netdev - Re: [PATCH bpf] bpf: fix sanitation rewrite in case of non-pointers

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Message-ID: <e1250fdc-96f7-e020-eaff-7e0b67be9ead@iogearbox.net>
Date:   Sat, 2 Mar 2019 00:22:59 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Song Liu <liu.song.a23@...il.com>
Cc:     Alexei Starovoitov <ast@...com>, afabre@...udflare.com,
        Marek Majkowski <marek@...udflare.com>, bpf@...r.kernel.org,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf] bpf: fix sanitation rewrite in case of non-pointers

On 03/02/2019 12:18 AM, Song Liu wrote:
> On Fri, Mar 1, 2019 at 1:06 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>>
>> Marek reported that he saw an issue with the below snippet in that
>> timing measurements where off when loaded as unpriv while results
>> were reasonable when loaded as privileged:
>>
>>     [...]
>>     uint64_t a = bpf_ktime_get_ns();
>>     uint64_t b = bpf_ktime_get_ns();
>>     uint64_t delta = b - a;
>>     if ((int64_t)delta > 0) {
>>     [...]
>>
>> Turns out there is a bug where a corner case is missing in the fix
>> d3bd7413e0ca ("bpf: fix sanitation of alu op with pointer / scalar
>> type from different paths"), namely fixup_bpf_calls() only checks
>> whether aux has a non-zero alu_state, but it also needs to test for
>> the case of BPF_ALU_NON_POINTER since in both occasions we need to
>> skip the masking rewrite (as there is nothing to mask).
>>
>> Fixes: d3bd7413e0ca ("bpf: fix sanitation of alu op with pointer / scalar type from different paths")
>> Reported-by: Marek Majkowski <marek@...udflare.com>
>> Reported-by: Arthur Fabre <afabre@...udflare.com>
>> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
>> Link: https://lore.kernel.org/netdev/CAJPywTJqP34cK20iLM5YmUMz9KXQOdu1-+BZrGMAGgLuBWz7fg@mail.gmail.com/T/
>> ---
>>  [ Test case will be routed via bpf-next to avoid useless merge churn
>>    due to test_verifier rework in bpf-next. ]
>>
>>  kernel/bpf/verifier.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 8f295b790297..5fcce2f4209d 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -6920,7 +6920,8 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
>>                         u32 off_reg;
>>
>>                         aux = &env->insn_aux_data[i + delta];
>> -                       if (!aux->alu_state)
>> +                       if (!aux->alu_state ||
>> +                           aux->alu_state == BPF_ALU_NON_POINTER)
> 
> alu_state is a bitmap. Shall we check "aux->alu_state &
> BPF_ALU_NON_POINTER" here?

The state in this case can only ever be BPF_ALU_NON_POINTER, any other
setting from sanitize_val_alu() would be a violation.