general protection fault in security_inode_getattr

[Dmitry Vyukov]

On Fri, Oct 30, 2020 at 8:21 PM Miklos Szeredi <miklos at szeredi.hu> wrote:
> > > On Mon, Aug 24, 2020 at 11:00 PM Ondrej Mosnacek <omosnace at redhat.com> wrote:
> > > >
> > > > On Mon, Aug 24, 2020 at 9:37 PM syzbot
> > > > <syzbot+f07cc9be8d1d226947ed at syzkaller.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()...
> >
> >
> > >   26: 48 c1 e8 03          shr    $0x3,%rax
> > >   2a:* 42 80 3c 38 00        cmpb   $0x0,(%rax,%r15,1) <-- trapping instruction
> >
> >
> > This access is part of KASAN check. But the original address kernel
> > tries to access is NULL, so it's not an issue with KASAN.
> >
> > The line is this:
> >
> > int security_inode_getattr(const struct path *path)
> > {
> >     if (unlikely(IS_PRIVATE(d_backing_inode(path->dentry))))
> >         return 0;
> >
> > So it's either path is NULL, or something in d_backing_inode
> > dereferences NULL path->dentry.
> >
> > The reproducer does involve overlayfs:
> >
> > mkdir(&(0x7f0000000240)='./file1\x00', 0x0)
> > mkdir(&(0x7f0000000300)='./bus\x00', 0x0)
> > r0 = creat(&(0x7f00000000c0)='./bus/file1\x00', 0x0)
> > mkdir(&(0x7f0000000080)='./file0\x00', 0x0)
> > mount$overlay(0x400002, &(0x7f0000000000)='./bus\x00',
> > &(0x7f0000000100)='overlay\x00', 0x0,
> > &(0x7f00000003c0)=ANY=[@ANYBLOB='upperdir=./file1,lowerdir=./bus,workdir=./file0,metacopy=on'])
> > link(&(0x7f0000000200)='./bus/file1\x00', &(0x7f00000002c0)='./bus/file0\x00')
> > write$RDMA_USER_CM_CMD_RESOLVE_ADDR(r0, 0x0, 0x0)
> > acct(&(0x7f0000000040)='./bus/file0\x00')
> >
> > Though, it may be overlayfs-related, or it may be a generic bug that
> > requires a tricky reproducer and the only reproducer syzbot come up
> > with happened to involve overlayfs.
> > But there are 4 reproducers on syzbot dashboard and all of them
> > involve overlayfs and they are somewhat different. So my bet would be
> > on overlayfs.
>
> Seems there's no C reproducer, though.   Can this be reproduced
> without KASAN obfuscating the oops?

I guess so.
If you are interest in what exact field is NULL, I think there is
enough info in the asm already:

> > >    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

The access via the NULL pointer happens with offset 0x68:

> > >   1f: 48 83 c3 68          add    $0x68,%rbx

So we just need to find what's here accesses with offset 0x68:

> >     if (unlikely(IS_PRIVATE(d_backing_inode(path->dentry))))

And that pointer itself was loaded from something at offset 0x8 previously:

> > >    2: 49 8d 5e 08          lea    0x8(%r14),%rbx