kernfs: fix use-after-free in __kernfs_remove
CVE: CVE-2022-50432
Component: kernel
CWE: CWE-362
Severity: Low
CVSS v3: 4.4/AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H
Release date: 2022-01-01T00:00:00+00:00

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

kernfs: fix use-after-free in __kernfs_remove

Syzkaller managed to trigger concurrent calls to
kernfs_remove_by_name_ns() for the same file resulting in
a KASAN detected use-after-free. The race occurs when the root
node is freed during kernfs_drain().

To prevent this acquire an additional reference for the root
of the tree that is removed before calling __kernfs_remove().

Found by syzkaller with the following reproducer (slab_nomerge is
required):

syz_mount_image$ext4(0x0, &(0x7f0000000100)='./file0\x00', 0x100000, 0x0, 0x0, 0x0, 0x0)
r0 = openat(0xffffffffffffff9c, &(0x7f0000000080)='/proc/self/exe\x00', 0x0, 0x0)
close(r0)
pipe2(&(0x7f0000000140)={0xffffffffffffffff, <r1=>0xffffffffffffffff}, 0x800)
mount$9p_fd(0x0, &(0x7f0000000040)='./file0\x00', &(0x7f00000000c0), 0x408, &(0x7f0000000280)={'trans=fd,', {'rfdno', 0x3d, r0}, 0x2c, {'wfdno', 0x3d, r1}, 0x2c, {[{@cache_loose}, {@mmap}, {@loose}, {@loose}, {@mmap}], [{@mask={'mask', 0x3d, '^MAY_EXEC'}}, {@fsmagic={'fsmagic', 0x3d, 0x10001}}, {@dont_hash}]}})

Sample report:

==================================================================
BUG: KASAN: use-after-free in kernfs_type include/linux/kernfs.h:335 [inline]
BUG: KASAN: use-after-free in kernfs_leftmost_descendant fs/kernfs/dir.c:1261 [inline]
BUG: KASAN: use-after-free in __kernfs_remove.part.0+0x843/0x960 fs/kernfs/dir.c:1369
Read of size 2 at addr ffff8880088807f0 by task syz-executor.2/857

CPU: 0 PID: 857 Comm: syz-executor.2 Not tainted 6.0.0-rc3-00363-g7726d4c3e60b #5
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0x6e/0x91 lib/dump_stack.c:106
 print_address_description mm/kasan/report.c:317 [inline]
 print_report.cold+0x5e/0x5e5 mm/kasan/report.c:433
 kasan_report+0xa3/0x130 mm/kasan/report.c:495
 kernfs_type include/linux/kernfs.h:335 [inline]
 kernfs_leftmost_descendant fs/kernfs/dir.c:1261 [inline]
 __kernfs_remove.part.0+0x843/0x960 fs/kernfs/dir.c:1369
 __kernfs_remove fs/kernfs/dir.c:1356 [inline]
 kernfs_remove_by_name_ns+0x108/0x190 fs/kernfs/dir.c:1589
 sysfs_slab_add+0x133/0x1e0 mm/slub.c:5943
 __kmem_cache_create+0x3e0/0x550 mm/slub.c:4899
 create_cache mm/slab_common.c:229 [inline]
 kmem_cache_create_usercopy+0x167/0x2a0 mm/slab_common.c:335
 p9_client_create+0xd4d/0x1190 net/9p/client.c:993
 v9fs_session_init+0x1e6/0x13c0 fs/9p/v9fs.c:408
 v9fs_mount+0xb9/0xbd0 fs/9p/vfs_super.c:126
 legacy_get_tree+0xf1/0x200 fs/fs_context.c:610
 vfs_get_tree+0x85/0x2e0 fs/super.c:1530
 do_new_mount fs/namespace.c:3040 [inline]
 path_mount+0x675/0x1d00 fs/namespace.c:3370
 do_mount fs/namespace.c:3383 [inline]
 __do_sys_mount fs/namespace.c:3591 [inline]
 __se_sys_mount fs/namespace.c:3568 [inline]
 __x64_sys_mount+0x282/0x300 fs/namespace.c:3568
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x38/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7f725f983aed
Code: 02 b8 ff ff ff ff c3 66 0f 1f 44 00 00 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f725f0f7028 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007f725faa3f80 RCX: 00007f725f983aed
RDX: 00000000200000c0 RSI: 0000000020000040 RDI: 0000000000000000
RBP: 00007f725f9f419c R08: 0000000020000280 R09: 0000000000000000
R10: 0000000000000408 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000006 R14: 00007f725faa3f80 R15: 00007f725f0d7000
 </TASK>

Allocated by task 855:
 kasan_save_stack+0x1e/0x40 mm/kasan/common.c:38
 kasan_set_track mm/kasan/common.c:45 [inline]
 set_alloc_info mm/kasan/common.c:437 [inline]
 __kasan_slab_alloc+0x66/0x80 mm/kasan/common.c:470
 kasan_slab_alloc include/linux/kasan.h:224 [inline]
 slab_post_alloc_hook mm/slab.h:7
---truncated---

Statement: This race condition in __kernfs_remove() can lead to a use-after-free when concurrent calls remove the same node. Exploitation requires high privileges (PR:H), since only kernel subsystems or privileged code paths (e.g., operations triggered by CAP_SYS_ADMIN users such as sysfs node manipulation or mounting complex filesystems) can invoke kernfs_remove_by_name_ns().

Remediation:
Mitigation for this issue is either not available or the currently
available options don't meet the Red Hat Product Security criteria
comprising ease of use and deployment, applicability to widespread
installation base or stability.