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

Eric Dumazet
2025-01-04T18:53:34Z

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.
*/