Re: [PATCH] bpf: fix misaligned access for BPF_PROG_TYPE_PERF_EVENT program type on x86_32 platform

[Daniel Borkmann <daniel@iogearbox.net>]

On 05/07/2018 09:23 AM, Wang YanQing wrote:
> On Sat, Apr 28, 2018 at 01:29:17PM +0800, Wang YanQing wrote:
>> On Sat, Apr 28, 2018 at 01:33:15AM +0200, Daniel Borkmann wrote:
>>> On 04/28/2018 12:48 AM, Alexei Starovoitov wrote:
>>>> On Thu, Apr 26, 2018 at 05:57:49PM +0800, Wang YanQing wrote:
>>>>> All the testcases for BPF_PROG_TYPE_PERF_EVENT program type in
>>>>> test_verifier(kselftest) report below errors on x86_32:
>>>>> "
>>>>> 172/p unpriv: spill/fill of different pointers ldx FAIL
>>>>> Unexpected error message!
>>>>> 0: (bf) r6 = r10
>>>>> 1: (07) r6 += -8
>>>>> 2: (15) if r1 == 0x0 goto pc+3
>>>>> R1=ctx(id=0,off=0,imm=0) R6=fp-8,call_-1 R10=fp0,call_-1
>>>>> 3: (bf) r2 = r10
>>>>> 4: (07) r2 += -76
>>>>> 5: (7b) *(u64 *)(r6 +0) = r2
>>>>> 6: (55) if r1 != 0x0 goto pc+1
>>>>> R1=ctx(id=0,off=0,imm=0) R2=fp-76,call_-1 R6=fp-8,call_-1 R10=fp0,call_-1 fp-8=fp
>>>>> 7: (7b) *(u64 *)(r6 +0) = r1
>>>>> 8: (79) r1 = *(u64 *)(r6 +0)
>>>>> 9: (79) r1 = *(u64 *)(r1 +68)
>>>>> invalid bpf_context access off=68 size=8
>>>>>
>>>>> 378/p check bpf_perf_event_data->sample_period byte load permitted FAIL
>>>>> Failed to load prog 'Permission denied'!
>>>>> 0: (b7) r0 = 0
>>>>> 1: (71) r0 = *(u8 *)(r1 +68)
>>>>> invalid bpf_context access off=68 size=1
>>>>>
>>>>> 379/p check bpf_perf_event_data->sample_period half load permitted FAIL
>>>>> Failed to load prog 'Permission denied'!
>>>>> 0: (b7) r0 = 0
>>>>> 1: (69) r0 = *(u16 *)(r1 +68)
>>>>> invalid bpf_context access off=68 size=2
>>>>>
>>>>> 380/p check bpf_perf_event_data->sample_period word load permitted FAIL
>>>>> Failed to load prog 'Permission denied'!
>>>>> 0: (b7) r0 = 0
>>>>> 1: (61) r0 = *(u32 *)(r1 +68)
>>>>> invalid bpf_context access off=68 size=4
>>>>>
>>>>> 381/p check bpf_perf_event_data->sample_period dword load permitted FAIL
>>>>> Failed to load prog 'Permission denied'!
>>>>> 0: (b7) r0 = 0
>>>>> 1: (79) r0 = *(u64 *)(r1 +68)
>>>>> invalid bpf_context access off=68 size=8
>>>>> "
>>>>>
>>>>> This patch fix it, the fix isn't only necessary for x86_32, it will fix the
>>>>> same problem for other platforms too, if their size of bpf_user_pt_regs_t
>>>>> can't divide exactly into 8.
>>>>>
>>>>> Signed-off-by: Wang YanQing <udknight@gmail.com>
>>>>> ---
>>>>>  Hi all!
>>>>>  After mainline accept this patch, then we need to submit a sync patch
>>>>>  to update the tools/include/uapi/linux/bpf_perf_event.h.
>>>>>
>>>>>  Thanks.
>>>>>
>>>>>  include/uapi/linux/bpf_perf_event.h | 2 +-
>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/include/uapi/linux/bpf_perf_event.h b/include/uapi/linux/bpf_perf_event.h
>>>>> index eb1b9d2..ff4c092 100644
>>>>> --- a/include/uapi/linux/bpf_perf_event.h
>>>>> +++ b/include/uapi/linux/bpf_perf_event.h
>>>>> @@ -12,7 +12,7 @@
>>>>>  
>>>>>  struct bpf_perf_event_data {
>>>>>  	bpf_user_pt_regs_t regs;
>>>>> -	__u64 sample_period;
>>>>> +	__u64 sample_period __attribute__((aligned(8)));
>>>>
>>>> I don't think this necessary.
>>>> imo it's a bug in pe_prog_is_valid_access
>>>> that should have allowed 8-byte access to 4-byte aligned sample_period.
>>>> The access rewritten by pe_prog_convert_ctx_access anyway,
>>>> no alignment issues as far as I can see.
>>>
>>> Right, good point. Wang, could you give the below a test run:
>>>
>>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
>>> index 56ba0f2..95b9142 100644
>>> --- a/kernel/trace/bpf_trace.c
>>> +++ b/kernel/trace/bpf_trace.c
>>> @@ -833,8 +833,14 @@ static bool pe_prog_is_valid_access(int off, int size, enum bpf_access_type type
>>>  		return false;
>>>  	if (type != BPF_READ)
>>>  		return false;
>>> -	if (off % size != 0)
>>> -		return false;
>>> +	if (off % size != 0) {
>>> +		if (sizeof(long) != 4)
>>> +			return false;
>>> +		if (size != 8)
>>> +			return false;
>>> +		if (off % size != 4)
>>> +			return false;
>>> +	}
>>>
>>>  	switch (off) {
>>>  	case bpf_ctx_range(struct bpf_perf_event_data, sample_period):
>> Hi all!
>>
>> I have tested this patch, but test_verifier reports the same errors
>> for the five testcases.
>>
>> The reason is they all failed to pass the test of bpf_ctx_narrow_access_ok.
>>
>> Thanks.
> Hi! Daniel Borkmann.
> 
> Do you have any plan to fix bpf_ctx_narrow_access_ok for these problems?

Yep, sorry for the delay, will get to it during this week.

Thanks,
Daniel