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

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Date:   Mon, 31 Aug 2020 11:46:51 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Mark Rutland <mark.rutland@....com>, nd <nd@....com>
Subject: Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

+++ 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