Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

From: peterz@infradead.org
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Jessica Yu <jeyu@kernel.org>,
	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>
Subject: Re: [PATCH v2] module: Harden STRICT_MODULE_RWX
Date: Wed, 12 Aug 2020 12:40:05 +0200	[thread overview]
Message-ID: <20200812104005.GN2674@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAMj1kXF8fm=9CdQykqDbgYCJSP88ezMs3EOosCW+SDi+Lve0zg@mail.gmail.com>

On Wed, Aug 12, 2020 at 10:56:56AM +0200, Ard Biesheuvel wrote:
> The module .lds has BYTE(0) in the section contents to prevent the
> linker from pruning them entirely. The (NOLOAD) is there to ensure
> that this byte does not end up in the .ko, which is more a matter of
> principle than anything else, so we can happily drop that if it helps.
> 
> However, this should only affect the PROGBITS vs NOBITS designation,
> and so I am not sure whether it makes a difference.
> 
> Depending on where the w^x check occurs, we might simply override the
> permissions of these sections, and strip the writable permission if it
> is set in the PLT handling init code, which manipulates the metadata
> of all these 3 sections before the module space is vmalloc'ed.

What's curious is that this seems the result of some recent binutils
change. Every build with binutils-2.34 (or older) does not seem to
generate these as WAX, but has the much more sensible WA.

I suppose we can change the kernel check and 'allow' W^X for 0 sized
sections, but I think we should still figure out why binutils-2.35 is
now generating WAX sections all of a sudden, it might come bite us
elsewhere.