[bpf-next,v2,2/2] selftests/bpf: check null propagation only neither reg is PTR_TO_BTF_ID

  Verify that nullness information is not porpagated in the branches
of register to register JEQ and JNE operations if one of them is
PTR_TO_BTF_ID.

Signed-off-by: Hao Sun <sunhao.th@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
---
 .../bpf/verifier/jeq_infer_not_null.c         | 22 +++++++++++++++++++
 1 file changed, 22 insertions(+)
  

On 12/12/22 7:04 PM, Hao Sun wrote:
> Verify that nullness information is not porpagated in the branches
> of register to register JEQ and JNE operations if one of them is
> PTR_TO_BTF_ID.

Thanks for the fix and test.

> 
> Signed-off-by: Hao Sun <sunhao.th@gmail.com>
> Acked-by: Yonghong Song <yhs@fb.com>
> ---
>   .../bpf/verifier/jeq_infer_not_null.c         | 22 +++++++++++++++++++
>   1 file changed, 22 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c b/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c
> index 67a1c07ead34..b2b215227d97 100644
> --- a/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c
> +++ b/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c
> @@ -172,3 +172,25 @@
>   	.prog_type = BPF_PROG_TYPE_XDP,
>   	.result = ACCEPT,
>   },
> +{
> +	"jne/jeq infer not null, PTR_TO_MAP_OR_NULL unchanged with PTR_TO_BTF_ID reg",
> +	.insns = {
> +	BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
> +	BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
> +	BPF_ST_MEM(BPF_DW, BPF_REG_2, 0, 0),
> +	BPF_LD_MAP_FD(BPF_REG_1, 0),
> +	/* r6 = bpf_map->inner_map_meta; */
> +	BPF_LDX_MEM(BPF_DW, BPF_REG_6, BPF_REG_1, 8),

This bpf_map->inner_map_meta requires CO-RE. It works now but could be fragile 
in different platform and in the future bpf_map changes. Take a look at the 
map_ptr_kern.c which uses "__attribute__((preserve_access_index))" at the 
"struct bpf_map".

Please translate this verifer test into a proper bpf prog in C code such that it 
can use the CO-RE in libbpf.  It should run under test_progs instead of 
test_verifier. The bpf prog can include the "vmlinux.h" to get the 
"__attribute__((preserve_access_index))" for free.  Take a look at 
https://lore.kernel.org/all/20221207201648.2990661-2-andrii@kernel.org/ which 
has example on how to check verifier message in test_progs.


> +	/* r0 = map_lookup_elem(r1, r2); */
> +	BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
> +	/* if (r0 == r6) read *r0; */
> +	BPF_JMP_REG(BPF_JEQ, BPF_REG_6, BPF_REG_0, 1),
> +	BPF_EXIT_INSN(),
> +	BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0),
> +	BPF_EXIT_INSN(),
> +	},
> +	.fixup_map_hash_8b = { 3 },
> +	.prog_type = BPF_PROG_TYPE_XDP,
> +	.result = REJECT,
> +	.errstr = "R0 invalid mem access 'map_value_or_null'",
> +},
  

> On 20 Dec 2022, at 6:01 AM, Martin KaFai Lau <martin.lau@linux.dev> wrote:
> 
> On 12/12/22 7:04 PM, Hao Sun wrote:
>> Verify that nullness information is not porpagated in the branches
>> of register to register JEQ and JNE operations if one of them is
>> PTR_TO_BTF_ID.
> 
> Thanks for the fix and test.
> 
>> Signed-off-by: Hao Sun <sunhao.th@gmail.com>
>> Acked-by: Yonghong Song <yhs@fb.com>
>> ---
>>  .../bpf/verifier/jeq_infer_not_null.c         | 22 +++++++++++++++++++
>>  1 file changed, 22 insertions(+)
>> diff --git a/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c b/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c
>> index 67a1c07ead34..b2b215227d97 100644
>> --- a/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c
>> +++ b/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c
>> @@ -172,3 +172,25 @@
>>   .prog_type = BPF_PROG_TYPE_XDP,
>>   .result = ACCEPT,
>>  },
>> +{
>> + "jne/jeq infer not null, PTR_TO_MAP_OR_NULL unchanged with PTR_TO_BTF_ID reg",
>> + .insns = {
>> + BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
>> + BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
>> + BPF_ST_MEM(BPF_DW, BPF_REG_2, 0, 0),
>> + BPF_LD_MAP_FD(BPF_REG_1, 0),
>> + /* r6 = bpf_map->inner_map_meta; */
>> + BPF_LDX_MEM(BPF_DW, BPF_REG_6, BPF_REG_1, 8),
> 
> This bpf_map->inner_map_meta requires CO-RE. It works now but could be fragile in different platform and in the future bpf_map changes. Take a look at the map_ptr_kern.c which uses "__attribute__((preserve_access_index))" at the "struct bpf_map".
> 
> Please translate this verifer test into a proper bpf prog in C code such that it can use the CO-RE in libbpf.  It should run under test_progs instead of test_verifier. The bpf prog can include the "vmlinux.h" to get the "__attribute__((preserve_access_index))" for free.  Take a look at https://lore.kernel.org/all/20221207201648.2990661-2-andrii@kernel.org/ which has example on how to check verifier message in test_progs.
> 

