Re: [syzbot] [mptcp?] general protection fault in proc_scheduler

Matthieu Baerts
2025-01-06T14:27:53Z

Hi Joel, Eric, Al,

On 06/01/2025 14:32, Joel Granados wrote:
> On Sat, Jan 04, 2025 at 08:11:52PM +0100, Matthieu Baerts wrote:
>> Hi Eric,
>>
>> (+cc Joel)
>>
>> Thank you for your reply!
>>
>> On 04/01/2025 19:53, Eric Dumazet wrote:
>>> On Sat, Jan 4, 2025 at 7:38 PM Matthieu Baerts <matttbe@kernel.org> wrote:
>>>>
>>>> Hi Eric,
>>>>
>>>> Thank you for the bug report!
>>>>
>>>> On 02/01/2025 16:21, Eric Dumazet wrote:
>>>>> On Thu, Jan 2, 2025 at 3:12 PM syzbot
>>>>> <syzbot+e364f774c6f57f2c86d1@syzkaller.appspotmail.com> wrote:
>>>>>>
>>>>>> Hello,
>>>>>>
>>>>>> syzbot found the following issue on:
>>>>>>
>>>>>> HEAD commit: ccb98ccef0e5 Merge tag 'platform-drivers-x86-v6.13-4' of g..
>>>>>> git tree: upstream
>>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=128f6ac4580000
>>>>>> kernel config: https://syzkaller.appspot.com/x/.config?x=86dd15278dbfe19f
>>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=e364f774c6f57f2c86d1
>>>>>> compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
>>>>>> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1245eaf8580000
>>>>>>
>>>>>> Downloadable assets:
>>>>>> disk image: https://storage.googleapis.com/syzbot-assets/d24eb225cff7/disk-ccb98cce.raw.xz
>>>>>> vmlinux: https://storage.googleapis.com/syzbot-assets/dd81532f8240/vmlinux-ccb98cce.xz
>>>>>> kernel image: https://storage.googleapis.com/syzbot-assets/18b08e4bbf40/bzImage-ccb98cce.xz
>>>>>>
>>>>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>>>>> Reported-by: syzbot+e364f774c6f57f2c86d1@syzkaller.appspotmail.com
>>>>>>
>>>>>> Oops: general protection fault, probably for non-canonical address 0xdffffc0000000005: 0000 [#1] PREEMPT SMP KASAN PTI
>>>>>> KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
>>>>>> CPU: 1 UID: 0 PID: 5924 Comm: syz-executor Not tainted 6.13.0-rc5-syzkaller-00004-gccb98ccef0e5 #0
>>>>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
>>>>>> RIP: 0010:proc_scheduler+0xc6/0x3c0 net/mptcp/ctrl.c:125
>>>>>> Code: 03 42 80 3c 38 00 0f 85 fe 02 00 00 4d 8b a4 24 08 09 00 00 48 b8 00 00 00 00 00 fc ff df 49 8d 7c 24 28 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 cc 02 00 00 4d 8b 7c 24 28 48 8d 84 24 c8 00 00
>>>>>> RSP: 0018:ffffc900034774e8 EFLAGS: 00010206
>>>>>>
>>>>>> RAX: dffffc0000000000 RBX: 1ffff9200068ee9e RCX: ffffc90003477620
>>>>>> RDX: 0000000000000005 RSI: ffffffff8b08f91e RDI: 0000000000000028
>>>>>> RBP: 0000000000000001 R08: ffffc90003477710 R09: 0000000000000040
>>>>>> R10: 0000000000000040 R11: 00000000726f7475 R12: 0000000000000000
>>>>>> R13: ffffc90003477620 R14: ffffc90003477710 R15: dffffc0000000000
>>>>>> FS: 0000000000000000(0000) GS:ffff8880b8700000(0000) knlGS:0000000000000000
>>>>>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>>> CR2: 00007fee3cd452d8 CR3: 000000007d116000 CR4: 00000000003526f0
>>>>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>>>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>>>>> Call Trace:
>>>>>> <TASK>
>>>>>> proc_sys_call_handler+0x403/0x5d0 fs/proc/proc_sysctl.c:601
>>>>>> __kernel_write_iter+0x318/0xa80 fs/read_write.c:612
>>>>>> __kernel_write+0xf6/0x140 fs/read_write.c:632
>>>>>> do_acct_process+0xcb0/0x14a0 kernel/acct.c:539
>>>>>> acct_pin_kill+0x2d/0x100 kernel/acct.c:192
