On 10 January 2017 at 19:22, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > 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? If it does what it says on the tin, it should fix the issue, but after adding the attribute, I get the exact same object output, so there's something dodgy going on here. -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
