[PATCH] fanotify: Don't call fsnotify_destroy_group() when fsnotify_alloc_group() fails.

[Kuniyuki Iwashima <kuniyu@google.com>]

syzbot reported the splat in __do_sys_fanotify_init(). [0]

The cited commit introduced the fsnotify_group class.

The constructor is fsnotify_alloc_group() and could fail,
so the error is handled this way:

	CLASS(fsnotify_group, group)(&fanotify_fsnotify_ops,
				     FSNOTIFY_GROUP_USER);
	if (IS_ERR(group))
		return PTR_ERR(group);

Even we return from the path, the destructor is triggered,
and the condition does not take IS_ERR() into account.

	if (_T) fsnotify_destroy_group(_T),

Thus, fsnotify_destroy_group() could be called for ERR_PTR().

Let's fix the condition to !IS_ERR_OR_NULL(_T).

[0]:
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
CPU: 1 UID: 0 PID: 6016 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:kasan_byte_accessible+0x12/0x30 mm/kasan/generic.c:210
Code: 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 0f 1f 40 d6 48 c1 ef 03 48 b8 00 00 00 00 00 fc ff df <0f> b6 04 07 3c 08 0f 92 c0 e9 40 01 33 09 cc 66 66 66 66 66 66 2e
RSP: 0018:ffffc90003147c10 EFLAGS: 00010207
RAX: dffffc0000000000 RBX: ffffffff8b5a8b4e RCX: 707d8ea8101f1b00
RDX: 0000000000000000 RSI: ffffffff8b5a8b4e RDI: 0000000000000003
RBP: ffffffff824e37fd R08: 0000000000000001 R09: 0000000000000000
R10: dffffc0000000000 R11: fffffbfff1c0c6f3 R12: 0000000000000000
R13: 000000000000001c R14: 000000000000001c R15: 0000000000000001
FS:  000055556de07500(0000) GS:ffff888125f8b000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f09c332b5a0 CR3: 00000000750b0000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 __kasan_check_byte+0x12/0x40 mm/kasan/common.c:572
 kasan_check_byte include/linux/kasan.h:401 [inline]
 lock_acquire+0x84/0x340 kernel/locking/lockdep.c:5842
 __raw_spin_lock include/linux/spinlock_api_smp.h:133 [inline]
 _raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:154
 spin_lock include/linux/spinlock.h:351 [inline]
 fsnotify_group_stop_queueing fs/notify/group.c:39 [inline]
 fsnotify_destroy_group+0x8d/0x320 fs/notify/group.c:58
 class_fsnotify_group_destructor fs/notify/fanotify/fanotify_user.c:1600 [inline]
 __do_sys_fanotify_init fs/notify/fanotify/fanotify_user.c:1759 [inline]
 __se_sys_fanotify_init+0x991/0xbc0 fs/notify/fanotify/fanotify_user.c:1607
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f09c338f749
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffd8b6be3e8 EFLAGS: 00000246 ORIG_RAX: 000000000000012c
RAX: ffffffffffffffda RBX: 00007f09c35e5fa0 RCX: 00007f09c338f749
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000f00
RBP: 00007ffd8b6be440 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007f09c35e5fa0 R14: 00007f09c35e5fa0 R15: 0000000000000002
 </TASK>
Modules linked in:

Fixes: 3a6b564a6beb ("fanotify: convert fanotify_init() to FD_PREPARE()")
Reported-by: syzbot+321168dfa622eda99689@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/lkml/6928b121.a70a0220.d98e3.0110.GAE@google.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 fs/notify/fanotify/fanotify_user.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index be0a96ad4316..d0b9b984002f 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -1598,10 +1598,10 @@ static struct hlist_head *fanotify_alloc_merge_hash(void)
 }
 
 DEFINE_CLASS(fsnotify_group,
-	      struct fsnotify_group *,
-	      if (_T) fsnotify_destroy_group(_T),
-	      fsnotify_alloc_group(ops, flags),
-	      const struct fsnotify_ops *ops, int flags)
+	     struct fsnotify_group *,
+	     if (!IS_ERR_OR_NULL(_T)) fsnotify_destroy_group(_T),
+	     fsnotify_alloc_group(ops, flags),
+	     const struct fsnotify_ops *ops, int flags)
 
 /* fanotify syscalls */
 SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
-- 
2.52.0.158.g65b55ccf14-goog