Sure, thanks for the hint, will send patch v3 soon.

Thanks
Hao
  

> On 20 Dec 2022, at 6:01 AM, Martin KaFai Lau <martin.lau@linux.dev> wrote:
> 
> On 12/12/22 7:04 PM, Hao Sun wrote:
>> Verify that nullness information is not porpagated in the branches
>> of register to register JEQ and JNE operations if one of them is
>> PTR_TO_BTF_ID.
> 
> Thanks for the fix and test.
> 
>> Signed-off-by: Hao Sun <sunhao.th@gmail.com>
>> Acked-by: Yonghong Song <yhs@fb.com>
>> ---
>>  .../bpf/verifier/jeq_infer_not_null.c         | 22 +++++++++++++++++++
>>  1 file changed, 22 insertions(+)
>> diff --git a/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c b/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c
>> index 67a1c07ead34..b2b215227d97 100644
>> --- a/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c
>> +++ b/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c
>> @@ -172,3 +172,25 @@
>>   .prog_type = BPF_PROG_TYPE_XDP,
>>   .result = ACCEPT,
>>  },
>> +{
>> + "jne/jeq infer not null, PTR_TO_MAP_OR_NULL unchanged with PTR_TO_BTF_ID reg",
>> + .insns = {
>> + BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
>> + BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
>> + BPF_ST_MEM(BPF_DW, BPF_REG_2, 0, 0),
>> + BPF_LD_MAP_FD(BPF_REG_1, 0),
>> + /* r6 = bpf_map->inner_map_meta; */
>> + BPF_LDX_MEM(BPF_DW, BPF_REG_6, BPF_REG_1, 8),
> 
> This bpf_map->inner_map_meta requires CO-RE. It works now but could be fragile in different platform and in the future bpf_map changes. Take a look at the map_ptr_kern.c which uses "__attribute__((preserve_access_index))" at the "struct bpf_map".
> 
> Please translate this verifer test into a proper bpf prog in C code such that it can use the CO-RE in libbpf.  It should run under test_progs instead of test_verifier. The bpf prog can include the "vmlinux.h" to get the "__attribute__((preserve_access_index))" for free.  Take a look at https://lore.kernel.org/all/20221207201648.2990661-2-andrii@kernel.org/ which has example on how to check verifier message in test_progs.
> 

Hi,

I’ve tried something like the bellow, but soon realized that this
won’t work because once compiler figures out `inner_map` equals
to `val`, it can choose either reg to write into in the following
path, meaning that this program can be rejected due to writing
into read-only PTR_TO_BTF_ID reg, and this makes the test useless.

Essentially, we want two regs, one points to PTR_TO_BTD_ID, one
points to MAP_VALUR_OR_NULL, then compare them and deref map val.
It’s hard to implement this in C level because compilers decide
which reg to use but not us, maybe we can just drop this test. 

thoughts? 
  
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 1);
+	__type(key, u64);
+	__type(value, u64);
+} m_hash SEC(".maps");
+
+SEC("?raw_tp")
+__failure __msg("invalid mem access 'map_value_or_null")
+int jeq_infer_not_null_ptr_to_btfid(void *ctx)
+{
+	struct bpf_map *map = (struct bpf_map *)&m_hash;
+	struct bpf_map *inner_map = map->inner_map_meta;
+	u64 key = 0, ret = 0, *val;
+
+	val = bpf_map_lookup_elem(map, &key);
+	/* Do not mark ptr as non-null if one of them is
+	 * PTR_TO_BTF_ID, reject because of invalid access
+	 * to map value.
+	 */
+	if (val == inner_map)
+		ret = *val;
+
+	return ret;
+}
  

On 12/21/22 5:46 AM, Hao Sun wrote:
> Hi,
> 
> I’ve tried something like the bellow, but soon realized that this
> won’t work because once compiler figures out `inner_map` equals
> to `val`, it can choose either reg to write into in the following
> path, meaning that this program can be rejected due to writing
> into read-only PTR_TO_BTF_ID reg, and this makes the test useless.

hmm... I read the above a few times but I still don't quite get it.  In 
particular, '...can be rejected due to writing into read-only PTR_TO_BTF_ID 
reg...'.  Where is it writing into a read-only PTR_TO_BTF_ID reg in the 
following bpf prog?  Did I overlook something?

