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Date:   Tue, 15 Jun 2021 09:30:38 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Jessica Yu <jeyu@kernel.org>
Cc:     Nicholas Piggin <npiggin@gmail.com>,
        Michal =?iso-8859-1?Q?Such=E1nek?= <msuchanek@suse.de>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 1/2] module: add elf_check_module_arch for module specific elf arch checks
Message-ID: <20210615143038.GH5077@gate.crashing.org>
References: <20210611093959.821525-1-npiggin@gmail.com> <20210611093959.821525-2-npiggin@gmail.com> <YMdGWjBOmcstBwOl@p200300cbcf109700df096d564fe976c3.dip0.t-ipconnect.de> <1623722110.amu32mwaqs.astroid@bobo.none> <YMiaZOqhHck9iy0n@p200300cbcf109700df096d564fe976c3.dip0.t-ipconnect.de> <20210615125057.GF5077@gate.crashing.org> <YMit7PZwiB87ig2u@p200300cbcf109700df096d564fe976c3.dip0.t-ipconnect.de>
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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 :-/


Segher