linux-kernel - Re: [PATCH bpf v1] bpf: Fix undefined behavior in narrow load handling

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Date:   Thu, 9 May 2019 23:30:31 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Krzesimir Nowak <krzesimir@...volk.io>, bpf@...r.kernel.org
Cc:     Alban Crequy <alban@...volk.io>,
        Iago López Galeiras <iago@...volk.io>,
        Yonghong Song <yhs@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf v1] bpf: Fix undefined behavior in narrow load
 handling

On 05/08/2019 06:08 PM, Krzesimir Nowak wrote:
> Commit 31fd85816dbe ("bpf: permits narrower load from bpf program
> context fields") made the verifier add AND instructions to clear the
> unwanted bits with a mask when doing a narrow load. The mask is
> computed with
> 
> (1 << size * 8) - 1
> 
> where "size" is the size of the narrow load. When doing a 4 byte load
> of a an 8 byte field the verifier shifts the literal 1 by 32 places to
> the left. This results in an overflow of a signed integer, which is an
> undefined behavior. Typically the computed mask was zero, so the
> result of the narrow load ended up being zero too.
> 
> Cast the literal to long long to avoid overflows. Note that narrow
> load of the 4 byte fields does not have the undefined behavior,
> because the load size can only be either 1 or 2 bytes, so shifting 1
> by 8 or 16 places will not overflow it. And reading 4 bytes would not
> be a narrow load of a 4 bytes field.
> 
> Reviewed-by: Alban Crequy <alban@...volk.io>
> Reviewed-by: Iago López Galeiras <iago@...volk.io>
> Fixes: 31fd85816dbe ("bpf: permits narrower load from bpf program context fields")
> Cc: Yonghong Song <yhs@...com>
> Signed-off-by: Krzesimir Nowak <krzesimir@...volk.io>
> ---
>  kernel/bpf/verifier.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 09d5d972c9ff..950fac024fbb 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -7296,7 +7296,7 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
>  									insn->dst_reg,
>  									shift);
>  				insn_buf[cnt++] = BPF_ALU64_IMM(BPF_AND, insn->dst_reg,
> -								(1 << size * 8) - 1);
> +								(1ULL << size * 8) - 1);
>  			}

Makes sense, good catch & thanks for the fix!

Could you also add a test case to test_verifier.c so we keep track of this?

Thanks,
Daniel