Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

From: Jessica Yu <jeyu@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	keescook@chromium.org, Josh Poimboeuf <jpoimboe@redhat.com>,
	Miroslav Benes <mbenes@suse.cz>
Subject: Re: [PATCH v2] module: Harden STRICT_MODULE_RWX
Date: Wed, 8 Apr 2020 17:32:49 +0200	[thread overview]
Message-ID: <20200408153249.GA26619@linux-8ccs> (raw)
In-Reply-To: <20200403171303.GK20760@hirez.programming.kicks-ass.net>

+++ Peter Zijlstra [03/04/20 19:13 +0200]:
>
>We're very close to enforcing W^X memory, refuse to load modules that
>violate this principle per construction.
>
>Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>---
> kernel/module.c |   25 +++++++++++++++++++++++++
> 1 file changed, 25 insertions(+)
>
>--- a/kernel/module.c
>+++ b/kernel/module.c
>@@ -2044,9 +2044,29 @@ static void module_enable_x(const struct
> 	frob_text(&mod->core_layout, set_memory_x);
> 	frob_text(&mod->init_layout, set_memory_x);
> }
>+
>+static int module_enforce_rwx_sections(Elf_Ehdr *hdr, Elf_Shdr *sechdrs,
>+				       char *secstrings, struct module *mod)
>+{
>+	const unsigned long shf_wx = SHF_WRITE|SHF_EXECINSTR;
>+	int i;
>+
>+	for (i = 0; i < hdr->e_shnum; i++) {
>+		if ((sechdrs[i].sh_flags & shf_wx) == shf_wx)
>+			return -ENOEXEC;
>+	}
>+
>+	return 0;
>+}

Just to clarify, did we want to enforce this only when
CONFIG_STRICT_MODULE_RWX=y?  Because here it's still in the
CONFIG_ARCH_HAS_STRICT_MODULE_RWX block.

Unfortunately, when we add module_enforce_rwx_sections() in the
CONFIG_STRICT_MODULE_RWX block, we'll need two empty stubs, one for
!CONFIG_ARCH_HAS_STRICT_MODULE_RWX and one for !CONFIG_STRICT_MODULE_RWX.

This is because the CONFIG_STRICT_MODULE_RWX block is currently nested
within ARCH_HAS_STRICT_MODULE_RWX :/

That unholy nested ifdef mess needs to be fixed. I will post a patch
shortly that gets rid of this nesting and you could rebase on top of
it if you want to move module_enforce_rwx_sections() under
CONFIG_STRICT_MODULE_RWX. Then you'll only need to provide one empty
stub for !CONFIG_STRICT_MODULE_RWX.

Thanks,

Jessica