Messages in this thread | | | Date | Sat, 03 May 2014 12:00:24 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH v2] x86: Return to kernel without IRET |
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On 05/03/2014 06:54 AM, Linus Torvalds wrote: > On Fri, May 2, 2014 at 11:12 PM, H. Peter Anvin <hpa@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
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