Re: [PATCH 2/3 v3] x86: entry_64.S: always allocate complete "struct pt_regs"

From: Andy Lutomirski <luto@amacapital.net>
To: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Borislav Petkov <bp@alien8.de>, X86 ML <x86@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrey Wagin <avagin@gmail.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Alexei Starovoitov <ast@plumgrid.com>,
	Kees Cook <keescook@chromium.org>, Will Drewry <wad@chromium.org>
Subject: Re: [PATCH 2/3 v3] x86: entry_64.S: always allocate complete "struct pt_regs"
Date: Thu, 26 Feb 2015 07:14:52 -0800	[thread overview]
Message-ID: <CALCETrWhOcfECEy+c2CcKnQ+gVh3yjB7ydMv-WkR9wcFTjpzog@mail.gmail.com> (raw)
In-Reply-To: <CAK1hOcN2jjfjNArnD8Uw68GCgHCXui59M3HETYE5YaR_Apxa4A@mail.gmail.com>

On Feb 26, 2015 1:55 AM, "Denys Vlasenko" <vda.linux@googlemail.com> wrote:
>
> On Wed, Feb 25, 2015 at 10:59 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> > On Wed, Feb 25, 2015 at 1:28 PM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
> >> On 02/25/2015 09:10 PM, Andy Lutomirski wrote:
> >> This part?
> >>
> >>         .macro FORK_LIKE func
> >>  ENTRY(stub_\func)
> >>         CFI_STARTPROC
> >> -       popq    %r11                    /* save return address */
> >> -       PARTIAL_FRAME 0
> >> -       SAVE_REST
> >> -       pushq   %r11                    /* put it back on stack */
> >> +       DEFAULT_FRAME 0, 8              /* offset 8: return address */
> >> +       SAVE_EXTRA_REGS 8
> >>         FIXUP_TOP_OF_STACK %r11, 8
> >> -       DEFAULT_FRAME 0 8               /* offset 8: return address */
> >>         call sys_\func
> >>         RESTORE_TOP_OF_STACK %r11, 8
> >> -       ret $REST_SKIP          /* pop extended registers */
> >> +       ret
> >>         CFI_ENDPROC
> >>  END(stub_\func)
> >>         .endm
> >>
> >>         FORK_LIKE  clone
> >>         FORK_LIKE  fork
> >>         FORK_LIKE  vfork
> >>
> >> But the old code (SAVE_REST thing) was also saving registers here.
> >> It had to jump through hoops (pop return address, SAVE_REST,
> >> push return address) to do that.
> >> After the patch, "SAVE_EXTRA_REGS 8" does the same, just without
> >> pop/push pair.
> >>
> >> I just don't see what's wrong with it. Can you elaborate?
> >
> > SAVE_REST pushed the regs onto the stack, whereas SAVE_EXTRA_REGS just
> > writes them in place.  It's possible for this to be called when the
> > regs have already been saved.
>
> If that would be the case - that is, if SAVE_REST was saving extra copy
> of registers on stack, then FIXUP_TOP_OF_STACK %r11, 8 would be working
> on wrong locations. The "8" there says "we have full pt_regs on stack,
> plus extra 8 bytes (the return address)". Your conjecture would mean
> that in fact there would be more bytes on stack, and FIXUP_TOP_OF_STACK
> would corrupt iret stack. Evidently, since old code was not crashing,
> this wasn't happening. SAVE_REST was really creating the "tail" of pt_regs

Ugh, you're right.

The FIXUP_TOP_OF_STACK indeed looks duplicated, bit t that's less
harmful and was already the case.

--Andy
.
>
> In addition to my previous tests, I ran my home machine with
> patched kernel. Unfortunately, it works for me :(
>
> Will try on yet another machine.
>
> --
> vda