CVE-2024-46734

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix race between direct IO write and fsync when using same fd

If we have 2 threads that are using the same file descriptor and one of

them is doing direct IO writes while the other is doing fsync, we have a

race where we can end up either:

1) Attempt a fsync without holding the inode's lock, triggering an

assertion failures when assertions are enabled;

2) Do an invalid memory access from the fsync task because the file private

points to memory allocated on stack by the direct IO task and it may be

used by the fsync task after the stack was destroyed.

The race happens like this:

1) A user space program opens a file descriptor with O_DIRECT;

2) The program spawns 2 threads using libpthread for example;

3) One of the threads uses the file descriptor to do direct IO writes,

while the other calls fsync using the same file descriptor.

4) Call task A the thread doing direct IO writes and task B the thread

doing fsyncs;

5) Task A does a direct IO write, and at btrfs_direct_write() sets the

file's private to an on stack allocated private with the member

'fsync_skip_inode_lock' set to true;

6) Task B enters btrfs_sync_file() and sees that there's a private

structure associated to the file which has 'fsync_skip_inode_lock' set

to true, so it skips locking the inode's VFS lock;

7) Task A completes the direct IO write, and resets the file's private to

NULL since it had no prior private and our private was stack allocated.

Then it unlocks the inode's VFS lock;

8) Task B enters btrfs_get_ordered_extents_for_logging(), then the

assertion that checks the inode's VFS lock is held fails, since task B

never locked it and task A has already unlocked it.

The stack trace produced is the following:

assertion failed: inode_is_locked(&inode->vfs_inode), in fs/btrfs/ordered-data.c:983

------------[ cut here ]------------

kernel BUG at fs/btrfs/ordered-data.c:983!

Oops: invalid opcode: 0000 [#1] PREEMPT SMP PTI

CPU: 9 PID: 5072 Comm: worker Tainted: G U OE 6.10.5-1-default #1 openSUSE Tumbleweed 69f48d427608e1c09e60ea24c6c55e2ca1b049e8

Hardware name: Acer Predator PH315-52/Covini_CFS, BIOS V1.12 07/28/2020

RIP: 0010:btrfs_get_ordered_extents_for_logging.cold+0x1f/0x42 [btrfs]

Code: 50 d6 86 c0 e8 (...)

RSP: 0018:ffff9e4a03dcfc78 EFLAGS: 00010246

RAX: 0000000000000054 RBX: ffff9078a9868e98 RCX: 0000000000000000

RDX: 0000000000000000 RSI: ffff907dce4a7800 RDI: ffff907dce4a7800

RBP: ffff907805518800 R08: 0000000000000000 R09: ffff9e4a03dcfb38

R10: ffff9e4a03dcfb30 R11: 0000000000000003 R12: ffff907684ae7800

R13: 0000000000000001 R14: ffff90774646b600 R15: 0000000000000000

FS: 00007f04b96006c0(0000) GS:ffff907dce480000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 00007f32acbfc000 CR3: 00000001fd4fa005 CR4: 00000000003726f0

Call Trace:

<TASK>

? __die_body.cold+0x14/0x24

? die+0x2e/0x50

? do_trap+0xca/0x110

? do_error_trap+0x6a/0x90

? btrfs_get_ordered_extents_for_logging.cold+0x1f/0x42 [btrfs bb26272d49b4cdc847cf3f7faadd459b62caee9a]

? exc_invalid_op+0x50/0x70

? btrfs_get_ordered_extents_for_logging.cold+0x1f/0x42 [btrfs bb26272d49b4cdc847cf3f7faadd459b62caee9a]

? asm_exc_invalid_op+0x1a/0x20

? btrfs_get_ordered_extents_for_logging.cold+0x1f/0x42 [btrfs bb26272d49b4cdc847cf3f7faadd459b62caee9a]

? btrfs_get_ordered_extents_for_logging.cold+0x1f/0x42 [btrfs bb26272d49b4cdc847cf3f7faadd459b62caee9a]

btrfs_sync_file+0x21a/0x4d0 [btrfs bb26272d49b4cdc847cf3f7faadd459b62caee9a]

? __seccomp_filter+0x31d/0x4f0

__x64_sys_fdatasync+0x4f/0x90

do_syscall_64+0x82/0x160

? do_futex+0xcb/0x190

? __x64_sys_futex+0x10e/0x1d0

? switch_fpu_return+0x4f/0xd0

? syscall_exit_to_user_mode+0x72/0x220

? do_syscall_64+0x8e/0x160

? syscall_exit_to_user_mod

---truncated---