> 
> Essentially, we want two regs, one points to PTR_TO_BTD_ID, one
> points to MAP_VALUR_OR_NULL, then compare them and deref map val.

If I read this request correctly, I guess the compiler has changed 'ret = *val' 
to 'ret = *inner_map'?  Thus, the verifier did not reject because it deref a 
PTR_TO_BTF_ID?

> It’s hard to implement this in C level because compilers decide
> which reg to use but not us, maybe we can just drop this test.

Have you tried inline assembly.  Something like this (untested):

         asm volatile (
                 "r8 = %[val];\n"
                 "r9 = %[inner_map];\n"
		"if r8 != r9 goto +1;\n"
                 "%[ret] = *(u64 *)(r8 +0);\n"
                 :[ret] "+r"(ret)
                 : [inner_map] "r"(inner_map), [val] "r"(val)
                 :"r8", "r9");

Please attach the verifier output in the future.  It will be easier to understand.

> 
> thoughts?
>    
> +struct {
> +	__uint(type, BPF_MAP_TYPE_HASH);
> +	__uint(max_entries, 1);
> +	__type(key, u64);
> +	__type(value, u64);
> +} m_hash SEC(".maps");
> +
> +SEC("?raw_tp")
> +__failure __msg("invalid mem access 'map_value_or_null")
> +int jeq_infer_not_null_ptr_to_btfid(void *ctx)
> +{
> +	struct bpf_map *map = (struct bpf_map *)&m_hash;
> +	struct bpf_map *inner_map = map->inner_map_meta;
> +	u64 key = 0, ret = 0, *val;
> +
> +	val = bpf_map_lookup_elem(map, &key);
> +	/* Do not mark ptr as non-null if one of them is
> +	 * PTR_TO_BTF_ID, reject because of invalid access
> +	 * to map value.
> +	 */
> +	if (val == inner_map)
> +		ret = *val;
> +
> +	return ret;
> +}
  

Martin KaFai Lau <martin.lau@linux.dev> 于2022年12月22日周四 05:21写道：
>
> On 12/21/22 5:46 AM, Hao Sun wrote:
> > Hi,
> >
> > I’ve tried something like the bellow, but soon realized that this
> > won’t work because once compiler figures out `inner_map` equals
> > to `val`, it can choose either reg to write into in the following
> > path, meaning that this program can be rejected due to writing
> > into read-only PTR_TO_BTF_ID reg, and this makes the test useless.
>
> hmm... I read the above a few times but I still don't quite get it.  In
> particular, '...can be rejected due to writing into read-only PTR_TO_BTF_ID
> reg...'.  Where is it writing into a read-only PTR_TO_BTF_ID reg in the
> following bpf prog?  Did I overlook something?
>
> >
> > Essentially, we want two regs, one points to PTR_TO_BTD_ID, one
> > points to MAP_VALUR_OR_NULL, then compare them and deref map val.
>
> If I read this request correctly, I guess the compiler has changed 'ret = *val'
> to 'ret = *inner_map'?  Thus, the verifier did not reject because it deref a
> PTR_TO_BTF_ID?
>

Yes, and if we do "*val = 0", it's rejected due to writing to read-only
PTR_TO_BTF_ID reg.

> > It’s hard to implement this in C level because compilers decide
> > which reg to use but not us, maybe we can just drop this test.
>
> Have you tried inline assembly.  Something like this (untested):
>
>          asm volatile (
>                  "r8 = %[val];\n"
>                  "r9 = %[inner_map];\n"
>                 "if r8 != r9 goto +1;\n"
>                  "%[ret] = *(u64 *)(r8 +0);\n"
>                  :[ret] "+r"(ret)
>                  : [inner_map] "r"(inner_map), [val] "r"(val)
>                  :"r8", "r9");
>

This would work, didn't realize that I can inline BPF insns this way.
Thanks!

> Please attach the verifier output in the future.  It will be easier to understand.
>

Will do the next time.