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

[Pavel Machek]

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

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