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

Joel Granados
2025-01-08T14:37:24Z

On Mon, Jan 06, 2025 at 03:27:47PM +0100, Matthieu Baerts wrote:
> 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'.
Perfect. Thx for the summary. I'll remove this thread from my radar as
it seems that a fix has already been found.

Best

--

Joel Granados