Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

From: Peter Zijlstra <peterz@infradead.org>
To: Jessica Yu <jeyu@kernel.org>
Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
	linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	keescook@chromium.org, Josh Poimboeuf <jpoimboe@redhat.com>,
	Miroslav Benes <mbenes@suse.cz>,
	Mark Rutland <mark.rutland@arm.com>,
	Ard Biesheuvel <ardb@kernel.org>
Subject: Re: [PATCH v2] module: Harden STRICT_MODULE_RWX
Date: Tue, 11 Aug 2020 23:29:08 +0200	[thread overview]
Message-ID: <20200811212908.GB5637@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20200811175912.GF2674@hirez.programming.kicks-ass.net>

On Tue, Aug 11, 2020 at 07:59:12PM +0200, peterz@infradead.org wrote:
> On Tue, Aug 11, 2020 at 06:01:35PM +0200, Jessica Yu wrote:
> 
> > > > On Tue, Aug 11, 2020 at 04:34:27PM +0200, Mauro Carvalho Chehab wrote:
> > > > >   [33] .plt              PROGBITS         0000000000000340  00035c80
> > > > >        0000000000000001  0000000000000000 WAX       0     0     1
> > > > >   [34] .init.plt         NOBITS           0000000000000341  00035c81
> > > > >        0000000000000001  0000000000000000  WA       0     0     1
> > > > >   [35] .text.ftrace[...] PROGBITS         0000000000000342  00035c81
> > > > >        0000000000000001  0000000000000000 WAX       0     0     1
> 
> > Interesting, my cross-compiled modules do not have the executable flag -
> > 
> >  [38] .plt              NOBITS           0000000000000340  00038fc0
> >       0000000000000001  0000000000000000  WA       0     0     1
> >  [39] .init.plt         NOBITS           0000000000000341  00038fc0
> >       0000000000000001  0000000000000000  WA       0     0     1
> >  [40] .text.ftrace_tram NOBITS           0000000000000342  00038fc0
> >       0000000000000001  0000000000000000  WA       0     0     1
> 
> > I'm a bit confused about what NOLOAD actually implies in this context. From the
> > ld documentation - "The `(NOLOAD)' directive will mark a section to not be
> > loaded at run time." But these sections are marked SHF_ALLOC and are referenced
> > to in the module plt code. Or does it just tell the linker to not initialize it?
> 
> Yeah, that confusion is wide-spread, so much so that bfd-ld and gold,
> both in bintils, had different behaviour at some point.
> 
> Anyway, another clue is that your build has all NOBITS, while Mauro's
> build has PROGBITS for the broken sections.
> 
> Anyway, my gcc-10.1/binutils-2.34 cross tool chain (from k.org)
> generates the same as Jessica's too. I wonder if binutils-2.35 is
> wonky...

When I use the Debian provided cross compiler which uses:

  binutils-aarch64-linux-gnu           2.35-1

I do indeed see the same thing Mauro does, which seems to suggest
there's something really dodgy with that toolchain. Some tools person
should have a look.