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

Joel Granados
2025-01-06T13:32:56Z

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.

Best

--

Joel Granados