On 05/03/2014 06:54 AM, Linus Torvalds wrote: > On Fri, May 2, 2014 at 11:12 PM, H. Peter Anvin <h...@zytor.com> wrote: >> On 05/02/2014 09:32 PM, Linus Torvalds wrote: >>> >>> At least as a proof-of-concept, having a code sequence in user mode >>> trampoline that does >>> >>> popq %rsi >>> popq %r11 >>> retq $128 >>> >>> and building up a stack in user space at '%rsp-128' that has the >>> values or rsi/r11/rip should allow us to use 'sysret'. Hmm? >> >> That would be a security hole if another userspace thread could muck >> with the stack. > > No, all of the above is in user space, and the pre-restore register > values for rsi/r11/rip/rsp are all user space values (just not the > right ones for the "real" return point). So no security issue. > > Now, replacing "iret" with "sysret + user-space trampoline" doesn't > work in general (it gets RF wrong, for example, so it's useless for > single-stepping and breakpoint handling), but I was more thinking that > it would be an interesting way to see what the performance impact of a > faster iret would be. >
Right, brain failure on my part. I somehow got it in my head you'd run the above off the user stack while in CPL 0, which would be obviously crazy. I think this would be rather interesting experiment. -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/