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From: Daniel Borkmann <daniel@iogearbox.net>
To: Krzesimir Nowak <krzesimir@kinvolk.io>, bpf@vger.kernel.org
Cc: "Alban Crequy" <alban@kinvolk.io>,
	"Iago López Galeiras" <iago@kinvolk.io>,
	"Yonghong Song" <yhs@fb.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Martin KaFai Lau" <kafai@fb.com>,
	"Song Liu" <songliubraving@fb.com>,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH bpf v1] bpf: Fix undefined behavior in narrow load handling
Date: Thu, 9 May 2019 23:30:31 +0200	[thread overview]
Message-ID: <46056c60-f106-e539-b614-498cb1e9e3d0@iogearbox.net> (raw)
In-Reply-To: <20190508160859.4380-1-krzesimir@kinvolk.io>

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@kinvolk.io>
> Reviewed-by: Iago López Galeiras <iago@kinvolk.io>
> Fixes: 31fd85816dbe ("bpf: permits narrower load from bpf program context fields")
> Cc: Yonghong Song <yhs@fb.com>
> Signed-off-by: Krzesimir Nowak <krzesimir@kinvolk.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

  reply	other threads:[~2019-05-09 21:30 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-08 16:08 Krzesimir Nowak
2019-05-09 21:30 ` Daniel Borkmann [this message]
2019-05-10 10:16   ` Krzesimir Nowak
2019-05-13  0:01     ` Daniel Borkmann

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