CVE-2021-47335

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

f2fs: fix to avoid racing on fsync_entry_slab by multi filesystem instances

As syzbot reported, there is an use-after-free issue during f2fs recovery:

Use-after-free write at 0xffff88823bc16040 (in kfence-#10): kmem_cache_destroy+0x1f/0x120 mm/slab_common.c:486 f2fs_recover_fsync_data+0x75b0/0x8380 fs/f2fs/recovery.c:869 f2fs_fill_super+0x9393/0xa420 fs/f2fs/super.c:3945 mount_bdev+0x26c/0x3a0 fs/super.c:1367 legacy_get_tree+0xea/0x180 fs/fs_context.c:592 vfs_get_tree+0x86/0x270 fs/super.c:1497 do_new_mount fs/namespace.c:2905 [inline] path_mount+0x196f/0x2be0 fs/namespace.c:3235 do_mount fs/namespace.c:3248 [inline] __do_sys_mount fs/namespace.c:3456 [inline] __se_sys_mount+0x2f9/0x3b0 fs/namespace.c:3433 do_syscall_64+0x3f/0xb0 arch/x86/entry/common.c:47 entry_SYSCALL_64_after_hwframe+0x44/0xae

The root cause is multi f2fs filesystem instances can race on accessing global fsync_entry_slab pointer, result in use-after-free issue of slab cache, fixes to init/destroy this slab cache only once during module init/destroy procedure to avoid this issue.

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 Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.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/79fa5d944c875711253a23b8155b36883c696409
• https://git.kernel.org/stable/c/86786603014e0a22d0d6af8e80ae4b8687927048
• https://git.kernel.org/stable/c/cad83c968c2ebe97905f900326988ed37146c347
• https://git.kernel.org/stable/c/e472b276a0d2180808009be38105e12754432e2a
• https://git.kernel.org/stable/c/79fa5d944c875711253a23b8155b36883c696409
• https://git.kernel.org/stable/c/86786603014e0a22d0d6af8e80ae4b8687927048
• https://git.kernel.org/stable/c/cad83c968c2ebe97905f900326988ed37146c347
• https://git.kernel.org/stable/c/e472b276a0d2180808009be38105e12754432e2a