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

Pavel Machek
2021-08-31T13:30:57Z

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Hi!

> > 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?

> I have just sent the fixes for 4.14. I sent fixes for 4.19 last Friday.
>
> The problem here is that the verifier assumes the source register has a given
> value, but the fixups change that value to something else when it is truncated.
>
> The fixups run after the verifier, so a similar sequence produced by userspace
> will be correctly verified, so no fixes are necessary on adjust_scalar_min_max
> for this specific issue. The fixed-up code is not verified again.

Thanks, understood.

> The challenge in providing those fixes to 4.14 and 4.19 is the absence of JMP32
> in those kernels. So, AX was taken as a temporary, so it would still work on
> JITs.

Yes, I got that far. I have seen the patches for 4.19 and 4.14, and
they should fix my problems. Thanks a lot for them.

Best regards,
Pavel
--
DENX Software Engineering GmbH, Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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