Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

From: Jessica Yu <jeyu@kernel.org>
To: Will Deacon <will@kernel.org>
Cc: Ard Biesheuvel <ardb@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: Mon, 31 Aug 2020 11:46:51 +0200	[thread overview]
Message-ID: <20200831094651.GA16385@linux-8ccs> (raw)
In-Reply-To: <20200821123036.GA21158@willie-the-truck>

+++ Will Deacon [21/08/20 13:30 +0100]:
[snipped]
>> > > > 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?
>> >
>>
>> That should work as well, I just haven't checked how straight-forward
>> it is to change that.
>
>Ok, if it's _not_ straightforward, then let's just drop the IS_ENABLED()
>altogether.

Unfortunately I've been getting more reports about this issue, so let's just
get the aforementioned workaround merged first. Does the following look OK?

diff --git a/arch/arm64/kernel/module-plts.c b/arch/arm64/kernel/module-plts.c
index 0ce3a28e3347..2e224435c024 100644
--- a/arch/arm64/kernel/module-plts.c
+++ b/arch/arm64/kernel/module-plts.c
@@ -305,8 +305,7 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
                        mod->arch.core.plt_shndx = i;
                else if (!strcmp(secstrings + sechdrs[i].sh_name, ".init.plt"))
                        mod->arch.init.plt_shndx = i;
-               else if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) &&
-                        !strcmp(secstrings + sechdrs[i].sh_name,
+               else if (!strcmp(secstrings + sechdrs[i].sh_name,
                                 ".text.ftrace_trampoline"))
                        tramp = sechdrs + i;
                else if (sechdrs[i].sh_type == SHT_SYMTAB)

If so I'll turn it into a formal patch and we can get that merged in the next -rc.

Thanks,

Jessica
