netdev - Re: Self-XORing BPF registers is undefined behavior

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Message-ID: <def5d586-3a8c-01b7-c6dd-bc284336b76e@iogearbox.net>
Date:   Thu, 13 Dec 2018 14:18:52 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexander Potapenko <glider@...gle.com>, mkubecek@...e.cz
Cc:     ast@...nel.org, Dmitriy Vyukov <dvyukov@...gle.com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: Self-XORing BPF registers is undefined behavior

On 12/13/2018 01:24 PM, Alexander Potapenko wrote:
> On Thu, Dec 13, 2018 at 1:20 PM Michal Kubecek <mkubecek@...e.cz> wrote:
>> On Thu, Dec 13, 2018 at 12:59:36PM +0100, Michal Kubecek wrote:
>>> On Thu, Dec 13, 2018 at 12:00:59PM +0100, Alexander Potapenko wrote:
>>>> Hi BPF maintainers,
>>>>
>>>> some time ago KMSAN found an issue in BPF code which we decided to
>>>> suppress at that point, but now I'd like to bring it to your
>>>> attention.
>>>> Namely, some BPF programs may contain instructions that XOR a register
>>>> with itself.
>>>> This effectively results in the following C code:
>>>>   regs[BPF_REG_A] = regs[BPF_REG_A] ^ regs[BPF_REG_A];
>>>> or
>>>>   regs[BPF_REG_X] = regs[BPF_REG_X] ^ regs[BPF_REG_X];
>>>> being executed.
>>>>
>>>> According to the C11 standard this is undefined behavior, so KMSAN
>>>> reports an error in this case.

Can you elaborate / quote the exact bit from C11 that KMSAN is referring
to? (The below that Michal was quoting or something else?)

Does that only refer to C11 standard? Note that kernel's Makefile +430
explicitly states 'std=gnu89' and not 'std=c11' [0].

  [0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=51b97e354ba9fce1890cf38ecc754aa49677fc89

>>> Can you quote the part of the standard saying this is undefined
>>> behavior? I couldn't find anything else than
>>>
>>>   If the value being stored in an object is read from another object
>>>   that overlaps in any way the storage of the first object, then the
>>>   overlap shall be exact and the two objects shall have qualified or
>>>   unqualified versions of a compatible type; otherwise, the behavior
>>>   is undefined.
>>>
>>> (but I only have a draft for obvious reasons). I'm not sure what exactly
>>> they mean by "exact overlap" and the standard doesn't seem to define
>>> the term but if the two objects are actually the same, they certainly
>>> have compatible types.

Here is an example for the overlap quoted above; I don't think this
applies to our case since it would be "exact". Quote [1]:

  struct S { int x; int y; };
  struct T { int z; struct S s; };
  union U { struct S f ; struct T g; } u;

  main(){
    u.f = u.g.s;
    return 0;
  }

  [1] https://bts.frama-c.com/print_bug_page.php?bug_id=945

>> I think I understand now. You didn't want to say that the statement
>>
>>   regs[BPF_REG_A] = regs[BPF_REG_A] ^ regs[BPF_REG_A];
>>
>> as such is undefined behavior but that it's UB when regs[BPF_REG_A] is
>> uninitialized. Right?
> Yes. Sorry for being unclear.
> By default regs[] is uninitialized, so we need to initialize it before
> using the register values.
> I am also wondering if it's possible to simply copy the uninitialized
> register values from regs[] to the userspace via maps.
>> Michal Kubecek
> 
> 
>