>>>>>> pin_kill+0x194/0x7c0 fs/fs_pin.c:44
>>>>>> mnt_pin_kill+0x61/0x1e0 fs/fs_pin.c:81
>>>>>> cleanup_mnt+0x3ac/0x450 fs/namespace.c:1366
>>>>>> task_work_run+0x14e/0x250 kernel/task_work.c:239
>>>>>> exit_task_work include/linux/task_work.h:43 [inline]
>>>>>> do_exit+0xad8/0x2d70 kernel/exit.c:938
>>>>>> do_group_exit+0xd3/0x2a0 kernel/exit.c:1087
>>>>>> get_signal+0x2576/0x2610 kernel/signal.c:3017
>>>>>> arch_do_signal_or_restart+0x90/0x7e0 arch/x86/kernel/signal.c:337
>>>>>> exit_to_user_mode_loop kernel/entry/common.c:111 [inline]
>>>>>> exit_to_user_mode_prepare include/linux/entry-common.h:329 [inline]
>>>>>> __syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline]
>>>>>> syscall_exit_to_user_mode+0x150/0x2a0 kernel/entry/common.c:218
>>>>>> do_syscall_64+0xda/0x250 arch/x86/entry/common.c:89
>>>>>> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>>>>> RIP: 0033:0x7fee3cb87a6a
>>>>>> Code: Unable to access opcode bytes at 0x7fee3cb87a40.
>>>>>> RSP: 002b:00007fffcccac688 EFLAGS: 00000202 ORIG_RAX: 0000000000000037
>>>>>> RAX: 0000000000000000 RBX: 00007fffcccac710 RCX: 00007fee3cb87a6a
>>>>>> RDX: 0000000000000041 RSI: 0000000000000000 RDI: 0000000000000003
>>>>>> RBP: 0000000000000003 R08: 00007fffcccac6ac R09: 00007fffcccacac7
>>>>>> R10: 00007fffcccac710 R11: 0000000000000202 R12: 00007fee3cd49500
>>>>>> R13: 00007fffcccac6ac R14: 0000000000000000 R15: 00007fee3cd4b000
>>>>>> </TASK>
>>>>>> Modules linked in:
>>>>>> ---[ end trace 0000000000000000 ]---
>>>>>> RIP: 0010:proc_scheduler+0xc6/0x3c0 net/mptcp/ctrl.c:125
>>>>>> Code: 03 42 80 3c 38 00 0f 85 fe 02 00 00 4d 8b a4 24 08 09 00 00 48 b8 00 00 00 00 00 fc ff df 49 8d 7c 24 28 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 cc 02 00 00 4d 8b 7c 24 28 48 8d 84 24 c8 00 00
>>>>>> RSP: 0018:ffffc900034774e8 EFLAGS: 00010206
>>>>>> RAX: dffffc0000000000 RBX: 1ffff9200068ee9e RCX: ffffc90003477620
>>>>>> RDX: 0000000000000005 RSI: ffffffff8b08f91e RDI: 0000000000000028
>>>>>> RBP: 0000000000000001 R08: ffffc90003477710 R09: 0000000000000040
>>>>>> R10: 0000000000000040 R11: 00000000726f7475 R12: 0000000000000000
>>>>>> R13: ffffc90003477620 R14: ffffc90003477710 R15: dffffc0000000000
>>>>>> FS: 0000000000000000(0000) GS:ffff8880b8700000(0000) knlGS:0000000000000000
>>>>>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>>> CR2: 00007fee3cd452d8 CR3: 000000007d116000 CR4: 00000000003526f0
>>>>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>>>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>>>>> ----------------
>>>>>> Code disassembly (best guess), 1 bytes skipped:
>>>>>> 0: 42 80 3c 38 00 cmpb $0x0,(%rax,%r15,1)
>>>>>> 5: 0f 85 fe 02 00 00 jne 0x309
>>>>>> b: 4d 8b a4 24 08 09 00 mov 0x908(%r12),%r12
>>>>>> 12: 00
>>>>>> 13: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax
>>>>>> 1a: fc ff df
>>>>>> 1d: 49 8d 7c 24 28 lea 0x28(%r12),%rdi
>>>>>> 22: 48 89 fa mov %rdi,%rdx
>>>>>> 25: 48 c1 ea 03 shr $0x3,%rdx
>>>>>> * 29: 80 3c 02 00 cmpb $0x0,(%rdx,%rax,1) <-- trapping instruction
>>>>>> 2d: 0f 85 cc 02 00 00 jne 0x2ff
>>>>>> 33: 4d 8b 7c 24 28 mov 0x28(%r12),%r15
>>>>>> 38: 48 rex.W
>>>>>> 39: 8d .byte 0x8d
>>>>>> 3a: 84 24 c8 test %ah,(%rax,%rcx,8)
>>>>
>>>> (...)
>>>>
>>>>> I thought acct(2) was only allowing regular files.
>>>>>
>>>>> acct_on() indeed has :
>>>>>
>>>>> if (!S_ISREG(file_inode(file)->i_mode)) {
