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SubjectRe: [bpf-next 3/3] bpf: Fix a use after free in bpf_check()
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在 2021/7/9 23:12, Alexei Starovoitov 写道:
> On Fri, Jul 9, 2021 at 4:11 AM He Fengqing <hefengqing@huawei.com> wrote:
>>
>>
>>
>> 在 2021/7/8 11:09, Alexei Starovoitov 写道:
>>> On Wed, Jul 7, 2021 at 8:00 PM He Fengqing <hefengqing@huawei.com> wrote:
>>>>
>>>> Ok, I will change this in next version.
>>>
>>> before you spam the list with the next version
>>> please explain why any of these changes are needed?
>>> I don't see an explanation in the patches and I don't see a bug in the code.
>>> Did you check what is the prog clone ?
>>> When is it constructed? Why verifier has anything to do with it?
>>> .
>>>
>>
>>
>> I'm sorry, I didn't describe these errors clearly.
>>
>> bpf_check(bpf_verifier_env)
>> |
>> |->do_misc_fixups(env)
>> | |
>> | |->bpf_patch_insn_data(env)
>> | | |
>> | | |->bpf_patch_insn_single(env->prog)
>> | | | |
>> | | | |->bpf_prog_realloc(env->prog)
>> | | | | |
>> | | | | |->construct new_prog
>> | | | | | free old_prog(env->prog)
>> | | | | |
>> | | | | |->return new_prog;
>> | | | |
>> | | | |->return new_prog;
>> | | |
>> | | |->adjust_insn_aux_data
>> | | | |
>> | | | |->return ENOMEM;
>> | | |
>> | | |->return NULL;
>> | |
>> | |->return ENOMEM;
>>
>> bpf_verifier_env->prog had been freed in bpf_prog_realloc function.
>>
>>
>> There are two errors here, the first is memleak in the
>> bpf_patch_insn_data function, and the second is use after free in the
>> bpf_check function.
>>
>> memleak in bpf_patch_insn_data:
>>
>> Look at the call chain above, if adjust_insn_aux_data function return
>> ENOMEM, bpf_patch_insn_data will return NULL, but we do not free the
>> new_prog.
>>
>> So in the patch 2, before bpf_patch_insn_data return NULL, we free the
>> new_prog.
>>
>> use after free in bpf_check:
>>
>> If bpf_patch_insn_data function return NULL, we will not assign new_prog
>> to the bpf_verifier_env->prog, but bpf_verifier_env->prog has been freed
>> in the bpf_prog_realloc function. Then in bpf_check function, we will
>> use bpf_verifier_env->prog after do_misc_fixups function.
>>
>> In the patch 3, I added a free_old parameter to bpf_prog_realloc, in
>> this scenario we don't free old_prog. Instead, we free it in the
>> do_misc_fixups function when bpf_patch_insn_data return a valid new_prog.
>
> Thanks for explaining.
> Why not to make adjust_insn_aux_data() in bpf_patch_insn_data() first then?
> Just changing the order will resolve both issues, no?
> .
>
adjust_insn_aux_data() need the new constructed new_prog as an input
parameter, so we must call bpf_patch_insn_single() before
adjust_insn_aux_data().

But we can make adjust_insn_aux_data() never return ENOMEM. In
bpf_patch_insn_data(), first we pre-malloc memory for new aux_data, then
call bpf_patch_insn_single() to constructed the new_prog, at last call
adjust_insn_aux_data() functin. In this way, adjust_insn_aux_data()
never fails.

bpf_patch_insn_data(env) {
struct bpf_insn_aux_data *new_data = vzalloc();
struct bpf_prog *new_prog;
if (new_data == NULL)
return NULL;

new_prog = bpf_patch_insn_single(env->prog);
if (new_prog == NULL) {
vfree(new_data);
return NULL;
}

adjust_insn_aux_data(new_prog, new_data);
return new_prog;
}
What do you think about it?

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