Messages in this thread | | | Date | Sat, 3 May 2014 06:54:18 -0700 | Subject | Re: [PATCH v2] x86: Return to kernel without IRET | From | Linus Torvalds <> |
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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.
Linus
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