netdev - Re: [PATCH bpf-next] bpf/verifier: enable ctx + const + 0.

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Date:   Tue, 1 May 2018 21:35:29 -0700
From:   William Tu <u9012063@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Yonghong Song <yhs@...com>, Yifeng Sun <pkusunyifeng@...il.com>
Subject: Re: [PATCH bpf-next] bpf/verifier: enable ctx + const + 0.

On Tue, May 1, 2018 at 4:16 PM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Mon, Apr 30, 2018 at 10:15:05AM -0700, William Tu wrote:
>> Existing verifier does not allow 'ctx + const + const'.  However, due to
>> compiler optimization, there is a case where BPF compilerit generates
>> 'ctx + const + 0', as shown below:
>>
>>   599: (1d) if r2 == r4 goto pc+2
>>    R0=inv(id=0) R1=ctx(id=0,off=40,imm=0)
>>    R2=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff))
>>    R3=inv(id=0,umax_value=65535,var_off=(0x0; 0xffff)) R4=inv0
>>    R6=ctx(id=0,off=0,imm=0) R7=inv2
>>   600: (bf) r1 = r6                   // r1 is ctx
>>   601: (07) r1 += 36                  // r1 has offset 36
>>   602: (61) r4 = *(u32 *)(r1 +0)      // r1 + 0
>>   dereference of modified ctx ptr R1 off=36+0, ctx+const is allowed,
>>   ctx+const+const is not
>>
>> The reason for BPF backend generating this code is due optimization
>> likes this, explained from Yonghong:
>>     if (...)
>>         *(ctx + 60)
>>     else
>>         *(ctx + 56)
>>
>> The compiler translates it to
>>     if (...)
>>        ptr = ctx + 60
>>     else
>>        ptr = ctx + 56
>>     *(ptr + 0)
>>
>> So load ptr memory become an example of 'ctx + const + 0'.  This patch
>> enables support for this case.
>>
>> Fixes: f8ddadc4db6c7 ("Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net")
>> Cc: Yonghong Song <yhs@...com>
>> Signed-off-by: Yifeng Sun <pkusunyifeng@...il.com>
>> Signed-off-by: William Tu <u9012063@...il.com>
>> ---
>>  kernel/bpf/verifier.c                       |  2 +-
>>  tools/testing/selftests/bpf/test_verifier.c | 13 +++++++++++++
>>  2 files changed, 14 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 712d8655e916..c9a791b9cf2a 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -1638,7 +1638,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
>>               /* ctx accesses must be at a fixed offset, so that we can
>>                * determine what type of data were returned.
>>                */
>> -             if (reg->off) {
>> +             if (reg->off && off != reg->off) {
>>                       verbose(env,
>>                               "dereference of modified ctx ptr R%d off=%d+%d, ctx+const is allowed, ctx+const+const is not\n",
>>                               regno, reg->off, off - reg->off);
>> diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
>> index 1acafe26498b..95ad5d5723ae 100644
>> --- a/tools/testing/selftests/bpf/test_verifier.c
>> +++ b/tools/testing/selftests/bpf/test_verifier.c
>> @@ -8452,6 +8452,19 @@ static struct bpf_test tests[] = {
>>               .prog_type = BPF_PROG_TYPE_SCHED_CLS,
>>       },
>>       {
>> +             "arithmetic ops make PTR_TO_CTX + const + 0 valid",
>> +             .insns = {
>> +                     BPF_ALU64_IMM(BPF_ADD, BPF_REG_1,
>> +                                   offsetof(struct __sk_buff, data) -
>> +                                   offsetof(struct __sk_buff, mark)),

This is:
   r1 += N     // r1 has offset N: the offset between data and mark)

>> +                     BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1, 0),

This is:
   r0 = *(u32 *)(r1 +0)      // r1 + 0

So the above two lines create similar case I hit
  601: (07) r1 += 36                       // r1 has offset 36
  602: (61) r4 = *(u32 *)(r1 +0)      // r1 + 0

>
> How rewritten code looks here?
>
> The patch is allowing check_ctx_access() to proceed with sort-of
> correct 'off' and remember ctx_field_size,
> but in convert_ctx_accesses() it's using insn->off to do conversion.
> Which is zero in this case, so it will convert
> struct __sk_buff {
>         __u32 len; // offset 0
>
> into access of 'struct sk_buff'->len
> and then will add __sk_buff's &data - &mark delta to in-kernel len field.
> Which will point to some random field further down in struct sk_buff.
> Doesn't look correct at all.

why?
So it points to ctx + "offsetof(struct __sk_buff, data) -
offsetof(struct __sk_buff, mark)",
which is ctx + const
then I tested that 'ctx + const + 0' should pass the verifier

> How did you test this patch?
>
Without the patch, the test case will fail.
With the patch, the test case passes.

William