Re: x86-64: Maintain 16-byte stack alignment

[Andy Lutomirski]

On Tue, Jan 10, 2017 at 11:16 AM, Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
> On 10 January 2017 at 19:00, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Tue, Jan 10, 2017 at 9:30 AM, Ard Biesheuvel
>> <ard.biesheuvel@xxxxxxxxxx> wrote:
>>> On 10 January 2017 at 14:33, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
>>>> I recently applied the patch
>>>>
>>>>         https://patchwork.kernel.org/patch/9468391/
>>>>
>>>> and ended up with a boot crash when it tried to run the x86 chacha20
>>>> code.  It turned out that the patch changed a manually aligned
>>>> stack buffer to one that is aligned by gcc.  What was happening was
>>>> that gcc can stack align to any value on x86-64 except 16.  The
>>>> reason is that gcc assumes that the stack is always 16-byte aligned,
>>>> which is not actually the case in the kernel.
>>>>
>>>
>>> Apologies for introducing this breakage. It seemed like an obvious and
>>> simple cleanup, so I didn't even bother to mention it in the commit
>>> log, but if the kernel does not guarantee 16 byte alignment, I guess
>>> we should revert to the old method. If SSE instructions are the only
>>> ones that require this alignment, then I suppose not having a ABI
>>> conforming stack pointer should not be an issue in general.
>>
>> Here's what I think is really going on.  This is partially from
>> memory, so I could be off base.  The kernel is up against
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383, which means that,
>> on some GCC versions (like the bad one and maybe even current ones),
>> things compiled without -mno-sse can't have the stack alignment set
>> properly.  IMO we should fix this in the affected code, not the entry
>> code.  In fact, I think that fixing it in the entry code won't even
>> fully fix it because modern GCC will compile the rest of the kernel
>> with 8-byte alignment and the stack will get randomly unaligned (GCC
>> 4.8 and newer).
>>
>> Can we just add __attribute__((force_align_arg_pointer)) to the
>> affected functions?  Maybe have:
>>
>> #define __USES_SSE __attribute__((force_align_arg_pointer))
>>
>> on affected gcc versions?
>>
>> ***HOWEVER***
>>
>> I think this is missing the tree for the supposed forest.  The actual
>> affected code appears to be:
>>
>> static int chacha20_simd(struct blkcipher_desc *desc, struct scatterlist *dst,
>>                          struct scatterlist *src, unsigned int nbytes)
>> {
>>         u32 *state, state_buf[16 + (CHACHA20_STATE_ALIGN / sizeof(u32)) - 1];
>>
>> ...
>>
>>         state = (u32 *)roundup((uintptr_t)state_buf, CHACHA20_STATE_ALIGN);
>>
>> gcc presumably infers (incorrectly) that state_buf is 16-byte aligned
>> and optimizes out the roundup.  How about just declaring an actual
>> __aligned(16) buffer, marking the function
>> __attribute__((force_align_arg_pointer)), and being done with it?
>> After all, we need that forcible alignment on *all* gcc versions.
>>
>
> Actually, the breakage is introduced by the patch Herbert refers to
>
> https://patchwork.kernel.org/patch/9468391/
>
> where the state is replaced by a simple
>
> u32 state[16] __aligned(CHACHA20_STATE_ALIGN);
>
> which seemed harmless enough to me. So the code above works fine.

So how about just the one-line patch of adding the
force_align_arg_pointer?  Would that solve the problem?