Re: [PATCH v4 1/2] module: add elf_check_module_arch for module specific elf arch checks

From: Jessica Yu <jeyu@kernel.org>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: "Michal Suchánek" <msuchanek@suse.de>,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	"Nicholas Piggin" <npiggin@gmail.com>
Subject: Re: [PATCH v4 1/2] module: add elf_check_module_arch for module specific elf arch checks
Date: Tue, 15 Jun 2021 15:41:00 +0200	[thread overview]
Message-ID: <YMit7PZwiB87ig2u@p200300cbcf109700df096d564fe976c3.dip0.t-ipconnect.de> (raw)
In-Reply-To: <20210615125057.GF5077@gate.crashing.org>

+++ 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.