netdev - Re: [bpf-next 3/3] bpf: Fix a use after free in bpf

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Date:   Mon, 12 Jul 2021 10:17:39 +0800
From:   He Fengqing <hefengqing@...wei.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Song Liu <song@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [bpf-next 3/3] bpf: Fix a use after free in bpf_check()



在 2021/7/9 23:12, Alexei Starovoitov 写道:
> On Fri, Jul 9, 2021 at 4:11 AM He Fengqing <hefengqing@...wei.com> wrote:
>>
>>
>>
>> 在 2021/7/8 11:09, Alexei Starovoitov 写道:
>>> On Wed, Jul 7, 2021 at 8:00 PM He Fengqing <hefengqing@...wei.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?