Re: [PATCH bpf] bpf: Fix a verifier bug due to incorrect branch offset comparison with cpu=v4

[Yonghong Song <yonghong.song@linux.dev>]

On 11/29/23 5:51 PM, Daniel Borkmann wrote:
> On 11/29/23 8:54 AM, Yonghong Song wrote:
>> Bpf cpu=v4 support is introduced in [1] and Commit 4cd58e9af8b9
>> ("bpf: Support new 32bit offset jmp instruction") added support for new
>> 32bit offset jmp instruction. Unfortunately, in function
>> bpf_adj_delta_to_off(), for new branch insn with 32bit offset, the 
>> offset
>> (plus/minor a small delta) compares to 16-bit offset bound
>> [S16_MIN, S16_MAX], which caused the following verification failure:
>>    $ ./test_progs-cpuv4 -t verif_scale_pyperf180
>>    ...
>>    insn 10 cannot be patched due to 16-bit range
>>    ...
>>    libbpf: failed to load object 'pyperf180.bpf.o'
>>    scale_test:FAIL:expect_success unexpected error: -12 (errno 12)
>>    #405     verif_scale_pyperf180:FAIL
>>
>> Note that due to recent llvm18 development, the patch [2] (already 
>> applied
>> in bpf-next) needs to be applied to bpf tree for testing purpose.
>>
>> The fix is rather simple. For 32bit offset branch insn, the adjusted
>> offset compares to [S32_MIN, S32_MAX] and then verification succeeded.
>>
>>    [1] 
>> https://lore.kernel.org/all/20230728011143.3710005-1-yonghong.song@linux.dev
>>    [2] 
>> https://lore.kernel.org/bpf/20231110193644.3130906-1-yonghong.song@linux.dev
>>
>> Fixes: 4cd58e9af8b9 ("bpf: Support new 32bit offset jmp instruction")
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
>>   kernel/bpf/core.c | 9 ++++++---
>>   1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
>> index cd3afe57ece3..74f2fd48148c 100644
>> --- a/kernel/bpf/core.c
>> +++ b/kernel/bpf/core.c
>> @@ -371,14 +371,17 @@ static int bpf_adj_delta_to_imm(struct bpf_insn 
>> *insn, u32 pos, s32 end_old,
>>   static int bpf_adj_delta_to_off(struct bpf_insn *insn, u32 pos, s32 
>> end_old,
>>                   s32 end_new, s32 curr, const bool probe_pass)
>>   {
>> -    const s32 off_min = S16_MIN, off_max = S16_MAX;
>> +    s32 off_min = S16_MIN, off_max = S16_MAX;
>>       s32 delta = end_new - end_old;
>>       s32 off;
>
> These should all be converted to s64, no? E.g. further below
> the test will never trigger then for jmp32:
>
>        if (off < off_min || off > off_max)
>                 return -ERANGE;


good point! Let us use s64 for potential overflows.
Will send v2 soon.

>
>> -    if (insn->code == (BPF_JMP32 | BPF_JA))
>> +    if (insn->code == (BPF_JMP32 | BPF_JA)) {
>>           off = insn->imm;
>> -    else
>> +        off_min = S32_MIN;
>> +        off_max = S32_MAX;
>> +    } else {
>>           off = insn->off;
>> +    }
>>         if (curr < pos && curr + off + 1 >= end_old)
>>           off += delta;
>>
>