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

Matthieu Baerts
2025-01-04T19:12:13Z

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 ?
>>
>> 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.
>
> 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;
> +
> /*
> * 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
--