Re: general protection fault in security_inode_getattr

Dmitry Vyukov
2020-10-30T19:56:33Z

On Fri, Oct 30, 2020 at 8:21 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
> > > On Mon, Aug 24, 2020 at 11:00 PM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> > > >
> > > > On Mon, Aug 24, 2020 at 9:37 PM syzbot
> > > > <syzbot+f07cc9be8d1d226947ed@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