Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

From: Will Deacon <will@kernel.org>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Jessica Yu <jeyu@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Szabolcs Nagy <szabolcs.nagy@arm.com>,
	Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Kees Cook <keescook@chromium.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Miroslav Benes <mbenes@suse.cz>,
	Mark Rutland <mark.rutland@arm.com>, nd <nd@arm.com>
Subject: Re: [PATCH v2] module: Harden STRICT_MODULE_RWX
Date: Fri, 21 Aug 2020 13:20:00 +0100	[thread overview]
Message-ID: <20200821121959.GC20833@willie-the-truck> (raw)
In-Reply-To: <CAMj1kXErCQYNN9r5siGNukc+9KC=QnER8LfFXVfbHdeDivYztg@mail.gmail.com>

On Thu, Aug 13, 2020 at 03:07:13PM +0200, Ard Biesheuvel wrote:
> On Thu, 13 Aug 2020 at 15:04, Jessica Yu <jeyu@kernel.org> wrote:
> >
> > +++ Ard Biesheuvel [13/08/20 10:36 +0200]:
> > >On Wed, 12 Aug 2020 at 22:00, Peter Zijlstra <peterz@infradead.org> wrote:
> > >>
> > >> On Wed, Aug 12, 2020 at 06:37:57PM +0200, Ard Biesheuvel wrote:
> > >> > I know there is little we can do at this point, apart from ignoring
> > >> > the permissions - perhaps we should just defer the w^x check until
> > >> > after calling module_frob_arch_sections()?
> > >>
> > >> My earlier suggestion was to ignore it for 0-sized sections.
> > >
> > >Only they are 1 byte sections in this case.
> > >
> > >We override the sh_type and sh_flags explicitly for these sections at
> > >module load time, so deferring the check seems like a reasonable
> > >alternative to me.
> >
> > So module_enforce_rwx_sections() is already called after
> > module_frob_arch_sections() - which really baffled me at first, since
> > sh_type and sh_flags should have been set already in
> > module_frob_arch_sections().
> >
> > I added some debug prints to see which section the module code was
> > tripping on, and it was .text.ftrace_trampoline. See this snippet from
> > arm64's module_frob_arch_sections():
> >
> >                 else if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) &&
> >                          !strcmp(secstrings + sechdrs[i].sh_name,
> >                                  ".text.ftrace_trampoline"))
> >                         tramp = sechdrs + i;
> >
> > Since Mauro's config doesn't have CONFIG_DYNAMIC_FTRACE enabled, tramp
> > is never set here and the if (tramp) check at the end of the function
> > fails, so its section flags are never set, so they remain WAX and fail
> > the rwx check.
> 
> Right. Our module.lds does not go through the preprocessor, so we
> cannot add the #ifdef check there currently. So we should either drop
> the IS_ENABLED() check here, or simply rename the section, dropping
> the .text prefix (which doesn't seem to have any significance outside
> this context)
> 
> I'll leave it to Will to make the final call here.

Why don't we just preprocess the linker script, like we do for the main
kernel?

Will