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Date: Fri, 22 May 2015 09:32:12 -0500
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Denys Vlasenko <dvlasenk@redhat.com>, Brian Gerst <brgerst@gmail.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v4 0/3] Compile-time stack frame pointer validation
Message-ID: <20150522143212.GB20555@treble.redhat.com>
References: <cover.1431966710.git.jpoimboe@redhat.com>
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On Fri, May 22, 2015 at 12:01:58AM +0200, Borislav Petkov wrote:
> On Thu, May 21, 2015 at 03:54:25PM -0500, Josh Poimboeuf wrote:
> > stackvalidate: arch/x86/lib/memmove_64.o: return instruction outside of a function at .altinstr_replacement+0x5
> 
> That must be something like this:
> 
> 0000000000000000 <.altinstr_replacement>:
>    0:   48 89 d1                mov    %rdx,%rcx
>    3:   f3 a4                   rep movsb %ds:(%rsi),%es:(%rdi)
>    5:   c3                      retq
> 
> right?
> 
> In any case, anything with alternatives is probably a false positive
> because even if instructions appear outside of the containing function,
> they get patched in and are actually inside. Jump offsets get fixed up
> properly too. Should, at least :-)

Hm, alternatives do complicate things a bit.  It *is* a false positive,
but not necessarily because its part of an alternative instruction
block.

The above code would be patched into memmove(), which is a leaf function
because it doesn't call any other functions.  Leaf functions don't need
frame pointer logic, so we can ignore them.

If instead the above code were patched into a non-leaf function, we'd
have to change it to restore the frame pointer before returning.

-- 
Josh