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

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Message-ID: <YMit7PZwiB87ig2u@p200300cbcf109700df096d564fe976c3.dip0.t-ipconnect.de>
Date:   Tue, 15 Jun 2021 15:41:00 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Segher Boessenkool <segher@...nel.crashing.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 [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.