: wed: use proper wed reference in mt76 wed driver callabacks 2025-12-18 14:03:13 +01:00
init printk changes for 6.18 2025-10-04 11:13:11 -07:00
io_uring io_uring/zcrx: call netdev_queue_get_dma_dev() under instance lock 2025-12-18 14:02:56 +01:00
ipc namespace-6.18-rc1 2025-09-29 11:20:29 -07:00
kernel bpf: properly verify tail call behavior 2025-12-18 14:03:11 +01:00
lib lib/vsprintf: Check pointer before dereferencing in time_and_date() 2025-12-18 14:02:58 +01:00
LICENSES LICENSES: Replace the obsolete address of the FSF in the GFDL-1.2 2025-07-24 11:15:39 +02:00
mm 8 hotfixes. 4 are cc:stable, 7 are against mm/. 2025-11-26 12:38:05 -08:00
net wifi: mac80211: fix CMAC functions not handling errors 2025-12-18 14:03:06 +01:00
rust rust: Add -fno-isolate-erroneous-paths-dereference to bindgen_skip_c_flags 2025-11-10 08:37:06 +08:00
samples samples: work around glibc redefining some of our defines wrong 2025-11-21 09:29:02 -08:00
scripts kbuild: don't enable CC_CAN_LINK if the dummy program generates warnings 2025-12-18 14:03:01 +01:00
security ima: Handle error code returned by ima_filter_rule_match() 2025-12-18 14:03:10 +01:00
sound ASoC: tas2781: correct the wrong period 2025-12-18 14:03:13 +01:00
tools selftests/bpf: Allow selftests to build with older xxd 2025-12-18 14:03:16 +01:00
usr gen_init_cpio: Ignore fsync() returning EINVAL on pipes 2025-10-07 09:53:05 -07:00
virt KVM: guest_memfd: Remove bindings on memslot deletion when gmem is dying 2025-11-04 09:16:53 -08:00
.clang-format memblock: drop for_each_free_mem_pfn_range_in_zone_from() 2025-09-14 08:49:03 +03:00
.clippy.toml rust: clean Rust 1.88.0's warning about clippy::disallowed_macros configuration 2025-05-07 00:11:47 +02:00
.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 MAINTAINERS: remove Alyssa Rosenzweig 2025-09-18 21:17:31 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore .gitignore: ignore compile_commands.json globally 2025-08-12 15:53:55 -07:00
.mailmap 8 hotfixes. 4 are cc:stable, 7 are against mm/. 2025-11-26 12:38:05 -08:00
.pylintrc tools: docs: parse-headers.py: move it from sphinx dir 2025-08-29 15:54:42 -06:00
.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: mark ISDN subsystem as orphan 2025-10-27 17:49:45 -07:00
Kbuild sched: Make migrate_{en,dis}able() inline 2025-09-25 09:57:16 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS soc: fixes for 6.18, part 4 2025-11-28 09:57:31 -08:00
Makefile Linux 6.18.1 2025-12-12 18:42:47 +01:00
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.