Re: [PATCH bpf-next 04/11] bpf: check_map_kptr_access() compute the offset from the reg state.

[Kui-Feng Lee <sinquersw@gmail.com>]

On 4/11/24 15:13, Eduard Zingerman wrote:
> On Tue, 2024-04-09 at 17:41 -0700, Kui-Feng Lee wrote:
>> Previously, check_map_kptr_access() assumed that the accessed offset was
>> identical to the offset in the btf_field. However, once field array is
>> supported, the accessed offset no longer matches the offset in the
>> bpf_field. It may refer to an element in an array while the offset in the
>> bpf_field refers to the beginning of the array.
>>
>> To handle arrays, it computes the offset from the reg state instead.
>>
>> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
>> ---
> 
> Acked-by: Eduard Zingerman <eddyz87@gmail.com>
> 
>>   kernel/bpf/verifier.c | 15 +++++++++------
>>   1 file changed, 9 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 86adacc5f76c..34e43220c6f0 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -5349,18 +5349,19 @@ static u32 btf_ld_kptr_type(struct bpf_verifier_env *env, struct btf_field *kptr
>>   }
>>   
>>   static int check_map_kptr_access(struct bpf_verifier_env *env, u32 regno,
>> -				 int value_regno, int insn_idx,
>> +				 u32 offset, int value_regno, int insn_idx,
>>   				 struct btf_field *kptr_field)
>>   {
>>   	struct bpf_insn *insn = &env->prog->insnsi[insn_idx];
>>   	int class = BPF_CLASS(insn->code);
>> -	struct bpf_reg_state *val_reg;
>> +	struct bpf_reg_state *val_reg, *reg;
>>   
>>   	/* Things we already checked for in check_map_access and caller:
> 
> Nit: at the moment when this patch is applied check_map_access is not
>       yet modified.


Yes, I will change the order of the patches.


> 
>>   	 *  - Reject cases where variable offset may touch kptr
>>   	 *  - size of access (must be BPF_DW)
>>   	 *  - tnum_is_const(reg->var_off)
>> -	 *  - kptr_field->offset == off + reg->var_off.value
>> +	 *  - kptr_field->offset + kptr_field->size * i / kptr_field->nelems
>> +	 *    == off + reg->var_off.value where n is an index into the array
>                                             ^^^ nit: this should be 'i'

Yes!


> 
>>   	 */
>>   	/* Only BPF_[LDX,STX,ST] | BPF_MEM | BPF_DW is supported */
>>   	if (BPF_MODE(insn->code) != BPF_MEM) {
> 
> [...]
>