CVE-2024-47689

Summary

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

f2fs: fix to don't set SB_RDONLY in f2fs_handle_critical_error()

syzbot reports a f2fs bug as below:

————[ cut here ]———— WARNING: CPU: 1 PID: 58 at kernel/rcu/sync.c:177 rcu_sync_dtor+0xcd/0x180 kernel/rcu/sync.c:177 CPU: 1 UID: 0 PID: 58 Comm: kworker/1:2 Not tainted 6.10.0-syzkaller-12562-g1722389b0d86 #0 Workqueue: events destroy_super_work RIP: 0010:rcu_sync_dtor+0xcd/0x180 kernel/rcu/sync.c:177 Call Trace: percpu_free_rwsem+0x41/0x80 kernel/locking/percpu-rwsem.c:42 destroy_super_work+0xec/0x130 fs/super.c:282 process_one_work kernel/workqueue.c:3231 [inline] process_scheduled_works+0xa2c/0x1830 kernel/workqueue.c:3312 worker_thread+0x86d/0xd40 kernel/workqueue.c:3390 kthread+0x2f0/0x390 kernel/kthread.c:389 ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

As Christian Brauner pointed out [1]: the root cause is f2fs sets SB_RDONLY flag in internal function, rather than setting the flag covered w/ sb->s_umount semaphore via remount procedure, then below race condition causes this bug:

• freeze_super()
• sb_wait_write(sb, SB_FREEZE_WRITE)
• sb_wait_write(sb, SB_FREEZE_PAGEFAULT)
• sb_wait_write(sb, SB_FREEZE_FS) - f2fs_handle_critical_error - sb->s_flags |= SB_RDONLY
• thaw_super
• thaw_super_locked
• sb_rdonly() is true, so it skips sb_freeze_unlock(sb, SB_FREEZE_FS)
• deactivate_locked_super

Since f2fs has almost the same logic as ext4 [2] when handling critical error in filesystem if it mounts w/ errors=remount-ro option:

• set CP_ERROR_FLAG flag which indicates filesystem is stopped
• record errors to superblock
• set SB_RDONLY falg Once we set CP_ERROR_FLAG flag, all writable interfaces can detect the flag and stop any further updates on filesystem. So, it is safe to not set SB_RDONLY flag, let's remove the logic and keep in line w/ ext4 [3].

[1] https://lore.kernel.org/all/20240729-himbeeren-funknetz-96e62f9c7aee@brauner [2] https://lore.kernel.org/all/20240729132721.hxih6ehigadqf7wx@quack3 [3] https://lore.kernel.org/linux-ext4/20240805201241.27286-1-jack@suse.cz

Affected Software

Weaknesses

ADP Enrichment

CISA ADP Vulnrichment

• SSVC:
• Exploitation: none
  • Automatable: no
  • Technical Impact: partial

References

• https://git.kernel.org/stable/c/649ec8b30df113042588bd3d3cd4e98bcb1091e0
• https://git.kernel.org/stable/c/de43021c72993877a8f86f9fddfa0687609da5a4
• https://git.kernel.org/stable/c/1f63f405c1a1a64b9c310388aad7055fb86b245c
• https://git.kernel.org/stable/c/930c6ab93492c4b15436524e704950b364b2930c

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