Re: [PATCH bpf-next v3 06/11] bpf: Fix compare error in function retval_range_within

[Xu Kuohai <xukuohai@huaweicloud.com>]

On 4/26/2024 7:41 AM, Andrii Nakryiko wrote:
> On Thu, Apr 11, 2024 at 5:24 AM Xu Kuohai <xukuohai@huaweicloud.com> wrote:
>>
>> From: Xu Kuohai <xukuohai@huawei.com>
>>
>> After checking lsm hook return range in verifier, the test case
>> "test_progs -t test_lsm" failed, and the failure log says:
>>
>> libbpf: prog 'test_int_hook': BPF program load failed: Invalid argument
>> libbpf: prog 'test_int_hook': -- BEGIN PROG LOAD LOG --
>> 0: R1=ctx() R10=fp0
>> ; int BPF_PROG(test_int_hook, struct vm_area_struct *vma, @ lsm.c:89
>> 0: (79) r0 = *(u64 *)(r1 +24)         ; R0_w=scalar(smin=smin32=-4095,smax=smax32=0) R1=ctx()
>>
>> [...]
>>
>> 24: (b4) w0 = -1                      ; R0_w=0xffffffff
>> ; int BPF_PROG(test_int_hook, struct vm_area_struct *vma, @ lsm.c:89
>> 25: (95) exit
>> At program exit the register R0 has smin=4294967295 smax=4294967295 should have been in [-4095, 0]
>>
>> It can be seen that instruction "w0 = -1" zero extended -1 to 64-bit
>> register r0, setting both smin and smax values of r0 to 4294967295.
>> This resulted in a false reject when r0 was checked with range [-4095, 0].
>>
>> Given bpf_retval_range is a 32-bit range, this patch fixes it by
>> changing the compare between r0 and return range from 64-bit
>> operation to 32-bit operation.
>>
>> Fixes: 8fa4ecd49b81 ("bpf: enforce exact retval range on subprog/callback exit")
>> Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
>> ---
>>   kernel/bpf/verifier.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 05c7c5f2bec0..5393d576c76f 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -9879,7 +9879,7 @@ static bool in_rbtree_lock_required_cb(struct bpf_verifier_env *env)
>>
>>   static bool retval_range_within(struct bpf_retval_range range, const struct bpf_reg_state *reg)
>>   {
>> -       return range.minval <= reg->smin_value && reg->smax_value <= range.maxval;
>> +       return range.minval <= reg->s32_min_value && reg->s32_max_value <= range.maxval;
> 
> are all BPF programs treated as if they return int instead of long? If
> not, we probably should have a bool flag in bpf_retval_range whether
> comparison should be 32-bit or 64-bit?
>

It seems that when a fmod_return prog is attached to a kernel function
that returns long value, the bpf prog should also return long value.
To confirm it, I'll try to find an example or construct a case for this.

>>   }
>>
>>   static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
>> --
>> 2.30.2
>>
>