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

From: Matthieu Baerts <[email-masked]>
Date: Sat, 4 Jan 2025 19:38:39 +0100

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
> <[email-masked]> 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: [email-masked]
>>
>> 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?

Cheers,
Matt
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