linux-kernel - Re: [PATCH] bpf, x32: Fix bug for BPF_JMP | {BPF_JSGT, BPF_JSLE, BPF_JSLT, BPF

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Message-ID: <6735a15f-57af-2d09-016e-d81b1877eca6@iogearbox.net>
Date:   Fri, 26 Apr 2019 14:08:18 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Wang YanQing <udknight@...il.com>, ast@...nel.org,
        davem@...emloft.net, kuznet@....inr.ac.ru, tglx@...utronix.de,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf, x32: Fix bug for BPF_JMP | {BPF_JSGT, BPF_JSLE,
 BPF_JSLT, BPF_JSGE}

Hi Wang,

On 04/26/2019 12:56 PM, Wang YanQing wrote:
> The current method to compare 64-bit numbers for conditional jump is:
> 
> 1) Compare the high 32-bit first.
> 
> 2) If the high 32-bit isn't the same, then goto step 4.
> 
> 3) Compare the low 32-bit.
> 
> 4) Check the desired condition.
> 
> This method is right for unsigned comparison, but it is buggy for signed
> comparison, because it does signed comparison for low 32-bit too.
> 
> There is only one sign bit in 64-bit number, that is the MSB in the 64-bit
> number, it is wrong to treat low 32-bit as signed number and do the signed
> comparison for it.
> 
> This patch fixes the bug.
> 
> Signed-off-by: Wang YanQing <udknight@...il.com>

I presume this issue has coverage in our BPF kselftest suite, right? (If
not, please also add a test into the test_verifier tool.)

Thanks,
Daniel