linux-kernel - Re: [PATCH 0/4] arm64: Make Aarch32 compatibility enablement optional at boot

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Date:   Thu, 19 Oct 2023 11:17:00 +0200
From:   Andrea della Porta <aporta@...e.de>
To:     Will Deacon <will@...nel.org>
Cc:     Andrea della Porta <andrea.porta@...e.com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        nik.borisov@...e.com, arnd@...db.de, keescook@...omium.org
Subject: Re: [PATCH 0/4] arm64: Make Aarch32 compatibility enablement
 optional at boot

On 13:27 Wed 18 Oct     , Will Deacon wrote:
> Hi,
> 
> On Wed, Oct 18, 2023 at 01:13:18PM +0200, Andrea della Porta wrote:
> > Aarch32 compatibility mode is enabled at compile time through
> > CONFIG_COMPAT Kconfig option. This patchset lets 32-bit support
> > (for both processes and syscalls) be enabled at boot time using
> > a kernel parameter. Also, it provides a mean for distributions 
> > to set their own default without sacrificing compatibility support,
> > that is users can override default behaviour through the kernel
> > parameter.
> 
> I proposed something similar in the past:
> 
> https://lkml.kernel.org/linux-fsdevel/20210916131816.8841-1-will@kernel.org/
> 
> bu the conclusion there (see the reply from Kees) was that it was better
> to either use existing seccomp mechanisms or add something to control
> which binfmts can be loaded.
> 
> Will

I see. Seccomp sounds like a really good idea, since just blocking the compat
binfmt would not avoid the call to 32-bit syscalls per se: it's true that
ARM64 enforce the transition from A64 to A32 only on exception return and
PSTATE.nRW flag can change only from EL1, maybe though some exploitation
may arise in the future to do just that (I'm not aware of any or come up
with a proof off the top of my head, but I can't exclude it either). So,
assuming by absurd a switch to A32 is feasible, the further step of embedding
A32 instruction in a A64 ELF executable is a breeze. Hence blocking the 
syscall (and not only the binfmt loading) could prove necessary. I know all
of this is higly speculative right now, maybe it's worth thinking nonetheless.

Andrea