Re: [syzbot] [mm?] general protection fault in find_mergeable_anon_vma

[Jann Horn <jannh@google.com>]

On Mon, Dec 9, 2024 at 1:53 PM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
> On Mon, Dec 09, 2024 at 03:20:19AM -0800, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    feffde684ac2 Merge tag 'for-6.13-rc1-tag' of git://git.ker..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=17f85fc0580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=50c7a61469ce77e7
> > dashboard link: https://syzkaller.appspot.com/bug?extid=2d788f4f7cb660dac4b7
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
>
> Points to this being racey.
>
> >
> > Downloadable assets:
> > disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/7feb34a89c2a/non_bootable_disk-feffde68.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/6135c7297e8e/vmlinux-feffde68.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/6c154fdcc9cb/bzImage-feffde68.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+2d788f4f7cb660dac4b7@syzkaller.appspotmail.com
> >
> > Oops: general protection fault, probably for non-canonical address 0xdffffc0000000080: 0000 [#1] PREEMPT SMP KASAN NOPTI
> > KASAN: null-ptr-deref in range [0x0000000000000400-0x0000000000000407]
>
> This doesn't make a huge amount of sense to me, the VMA is not 0x400 (1,024)
> bytes in size... and the actual faulting offset seems to be 0xdffffc0000000080
> which is 0x80 off from some KASAN-specified value?

If you look at the disassembly, you can see this:

  13: 4d 89 ec              mov    %r13,%r12
  16: 49 c1 ec 03          shr    $0x3,%r12
  1a: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax
  21: fc ff df
* 24: 41 80 3c 04 00        cmpb   $0x0,(%r12,%rax,1) <-- trapping instruction

R13 is 0000000000000406, that's the address we're about to access.
This code is trying to read KASAN shadow memory for that address by
reading from 0xdffffc0000000000+address>>3, which for real kernel
addresses gives you an address in the "KASAN shadow memory" range (see
https://kernel.org/doc/html/latest/arch/x86/x86_64/mm.html), but for
addresses in the low half of the address space gives you non-canonical
addresses starting with 0xdfff that cause #GP on access.
The second line "KASAN: null-ptr-deref in range
[0x0000000000000400-0x0000000000000407]" is basically computed by
doing that calculation in reverse.

> This would be vma->vm_file. But that also doesn't really make any sense.
>
> But I wonder...
>
> I see in the report at [0] that there's a failure injection in vm_area_dup() on
> fork:
>
> [   73.842623][ T5318]  ? kmem_cache_alloc_noprof+0x48/0x380
> [   73.844725][ T5318]  ? __pfx___might_resched+0x10/0x10
> [   73.846687][ T5318]  should_fail_ex+0x3b0/0x4e0
> [   73.848496][ T5318]  should_failslab+0xac/0x100
> [   73.850232][ T5318]  ? vm_area_dup+0x27/0x290
> [   73.852017][ T5318]  kmem_cache_alloc_noprof+0x70/0x380
> [   73.854011][ T5318]  vm_area_dup+0x27/0x290
> [   73.855771][ T5318]  copy_mm+0xc1d/0x1f90
>
> I also see in the fork logic we have the following code on error path:
>
>         mas_set_range(&vmi.mas, mpnt->vm_start, mpnt->vm_end - 1);
>         mas_store(&vmi.mas, XA_ZERO_ENTRY);
>
> And XA_ZERO_ENTRY is 0x406.

That matches...