Re: general protection fault in unlink_file_vma

[Souptick Joarder]

Hi Hiff,

On Wed, Sep 9, 2020 at 9:45 AM Hillf Danton <hdanton@xxxxxxxx> wrote:
>
>
> Tue, 08 Sep 2020 17:19:17 -0700
> > syzbot found the following issue on:
> >
> > HEAD commit:    59126901 Merge tag 'perf-tools-fixes-for-v5.9-2020-09-03' ..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1166cb5d900000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=3c5f6ce8d5b68299
> > dashboard link: https://syzkaller.appspot.com/bug?extid=c5d5a51dcbb558ca0cb5
> > compiler:       gcc (GCC) 10.1.0-syz 20200507
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11901e95900000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15f56195900000
> >
> > Bisection is inconclusive: the issue happens on the oldest tested release.
> >
> > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1205faed900000
> > final oops:     https://syzkaller.appspot.com/x/report.txt?x=1105faed900000
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1605faed900000
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+c5d5a51dcbb558ca0cb5@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > general protection fault, probably for non-canonical address 0xe00eeaee0000003b: 0000 [#1] PREEMPT SMP KASAN
> > KASAN: maybe wild-memory-access in range [0x00777770000001d8-0x00777770000001df]
> > CPU: 1 PID: 10488 Comm: syz-executor721 Not tainted 5.9.0-rc3-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > RIP: 0010:unlink_file_vma+0x57/0xb0 mm/mmap.c:164
> > Code: 4c 8b a5 a0 00 00 00 4d 85 e4 74 4e e8 92 d7 cd ff 49 8d bc 24 d8 01 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 75 3d 4d 8b b4 24 d8 01 00 00 4d 8d 6e 78 4c 89 ef e8
> > RSP: 0018:ffffc9000ac0f9b0 EFLAGS: 00010202
> > RAX: dffffc0000000000 RBX: ffff88800010ceb0 RCX: ffffffff81592421
> > RDX: 000eeeee0000003b RSI: ffffffff81a6736e RDI: 00777770000001d8
> > RBP: ffff88800010ceb0 R08: 0000000000000001 R09: ffff88801291a50f
> > R10: ffffed10025234a1 R11: 0000000000000001 R12: 0077777000000000
> > R13: 00007f1eea0da000 R14: 00007f1eea0d9000 R15: 0000000000000000
> > FS:  0000000000000000(0000) GS:ffff8880ae700000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007f1eea11a9d0 CR3: 000000000007e000 CR4: 00000000001506e0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > Call Trace:
> >  free_pgtables+0x1b3/0x2f0 mm/memory.c:415
> >  exit_mmap+0x2c0/0x530 mm/mmap.c:3184
> >  __mmput+0x122/0x470 kernel/fork.c:1076
> >  mmput+0x53/0x60 kernel/fork.c:1097
> >  exit_mm kernel/exit.c:483 [inline]
> >  do_exit+0xa8b/0x29f0 kernel/exit.c:793
> >  do_group_exit+0x125/0x310 kernel/exit.c:903
> >  get_signal+0x428/0x1f00 kernel/signal.c:2757
> >  arch_do_signal+0x82/0x2520 arch/x86/kernel/signal.c:811
> >  exit_to_user_mode_loop kernel/entry/common.c:136 [inline]
> >  exit_to_user_mode_prepare+0x1ae/0x200 kernel/entry/common.c:167
> >  syscall_exit_to_user_mode+0x7e/0x2e0 kernel/entry/common.c:242
> >  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> Looks like d70cec898324 ("mm: mmap: merge vma after call_mmap() if possible")
> added an extra fput.

Can you please help me to understand how do you figure out this commit ?
>
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1781,7 +1781,6 @@ unsigned long mmap_region(struct file *f
>                         merge = vma_merge(mm, prev, vma->vm_start, vma->vm_end, vma->vm_flags,
>                                 NULL, vma->vm_file, vma->vm_pgoff, NULL, NULL_VM_UFFD_CTX);
>                         if (merge) {
> -                               fput(file);
>                                 vm_area_free(vma);
>                                 vma = merge;
>                                 /* Update vm_flags and possible addr to pick up the change. We don't
>
>