>>>>> kfree(acct);
>>>>> filp_close(file, NULL);
>>>>> return -EACCES;
>>>>> }
>>>>>
>>>>> It seems there are other ways to call do_acct_process() targeting a sysfs file ?
> If this is the case, can you point me to the place where this happens?
>
>>>>
>>>> Just to be sure I'm not misunderstanding your comment: do you mean that
>>>> here, the issue is *not* in MPTCP code where we get the 'struct net'
>>>> pointer via 'current->nsproxy->net_ns', but in the FS part, right?
>>>>
>>>> Here, we have an issue because 'current->nsproxy' is NULL, but is it
>>>> normal? Or should we simply exit with an error if it is the case because
>>>> we are in an exiting phase?
>>>>
>>>> I'm just a bit confused, because it looks like 'net' is retrieved from
>>>> different places elsewhere when dealing with sysfs: some get it from
>>>> 'current' like us, some assign 'net' to 'table->extra2', others get it
>>>> from 'table->data' (via a container_of()), etc. Maybe we should not use
>>>> 'current->nsproxy->net_ns' here then?
>>>
>>> I do think this is a bug in process accounting, not in networking.
>>>
>>> It might make sense to output a record on a regular file, but probably
>>> not on any other files.
> It for sure does not make sense to output a record on a sysctl file that
> has a maxlen of just 3*sizeof(int) (kernel/acct.c:79).
>
>>>
>>> diff --git a/kernel/acct.c b/kernel/acct.c
>>> index 179848ad33e978a557ce695a0d6020aa169177c6..a211305cb930f6860d02de7f45ebd260ae03a604
>>> 100644
>>> --- a/kernel/acct.c
>>> +++ b/kernel/acct.c
>>> @@ -495,6 +495,9 @@ static void do_acct_process(struct bsd_acct_struct *acct)
>>> const struct cred *orig_cred;
>>> struct file *file = acct->file;
>>>
>>> + if (S_ISREG(file_inode(file)->i_mode))
>>> + return;
>>> +
> This seems like it does not handle the actual culprit which is. Why is
> the sysctl file being used for the accounting.
>
>>> /*
>>> * Accounting records are not subject to resource limits.
>>> */
>>
>> OK, thank you, that's clearer.
>>
>> So this is then more a question for Joel, right?
>>
>> Do you plan to send this patch to him?
>>
>> #syz set subsystems: fs
>>
>> Cheers,
>> Matt
>> --
>> Sponsored by the NGI0 Core fund.
>>
>
> So what is happening is that:
> 1. The accounting file is set to a non-sysctl file.
> 2. And when accounting tries to write to this file, you get the
> behaviour explained in this mail?
>
> Please correct me if I have miss-read the situation.

@Joel: Thank you for your reply!

I'm sorry, I'm not sure whether I can help here. I hope Eric and/or Al
can jump in.

What I can say is that the original issue has been found by syzbot, and
the reproducer [1] shows that 3 syscalls have been used:
- openat('/proc/sys/net/mptcp/scheduler')
- mprotect()
- acct()

Please also note that the conversation continued in a sub-tread where
you are not in the Cc list, see [2]. In short, Eric suggested another
patch only for sysfs, and Al recommended dropping the use of
'current->nsproxy'.

On my side, I'm looking at dropping the use of 'current->nsproxy' in
sysctl callbacks. I guess such patches will be seen as fixes, except if
Eric's new patch is enough for stable?

[1] https://syzkaller.appspot.com/x/repro.syz?x=1245eaf8580000
[2]
https://lore.kernel.org/netdev/67769ecb.050a0220.3a8527.003f.GAE@google.com/T/#m862d0913ebfcec5e462a9c33b47bc3f6440a2900

Cheers,
Matt
--