linux-kernel - Re: [PATCH 6/8] i915,uaccess: Fix redundant CLAC

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Message-Id: <6D332336-71DF-463A-B07A-5D91087F133A@amacapital.net>
Date:   Fri, 1 Mar 2019 07:27:47 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Anvin <hpa@...or.com>,
        Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>, valentin.schneider@....com,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Chris Wilson <chris@...is-wilson.co.uk>
Subject: Re: [PATCH 6/8] i915,uaccess: Fix redundant CLAC



> On Mar 1, 2019, at 6:38 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
>> On Fri, Mar 01, 2019 at 01:57:18PM +0100, Peter Zijlstra wrote:
>>> On Fri, Mar 01, 2019 at 01:27:45PM +0100, Peter Zijlstra wrote:
>>> arch/x86/lib/usercopy_64.o: warning: objtool: .altinstr_replacement+0x30: redundant UACCESS disable
>> 
>>> The usercopy one is difficult, that's copy_user_handle_tail(), it is
>>> buggered though, because that lacks notrace and thus has a __fentry__
>>> call in.
>>> 
>>> Also, afaict all exception jumps into copy_user_handle_tail() will have
>>> AC=1, but the __{get,put}_user_nocheck() things do STAC/CLAC all over
>>> again.
>>> 
>>> So what do we do? Annotate that we start with AC=1 and then immediately
>>> do the clac, and then let __{get,put}_user_nocheck() do their own thing?
>>> or make it use the unsafe stuff?
>> 
>> Or.. we move the thing to assembly. Of course, I suck at (writing) asm,
>> so the below is probably broken in various ways.
> 
> The advantage is that it now all lives in the same .o file and objtool
> can actually follow and find the complete control flow.
> 
> I've made it ENDPROC() such that it becomes STT_FUNC and objtool does
> all the normal things. I've also moved the ALIGN_DESTINATION macro into
> the .S file.
> 
> Andy, do we have a sensible self-test for this path?

Not that I know of. Something like my (rejected) strncpy_from_user test could probably be added fairly easily.