Date: Mon, 27 Apr 2015 10:53:05 +0200
From: Borislav Petkov <bp@alien8.de>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Denys Vlasenko <vda.linux@googlemail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Brian Gerst <brgerst@gmail.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor
 attribute issue
Message-ID: <20150427085305.GB6774@pd.tnic>
References: <5d120f358612d73fc909f5bfa47e7bd082db0af0.1429841474.git.luto@kernel.org>
 <20150425211206.GE32099@pd.tnic>
 <CALCETrWvqyCYOzCYXz7ZnzaM0obbidqo5CxVp2Hn1ELEiC3m3g@mail.gmail.com>
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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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