Re: general protection fault in security_inode_getattr

Dmitry Vyukov
2020-10-30T18:42:39Z

On Fri, Oct 30, 2020 at 2:02 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.