From: Thadeu Lima de Souza Cascardo <firstname.lastname@example.org>
To: Pavel Machek <email@example.com>
kernel list <firstname.lastname@example.org>,
email@example.com, firstname.lastname@example.org, email@example.com
Subject: Re: CVE-2021-3600 aka bpf: Fix 32 bit src register truncation on div/mod
Date: Mon, 30 Aug 2021 15:38:34 -0300 [thread overview]
Message-ID: <YS0lqmZg5Lq0scVv@mussarela> (raw)
On Thu, Aug 26, 2021 at 12:23:37PM +0200, Pavel Machek wrote:
> As far as I can tell, CVE-2021-3600 is still a problem for 4.14 and
> 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,
> 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,
> DENX Software Engineering GmbH, Managing Director: Wolfgang Denk
> HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
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.
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
next prev parent reply other threads:[~2021-08-30 18:38 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-26 10:23 CVE-2021-3600 aka bpf: Fix 32 bit src register truncation on div/mod Pavel Machek
2021-08-26 11:06 ` Thadeu Lima de Souza Cascardo
2021-08-30 18:38 ` Thadeu Lima de Souza Cascardo [this message]
2021-08-31 13:30 ` Pavel Machek
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