linux-kernel - Re: general protection fault in security_inode

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Date:   Fri, 30 Oct 2020 14:02:22 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Ondrej Mosnacek <omosnace@...hat.com>
Cc:     syzbot <syzbot+f07cc9be8d1d226947ed@...kaller.appspotmail.com>,
        andriin@...com, Alexei Starovoitov <ast@...nel.org>,
        bpf <bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        James Morris <jmorris@...ei.org>, john.fastabend@...il.com,
        kafai@...com, KP Singh <kpsingh@...omium.org>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>,
        Linux Security Module list 
        <linux-security-module@...r.kernel.org>,
        network dev <netdev@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Song Liu <songliubraving@...com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>, yhs@...com,
        linux-fsdevel@...r.kernel.org,
        overlayfs <linux-unionfs@...r.kernel.org>
Subject: Re: general protection fault in security_inode_getattr

On Mon, Aug 24, 2020 at 11:00 PM Ondrej Mosnacek <omosnace@...hat.com> wrote:
>
> On Mon, Aug 24, 2020 at 9:37 PM syzbot
> <syzbot+f07cc9be8d1d226947ed@...kaller.appspotmail.com> wrote:
> > syzbot has found a reproducer for the following issue on:
>
> Looping in fsdevel and OverlayFS maintainers, as this seems to be
> FS/OverlayFS related...

Hmm, the oopsing code is always something like:

All code
========
   0: 1b fe                sbb    %esi,%edi
   2: 49 8d 5e 08          lea    0x8(%r14),%rbx
   6: 48 89 d8              mov    %rbx,%rax
   9: 48 c1 e8 03          shr    $0x3,%rax
   d: 42 80 3c 38 00        cmpb   $0x0,(%rax,%r15,1)
  12: 74 08                je     0x1c
  14: 48 89 df              mov    %rbx,%rdi
  17: e8 bc b4 5b fe        callq  0xfffffffffe5bb4d8
  1c: 48 8b 1b              mov    (%rbx),%rbx
  1f: 48 83 c3 68          add    $0x68,%rbx
  23: 48 89 d8              mov    %rbx,%rax
  26: 48 c1 e8 03          shr    $0x3,%rax
  2a:* 42 80 3c 38 00        cmpb   $0x0,(%rax,%r15,1) <-- trapping instruction
  2f: 74 08                je     0x39
  31: 48 89 df              mov    %rbx,%rdi
  34: e8 9f b4 5b fe        callq  0xfffffffffe5bb4d8
  39: 48 8b 1b              mov    (%rbx),%rbx
  3c: 48 83 c3 0c          add    $0xc,%rbx


And that looks (to me) like the unrolled loop in call_int_hook().  I
don't see how that could be related to overlayfs, though it's
definitely interesting why it only triggers from
overlay->vfs_getattr()->security_inode_getattr()...

Thanks,
Miklos