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

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Message-ID: <20210831133053.GA32426@duo.ucw.cz>
Date:   Tue, 31 Aug 2021 15:30:53 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
Cc:     Pavel Machek <pavel@...x.de>, stable@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        daniel@...earbox.net, john.fastabend@...il.com, ast@...nel.org
Subject: Re: CVE-2021-3600 aka bpf: Fix 32 bit src register truncation on
 div/mod

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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