linux-kernel - Re: [PATCH] x86/smap: Fix the smap

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Message-Id: <441AA771-A859-4145-9425-E9D041580FE4@amacapital.net>
Date:   Tue, 15 Sep 2020 16:11:03 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Bill Wendling <morbo@...gle.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Thelen <gthelen@...gle.com>,
        John Sperbeck <jsperbeck@...gle.com>,
        "# 3.4.x" <stable@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] x86/smap: Fix the smap_save() asm



> On Sep 15, 2020, at 2:24 PM, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> 
> On Tue, Sep 15, 2020 at 1:56 PM Andy Lutomirski <luto@...nel.org> wrote:
>> 
>> The old smap_save() code was:
>> 
>>  pushf
>>  pop %0
>> 
>> with %0 defined by an "=rm" constraint.  This is fine if the
>> compiler picked the register option, but it was incorrect with an
>> %rsp-relative memory operand.
> 
> It is incorrect because ... (I think mentioning the point about the
> red zone would be good, unless there were additional concerns?)

This isn’t a red zone issue — it’s a just-plain-wrong issue.  The popf is storing the result in the wrong place in memory — it’s RSP-relative, but RSP is whatever the compiler thinks it should be minus 8, because the compiler doesn’t know that pushfq changed RSP.

> 
> This is something we should fix.  Bill, James, and I are discussing
> this internally.  Thank you for filing a bug; I owe you a beer just
> for that.

I’m looking forward to the day that beers can be exchanged in person again :)

> 
>> 
>> Fixes: e74deb11931f ("x86/uaccess: Introduce user_access_{save,restore}()")
>> Cc: stable@...r.kernel.org
>> Reported-by: Bill Wendling <morbo@...gle.com> # I think
> 
> LOL, yes, the comment can be dropped...though I guess someone else may
> have reported the problem to Bill?

The “I think” is because I’m not sure whether Bill reported this particular issue. But I’m fine with dropping it.