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Date: Mon, 12 Aug 2024 13:02:15 -0700
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Subject: Re: [PATCH bpf 1/2] bpf: Fix a kernel verifier crash in stacksafe()
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To: Eduard Zingerman <eddyz87@gmail.com>,
 Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
 Andrii Nakryiko <andrii@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>,
 Kernel Team <kernel-team@fb.com>, Martin KaFai Lau <martin.lau@kernel.org>,
 Daniel Hodges <hodgesd@meta.com>
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From: Yonghong Song <yonghong.song@linux.dev>
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On 8/12/24 12:43 PM, Eduard Zingerman wrote:
> On Mon, 2024-08-12 at 12:29 -0700, Alexei Starovoitov wrote:
>
> [...]
>
>>> It does not seem correct to swap the order for these two checks:
>>>
>>>                  if (exact != NOT_EXACT && i < cur->allocated_stack &&
>>>                      old->stack[spi].slot_type[i % BPF_REG_SIZE] !=
>>>                      cur->stack[spi].slot_type[i % BPF_REG_SIZE])
>>>                          return false;
>>>
>>>                  if (!(old->stack[spi].spilled_ptr.live & REG_LIVE_READ)
>>>                      && exact == NOT_EXACT) {
>>>                          i += BPF_REG_SIZE - 1;
>>>                          /* explored state didn't use this */
>>>                          continue;
>>>                  }
>>>
>>> if we do, 'slot_type' won't be checked for 'cur' when 'old' register is not marked live.
>> I see. This is to compare states in open coded iter loops when liveness
>> is not propagated yet, right?
> Yes
>
>> Then when comparing for exact states we should probably do:
>> if (exact != NOT_EXACT &&
>>      (i >= cur->allocated_stack ||
>>       old->stack[spi].slot_type[i % BPF_REG_SIZE] !=
>>       cur->stack[spi].slot_type[i % BPF_REG_SIZE]))
>>     return false;
>>
>> ?
> Hm, right, otherwise the old slots in the interval
> [cur->allocated_stack..old->allocated_stack)
> won't be checked using exact rules.

Okay, for *exact* stack slot_type comparison. Will make the change
and send v2 soon.