Re: [PATCH bpf] selftests/bpf: fix test_align

[Alexei Starovoitov <ast@fb.com>]

On 1/8/18 8:38 AM, Edward Cree wrote:
> On 05/01/18 23:02, Alexei Starovoitov wrote:
>> since commit 82abbf8d2fc4 the verifier rejects the bit-wise
>> arithmetic on pointers earlier.
>> The test 'dubious pointer arithmetic' now has less output to match on.
>> Adjust it.
>>
>> Fixes: 82abbf8d2fc4 ("bpf: do not allow root to mangle valid pointers")
>> Reported-by: kernel test robot <xiaolong.ye@intel.com>
>> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
>> ---
>>  tools/testing/selftests/bpf/test_align.c | 22 +---------------------
>>  1 file changed, 1 insertion(+), 21 deletions(-)
>>
>> diff --git a/tools/testing/selftests/bpf/test_align.c b/tools/testing/selftests/bpf/test_align.c
>> index 8591c89c0828..471bbbdb94db 100644
>> --- a/tools/testing/selftests/bpf/test_align.c
>> +++ b/tools/testing/selftests/bpf/test_align.c
>> @@ -474,27 +474,7 @@ static struct bpf_align_test tests[] = {
>>  		.result = REJECT,
>>  		.matches = {
>>  			{4, "R5=pkt(id=0,off=0,r=0,imm=0)"},
>> -			/* ptr & 0x40 == either 0 or 0x40 */
>> -			{5, "R5=inv(id=0,umax_value=64,var_off=(0x0; 0x40))"},
>> -			/* ptr << 2 == unknown, (4n) */
>> -			{7, "R5=inv(id=0,smax_value=9223372036854775804,umax_value=18446744073709551612,var_off=(0x0; 0xfffffffffffffffc))"},
>> -			/* (4n) + 14 == (4n+2).  We blow our bounds, because
>> -			 * the add could overflow.
>> -			 */
>> -			{8, "R5=inv(id=0,var_off=(0x2; 0xfffffffffffffffc))"},
>> -			/* Checked s>=0 */
>> -			{10, "R5=inv(id=0,umin_value=2,umax_value=9223372036854775806,var_off=(0x2; 0x7ffffffffffffffc))"},
>> -			/* packet pointer + nonnegative (4n+2) */
>> -			{12, "R6=pkt(id=1,off=0,r=0,umin_value=2,umax_value=9223372036854775806,var_off=(0x2; 0x7ffffffffffffffc))"},
>> -			{14, "R4=pkt(id=1,off=4,r=0,umin_value=2,umax_value=9223372036854775806,var_off=(0x2; 0x7ffffffffffffffc))"},
>> -			/* NET_IP_ALIGN + (4n+2) == (4n), alignment is fine.
>> -			 * We checked the bounds, but it might have been able
>> -			 * to overflow if the packet pointer started in the
>> -			 * upper half of the address space.
>> -			 * So we did not get a 'range' on R6, and the access
>> -			 * attempt will fail.
>> -			 */
>> -			{16, "R6=pkt(id=1,off=0,r=0,umin_value=2,umax_value=9223372036854775806,var_off=(0x2; 0x7ffffffffffffffc))"},
>> +			/* R5 bitwise operator &= on pointer prohibited */
>>  		}
>>  	},
>>  	{
> Rather than neutering this test, we should change it to keep the part where
>  it tests that a large pkt_ptr offset prevents us getting a reg->range.
> Specifically, in this test we have
>     r2 = pkt
>     r5 = large unknown scalar
>     r6 = r2 + r5
>     r4 = r6 + 4
> Then we check r4 < pkt_end, which normally would give r6->range = 4, but in
>  this case must not do so since r6 could be (u64)(-2) in which case r4 = 2
>  < pkt_end despite r6 not pointing into the packet.
> AFAICT there is not other coverage of this case in test_align, and I don't
>  recall such a test being in test_verifier either.  So please instead replace
>  the insns that do prohibited ops on pointers with some other way of creating
>  a large unknown scalar, and keep the rest of the test case intact.

makes sense. will send a follow up patch when security dust settles.