On Thu, Jan 12, 2017 at 09:03:18AM -0600, Josh Poimboeuf wrote: > On Wed, Jan 11, 2017 at 11:51:10PM -0800, Andy Lutomirski wrote: > > On Wed, Jan 11, 2017 at 11:05 PM, Herbert Xu > > <herbert@xxxxxxxxxxxxxxxxxxx> wrote: > > > On Tue, Jan 10, 2017 at 09:05:28AM -0800, Linus Torvalds wrote: > > >> > > >> I'm pretty sure we have random asm code that may not maintain a > > >> 16-byte stack alignment when it calls other code (including, in some > > >> cases, calling C code). > > >> > > >> So I'm not at all convinced that this is a good idea. We shouldn't > > >> expect 16-byte alignment to be something trustworthy. > > > > > > So what if we audited all the x86 assembly code to fix this? Would > > > it then be acceptable to do a 16-byte aligned stack? > > > > > > On the face of it it doesn't seem to be a huge amount of code > > > assuming they mostly live under arch/x86. > > > > The problem is that we have nasties like TRACE_IRQS_OFF. Performance > > doesn't really matter for these macros, so we could probably rig up a > > helper for forcibly align the stack there. Maybe > > FRAME_BEGIN_FORCE_ALIGN? I also think I'd rather not to modify > > pt_regs. We should just fix the small number of code paths that > > create a pt_regs and then call into C code to align the stack. > > > > But if we can't do this with automatic verification, then I'm not sure > > I want to do it at all. The asm is already more precarious than I'd > > like, and having a code path that is misaligned is asking for obscure > > bugs down the road. > > For the entry code, could we just replace all calls with CALL_ALIGNED? > That might be less intrusive than trying to adjust all the pt_regs > accesses. > > Then to ensure that nobody ever uses 'call' directly: > > '#define call please-use-CALL-ALIGNED-instead-of-call' > > I think that would be possible if CALL_ALIGNED were a ".macro". To clarify, CALL_ALIGNED could be (completely untested): .macro CALL_ALIGNED \func push %rbp movq %rsp, %rbp and $0xfffffffffffffff0,%rsp call \func movq %rbp, %rsp pop %rbp .endm -- Josh -- 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
