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

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Message-ID: <20200206152949.GA3055637@rani.riverdale.lan>
Date:   Thu, 6 Feb 2020 10:29:50 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Kees Cook <keescook@...omium.org>
Cc:     Kristen Carlson Accardi <kristen@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        arjan@...ux.intel.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 Thu, Feb 06, 2020 at 09:39:43AM -0500, Arvind Sankar wrote:
> On Thu, Feb 06, 2020 at 04:26:23AM -0800, Kees Cook wrote:
> > I know x86_64 stack alignment is 16 bytes. I cannot find evidence for
> > what function start alignment should be. It seems the linker is 16 byte
> > aligning these functions, when I think no alignment is needed for
> > function starts, so we're wasting some memory (average 8 bytes per
> > function, at say 50,000 functions, so approaching 512KB) between
> > functions. If we can specify a 1 byte alignment for these orphan
> > sections, that would be nice, as mentioned in the cover letter: we lose
> > a 4 bits of entropy to this alignment, since all randomized function
> > addresses will have their low bits set to zero.
> > 
> 
> The default function alignment is 16-bytes for x64 at least with gcc.
> You can use -falign-functions to specify a different alignment.
> 
> There was some old discussion on reducing it [1] but it doesn't seem to
> have been merged.
> 
> [1] https://lore.kernel.org/lkml/tip-4874fe1eeb40b403a8c9d0ddeb4d166cab3f37ba@git.kernel.org/

Though I don't think the entropy loss is real. With 50k functions, you
can use at most log(50k!) = ~35 KiB worth of entropy in permuting them,
no matter what the alignment is. The only way you can get more is if you
have more than 50k slots to put them in.