linux-kernel - Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures through CHECK_DATA

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Message-ID: <20170207175542.GC26173@leverpostej>
Date:   Tue, 7 Feb 2017 17:55:42 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <h.peter.anvin@...el.com>,
        Will Deacon <will.deacon@....com>,
        David Windsor <dwindsor@...il.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Howells <dhowells@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures
 through CHECK_DATA_CORRUPTION

On Tue, Feb 07, 2017 at 06:30:36PM +0100, Peter Zijlstra wrote:
> On Tue, Feb 07, 2017 at 04:03:01PM +0000, Mark Rutland wrote:
> > For x86 it's a little painful due to '%' in the register names, but it looks
> > possible. The below appears to do the mangling correctly (then screams due to
> > the mangled result being nonexistent).
> 
> > asm(
> > "	.macro	reg_to_offset	r\n"
> > "	.irp rs,eax,ebx,ecx,edx\n"
> > "	.ifc \\r, %\\rs\n"
> > "	__offset_of_\\rs\n"
> > "	.endif\n"
> > "	.endr\n"
> > "	.endm\n"
> > );
> > 
> > #define asm_sym(var)		asm volatile("reg_to_offset %0\n" : : "r" (var))
> 
> Oh gawd that's a most gnarly hack.

:)

> Do we want to go do that for all archs or somehow cook a generic
> fallback that ends up doing a full function call or something?

Given the arch-specific reg->blah mapping is so "fun", I guess a generic
fallback would be a good start.

I haven't figured out all the plumbing details. It'd be nice to reuse
the bug infrastructure so that arches don't have to implement another
trap and callback pair, but I guess the reg details need to live in
another data structure.

Thanks,
Mark.