linux-kernel - Re: [PATCH 2/3 v3] x86: entry_64.S: always allocate complete "struct pt

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Date:	Thu, 26 Feb 2015 10:55:16 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Andrey Wagin <avagin@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	X86 ML <x86@...nel.org>, Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3 v3] x86: entry_64.S: always allocate complete "struct pt_regs"

On Wed, Feb 25, 2015 at 10:59 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Wed, Feb 25, 2015 at 1:28 PM, Denys Vlasenko <dvlasenk@...hat.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.

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
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