netdev - Re: [PATCH net-next 1/3 v2] bpf: improve verifier ARG_CONST_SIZE_OR

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Date:   Sun, 12 Nov 2017 20:20:21 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Yonghong Song <yhs@...com>, ast@...com, netdev@...r.kernel.org
Cc:     kernel-team@...com
Subject: Re: [PATCH net-next 1/3 v2] bpf: improve verifier
 ARG_CONST_SIZE_OR_ZERO semantics

On 11/12/2017 05:03 PM, Yonghong Song wrote:
> For helpers, the argument type ARG_CONST_SIZE_OR_ZERO permits the
> access size to be 0 when accessing the previous argument (arg).
> Right now, it requires the arg needs to be NULL when size passed
> is 0 or could be 0. It also requires a non-NULL arg when the size
> is proved to be non-0.
> 
> This patch changes verifier ARG_CONST_SIZE_OR_ZERO behavior
> such that for size-0 or possible size-0, it is not required
> the arg equal to NULL.
> 
> There are a couple of reasons for this semantics change, and
> all of them intends to simplify user bpf programs which
> may improve user experience and/or increase chances of
> verifier acceptance. Together with the next patch which
> changes bpf_probe_read arg2 type from ARG_CONST_SIZE to
> ARG_CONST_SIZE_OR_ZERO, the following two examples, which
> fail the verifier currently, are able to get verifier acceptance.
> 
> Example 1:

Looks like patchwork messes up your patch description, see:

   http://patchwork.ozlabs.org/patch/837213/

I think it matches the '===' line with start of the patch. Would be
good if this can be fixed.

> ==========
>     unsigned long len = pend - pstart;
>     len = len > MAX_PAYLOAD_LEN ? MAX_PAYLOAD_LEN : len;
>     len &= MAX_PAYLOAD_LEN;
>     bpf_probe_read(data->payload, len, pstart);
> 
> It does not have test for "len > 0" and it failed the verifier.
> Users may not be aware that they have to add this test.
> Converting the bpf_probe_read helper to have
> ARG_CONST_SIZE_OR_ZERO helps the above code get
> verifier acceptance.
> 
> Example 2:
> ==========
> Here is one example where llvm "messed up" the code and
> the verifier fails.
> 
> ......
>     unsigned long len = pend - pstart;
>     if (len > 0 && len <= MAX_PAYLOAD_LEN)
>       bpf_probe_read(data->payload, len, pstart);
> ......
> 
> The compiler generates the following code and verifier fails:
> ......
> 39: (79) r2 = *(u64 *)(r10 -16)
> 40: (1f) r2 -= r8
> 41: (bf) r1 = r2
> 42: (07) r1 += -1
> 43: (25) if r1 > 0xffe goto pc+3
>    R0=inv(id=0) R1=inv(id=0,umax_value=4094,var_off=(0x0; 0xfff))
>    R2=inv(id=0) R6=map_value(id=0,off=0,ks=4,vs=4095,imm=0) R7=inv(id=0)
>    R8=inv(id=0) R9=inv0 R10=fp0
> 44: (bf) r1 = r6
> 45: (bf) r3 = r8
> 46: (85) call bpf_probe_read#45
> R2 min value is negative, either use unsigned or 'var &= const'
> ......
> 
> The compiler optimization is correct. If r1 = 0,
> r1 - 1 = 0xffffffffffffffff > 0xffe.  If r1 != 0, r1 - 1 will not wrap.
> r1 > 0xffe at insn #43 can actually capture
> both "r1 > 0" and "len <= MAX_PAYLOAD_LEN".
> This however causes an issue in verifier as the value range of arg2
> "r2" does not properly get refined and lead to verification failure.
> 
> Relaxing bpf_prog_read arg2 from ARG_CONST_SIZE to ARG_CONST_SIZE_OR_ZERO
> allows the following simplied code:
>     unsigned long len = pend - pstart;
>     if (len <= MAX_PAYLOAD_LEN)
>       bpf_probe_read(data->payload, len, pstart);
> 
> The llvm compiler will generate less complex code and the
> verifier is able to verify that the program is okay.
> 
> Signed-off-by: Yonghong Song <yhs@...com>
> Acked-by: Alexei Starovoitov <ast@...nel.org>
Acked-by: Daniel Borkmann <daniel@...earbox.net>