Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

[Borislav Petkov]

On Sun, Apr 26, 2015 at 04:39:38PM -0700, Andy Lutomirski wrote:
> > +#define X86_BUG_CANONICAL_RCX  X86_BUG(8) /* SYSRET #GPs when %RCX non-canonical */
> 
> I think that "sysret" should appear in the name.

Yeah, I thought about it too, will fix.

> Oh no!  My laptop is currently bug-free, and you're breaking it! :)

Muahahahhahaha...

> > +
> > +       /*
> > +        * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
> > +        * in kernel space.  This essentially lets the user take over
> > +        * the kernel, since userspace controls RSP.
> > +        */
> > +       ALTERNATIVE "jmp 1f", "", X86_BUG_CANONICAL_RCX
> > +
> 
> I know it would be ugly, but would it be worth saving two bytes by
> using ALTERNATIVE "jmp 1f", "shl ...", ...?
> 
> >         /* Change top 16 bits to be the sign-extension of 47th bit */
> >         shl     $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
> >         sar     $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
> > @@ -432,6 +436,7 @@ syscall_return:
> >         cmpq    %rcx, %r11
> >         jne     opportunistic_sysret_failed

You want to stick all 4 insns in the alternative? Yeah, it should work
but it might even more unreadable than it is now.

Btw, we can do this too:

	ALTERNATIVE "",
		    "shl     $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx \
		     sar     $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx \
		     cmpq    %rcx, %r11 \
		     jne     opportunistic_sysret_failed"
		     X86_BUG_SYSRET_CANONICAL_RCX

which will replace the 2-byte JMP with a lot of NOPs on AMD.

I'll trace it again to see which one is worse :-)

Thanks.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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