CVE-2024-53218

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

f2fs: fix race in concurrent f2fs_stop_gc_thread

In my test case, concurrent calls to f2fs shutdown report the following stack trace:

Oops: general protection fault, probably for non-canonical address 0xc6cfff63bb5513fc: 0000 [#1] PREEMPT SMP PTI CPU: 0 UID: 0 PID: 678 Comm: f2fs_rep_shutdo Not tainted 6.12.0-rc5-next-20241029-g6fb2fa9805c5-dirty #85 Call Trace: ? show_regs+0x8b/0xa0 ? __die_body+0x26/0xa0 ? die_addr+0x54/0x90 ? exc_general_protection+0x24b/0x5c0 ? asm_exc_general_protection+0x26/0x30 ? kthread_stop+0x46/0x390 f2fs_stop_gc_thread+0x6c/0x110 f2fs_do_shutdown+0x309/0x3a0 f2fs_ioc_shutdown+0x150/0x1c0 __f2fs_ioctl+0xffd/0x2ac0 f2fs_ioctl+0x76/0xe0 vfs_ioctl+0x23/0x60 __x64_sys_ioctl+0xce/0xf0 x64_sys_call+0x2b1b/0x4540 do_syscall_64+0xa7/0x240 entry_SYSCALL_64_after_hwframe+0x76/0x7e

The root cause is a race condition in f2fs_stop_gc_thread() called from different f2fs shutdown paths:

[CPU0] [CPU1]

f2fs_stop_gc_thread f2fs_stop_gc_thread gc_th = sbi->gc_thread gc_th = sbi->gc_thread kfree(gc_th) sbi->gc_thread = NULL < gc_th != NULL > kthread_stop(gc_th->f2fs_gc_task) //UAF

The commit c7f114d864ac (f2fs: fix to avoid use-after-free in f2fs_stop_gc_thread()) attempted to fix this issue by using a read semaphore to prevent races between shutdown and remount threads, but it fails to prevent all race conditions.

Fix it by converting to write lock of s_umount in f2fs_do_shutdown().

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/60457ed6c67625c87861f96912b4179dc2293896
• https://git.kernel.org/stable/c/794fa8792d4eacac191f1cbcc2e81b7369e4662a
• https://git.kernel.org/stable/c/7b0033dbc48340a1c1c3f12448ba17d6587ca092
• https://git.kernel.org/stable/c/c631207897a9b3d41167ceca58e07f8f94720e42