Re: CVE-2021-3600 aka bpf: Fix 32 bit src register truncation on div/mod

Thadeu Lima de Souza Cascardo
2021-08-26T11:06:35Z

On Thu, Aug 26, 2021 at 12:23:37PM +0200, Pavel Machek wrote:
> Hi!
>
> As far as I can tell, CVE-2021-3600 is still a problem for 4.14 and
> 4.19.
>
> Unfortunately, those kernels lack BPF_JMP32 support, and that support
> is too big and intrusive to backport.
>
> So I tried to come up with solution without JMP32 support... only to
> end up with not seeing the bug in the affected code.
>
> Changelog says:
>
> bpf: Fix 32 bit src register truncation on div/mod
>
> While reviewing a different fix, John and I noticed an oddity in one of the
> BPF program dumps that stood out, for example:
>
> # bpftool p d x i 13
> 0: (b7) r0 = 808464450
> 1: (b4) w4 = 808464432
> 2: (bc) w0 = w0
> 3: (15) if r0 == 0x0 goto pc+1
> 4: (9c) w4 %= w0
> [...]
>
> In line 2 we noticed that the mov32 would 32 bit truncate the original src
> register for the div/mod operation. While for the two operations the dst
> register is typically marked unknown e.g. from adjust_scalar_min_max_vals()
> the src register is not, and thus verifier keeps tracking original bounds,
> simplified:
>
> So this explains "mov32 w0, w0" is problematic, and fixes the bug by
> replacing it with jmp32. Unfortunately, I can't do that in 4.19; plus
> I don't really see how the bug is solved -- we avoided adding mov32
> sequence that triggers the problem, but the problematic sequence could
> still be produced by the userspace, no?
>
> Does adjust_scalar_min_max_vals still need fixing?
>
> Do you have any hints how to solve this in 4.19?
>
> Best regards,
> Pavel
> --
> DENX Software Engineering GmbH, Managing Director: Wolfgang Denk
> HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

Hi, Pavel.

We have fixed this in our 4.15 kernels with help from Daniel, John, Alexei and
Salvatore. I am a little busy right now, but will make time to work on the
commits we had for 4.19. 4.19 is also affected by CVE-2021-3444, IIRC.

When I get back to it, I may try to answer some of your questions.

Cascardo.