Re: [PATCH v2 net 3/3] atm: clip: Fix infinite recursive call of clip_push().

[Simon Horman <horms@kernel.org>]

On Fri, Jul 04, 2025 at 06:23:53AM +0000, Kuniyuki Iwashima wrote:
> syzbot reported the splat below. [0]
> 
> This happens if we call ioctl(ATMARP_MKIP) more than once.
> 
> During the first call, clip_mkip() sets clip_push() to vcc->push(),
> and the second call copies it to clip_vcc->old_push().
> 
> Later, when the socket is close()d, vcc_destroy_socket() passes
> NULL skb to clip_push(), which calls clip_vcc->old_push(),
> triggering the infinite recursion.
> 
> Let's prevent the second ioctl(ATMARP_MKIP) by checking
> vcc->user_back, which is allocated by the first call as clip_vcc.
> 
> Note also that we use lock_sock() to prevent racy calls.
> 
> [0]:
> BUG: TASK stack guard page was hit at ffffc9000d66fff8 (stack is ffffc9000d670000..ffffc9000d678000)
> Oops: stack guard page: 0000 [#1] SMP KASAN NOPTI
> CPU: 0 UID: 0 PID: 5322 Comm: syz.0.0 Not tainted 6.16.0-rc4-syzkaller #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> RIP: 0010:clip_push+0x5/0x720 net/atm/clip.c:191
> Code: e0 8f aa 8c e8 1c ad 5b fa eb ae 66 2e 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 55 <41> 57 41 56 41 55 41 54 53 48 83 ec 20 48 89 f3 49 89 fd 48 bd 00
> RSP: 0018:ffffc9000d670000 EFLAGS: 00010246
> RAX: 1ffff1100235a4a5 RBX: ffff888011ad2508 RCX: ffff8880003c0000
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff888037f01000
> RBP: dffffc0000000000 R08: ffffffff8fa104f7 R09: 1ffffffff1f4209e
> R10: dffffc0000000000 R11: ffffffff8a99b300 R12: ffffffff8a99b300
> R13: ffff888037f01000 R14: ffff888011ad2500 R15: ffff888037f01578
> FS:  000055557ab6d500(0000) GS:ffff88808d250000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: ffffc9000d66fff8 CR3: 0000000043172000 CR4: 0000000000352ef0
> Call Trace:
>  <TASK>
>  clip_push+0x6dc/0x720 net/atm/clip.c:200
>  clip_push+0x6dc/0x720 net/atm/clip.c:200
>  clip_push+0x6dc/0x720 net/atm/clip.c:200
> ...
>  clip_push+0x6dc/0x720 net/atm/clip.c:200
>  clip_push+0x6dc/0x720 net/atm/clip.c:200
>  clip_push+0x6dc/0x720 net/atm/clip.c:200
>  vcc_destroy_socket net/atm/common.c:183 [inline]
>  vcc_release+0x157/0x460 net/atm/common.c:205
>  __sock_release net/socket.c:647 [inline]
>  sock_close+0xc0/0x240 net/socket.c:1391
>  __fput+0x449/0xa70 fs/file_table.c:465
>  task_work_run+0x1d1/0x260 kernel/task_work.c:227
>  resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
>  exit_to_user_mode_loop+0xec/0x110 kernel/entry/common.c:114
>  exit_to_user_mode_prepare include/linux/entry-common.h:330 [inline]
>  syscall_exit_to_user_mode_work include/linux/entry-common.h:414 [inline]
>  syscall_exit_to_user_mode include/linux/entry-common.h:449 [inline]
>  do_syscall_64+0x2bd/0x3b0 arch/x86/entry/syscall_64.c:100
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7ff31c98e929
> 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:00007fffb5aa1f78 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4
> RAX: 0000000000000000 RBX: 0000000000012747 RCX: 00007ff31c98e929
> RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003
> RBP: 00007ff31cbb7ba0 R08: 0000000000000001 R09: 0000000db5aa226f
> R10: 00007ff31c7ff030 R11: 0000000000000246 R12: 00007ff31cbb608c
> R13: 00007ff31cbb6080 R14: ffffffffffffffff R15: 00007fffb5aa2090
>  </TASK>
> Modules linked in:
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: syzbot+0c77cccd6b7cd917b35a@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=2371d94d248d126c1eb1
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>

Reviewed-by: Simon Horman <horms@kernel.org>