linux-kernel - Re: [PATCH v2] module: Harden STRICT_MODULE

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Date:   Tue, 11 Aug 2020 19:59:12 +0200
From:   peterz@...radead.org
To:     Jessica Yu <jeyu@...nel.org>
Cc:     Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        keescook@...omium.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

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...