linux-kernel - Re: [RFC PATCH 06/11] x86: make sure

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Date:   Fri, 21 Feb 2020 13:50:39 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Arjan van de Ven <arjan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        rick.p.edgecombe@...el.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [RFC PATCH 06/11] x86: make sure _etext includes function
 sections

On Mon, Feb 10, 2020 at 11:36:29AM -0500, Arvind Sankar wrote:
> On Mon, Feb 10, 2020 at 07:54:58AM -0800, Arjan van de Ven wrote:
> > > 
> > > I'll leave it to others to figure out the exact details. But afaict it
> > > should be possible to have fine-grained-randomization and preserve the
> > > workaround in the end.
> > > 
> > 
> > the most obvious "solution" is to compile with an alignment of 4 bytes (so tight packing)
> > and then in the randomizer preserve the offset within 32 bytes, no matter what it is
> > 
> > that would get you an average padding of 16 bytes which is a bit more than now but not too insane
> > (queue Kees' argument that tiny bits of padding are actually good)
> > 
> 
> With the patchset for adding the mbranches-within-32B-boundaries option,
> the section alignment gets forced to 32. With function-sections that
> means function alignment has to be 32 too.

We should be careful about enabling -mbranches-within-32B-boundaries.
It will hurt AMD, and presumably future Intel CPUs which don't need it.

-- 
Josh