linux-kernel - Re: [PATCH v4 1/2] module: add elf_check_module

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Date:   Wed, 16 Jun 2021 12:39:30 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Segher Boessenkool <segher@...nel.crashing.org>,
        Jessica Yu <jeyu@...nel.org>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        Michal Suchánek <msuchanek@...e.de>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] module: add elf_check_module_arch for module
 specific elf arch checks

Segher Boessenkool <segher@...nel.crashing.org> writes:
> On Tue, Jun 15, 2021 at 03:41:00PM +0200, Jessica Yu wrote:
>> +++ Segher Boessenkool [15/06/21 07:50 -0500]:
>> >On Tue, Jun 15, 2021 at 02:17:40PM +0200, Jessica Yu wrote:
>> >>+int __weak elf_check_module_arch(Elf_Ehdr *hdr)
>> >>+{
>> >>+       return 1;
>> >>+}
>> >
>> >But is this a good idea?  It isn't useful to be able to attempt to load
>> >a module not compiled for your architecture, and it increases the attack
>> >surface tremendously.  These checks are one of the few things that can
>> >*not* be weak symbols, imo.
>> 
>> Hm, could you please elaborate a bit more? This patchset is adding
>> extra Elf header checks specifically for powerpc, and the module
>> loader usually provides arch-specific hooks via weak symbols. We are
>> just providing an new hook here, which should act as a no-op if it
>> isn't used.
>> 
>> So if an architecture wants to provide extra header checks, it can do
>> so by overriding the new weak symbol. Otherwise, the weak function acts as
>> a noop. We also already have the existing elf_check_arch() check for each
>> arch and that is *not* a weak symbol.
>
> The way I read your patch the default elf_check_module_arch does not
> call elf_check_arch?  Is that clearly called elsewhere and I'm just
> dumb again?  Sorry for the distraction in that case :-/

Yeah elf_check_arch() is already called from elf_validity_check(), and
that call would remain.

cheers