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Date:   Thu, 19 Oct 2023 13:41:59 +0200
From:   "Arnd Bergmann" <arnd@arndb.de>
To:     "Andrea della Porta" <aporta@suse.de>
Cc:     "Will Deacon" <will@kernel.org>,
        "Andrea della Porta" <andrea.porta@suse.com>,
        "Catalin Marinas" <catalin.marinas@arm.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        nik.borisov@suse.com, "Kees Cook" <keescook@chromium.org>
Subject: Re: [PATCH 0/4] arm64: Make Aarch32 compatibility enablement optional at boot
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On Thu, Oct 19, 2023, at 12:52, Andrea della Porta wrote:
> On 14:44 Wed 18 Oct     , Arnd Bergmann wrote:
>> On Wed, Oct 18, 2023, at 14:27, Will Deacon wrote:
>> 
>> Right, I was going to reply along the same lines here: x86 is
>> a bit of a special case that needs this, but I believe all the
>> other architectures already guard the compat syscall execution
>> on test_thread_flag(TIF_32BIT) that is only set by the compat
>> binfmt loader.
>
> Are you referring to the fact that x86 can switch at will between 32- and 64-
> bit code?

No.

> Regarding the TIF_32BIT flag, thanks for the head-up. I still believe though
> that this mechanism can somehow break down in the future, since prohibiting
> 32 bit executable loading *and* blocking 32 bit compat syscall are two
> separate path of execution, held together by the architecture prohibiting
> to switch to A32 instructions by design. Breaking the first rule and embedding 
> wisely crafted A32 instruction in an executable is easy, while the difficult
> part is finding some 'reentrancy' to be able to do the execution state switch,
> as pinted out in https://lore.kernel.org/lkml/ZTD0DAes-J-YQ2eu@apocalypse/.
> I agree it's highly speculative and not something to be concerned right
> now, it's just a head up, should the need arise in the future.

There are (at least) five separate aspects to compat mode that are easy
to mix up:

1. Instruction decoding -- switching between the modes supported by the
   CPU (A64/A32/T32)
2. Word size -- what happens to the upper 32 bits of a register in
   an arithmetic operation
3. Personality -- Which architecture string gets returned by the
   uname syscall (aarch64 vs armv8) as well as the format of
   /proc/cpuinfo
4. system call entry points -- how a process calls into native or
   compat syscalls, or possibly foreign OS emulation
5. Binary format -- elf32 vs elf64 executables

On most architectures with compat mode, 4. and 5. are fundamentally
tied together today: a compat task can only call compat syscalls
and a native task can only call native syscalls. x86 is the exception
here, as it uses different instructions (int80, syscall, sysenter)
and picks the syscall table based on that instruction.

I think 1. and 2. are also always tied to 5 on arm, but this is
not necessarily true for other architectures. 3. used to be tied
to 5 on some architectures in the past, but should be independent
now.

>> Doing the reverse is something that has however come up in the
>> past several times and that could be interesting: In order to
>> run userspace emulation (qemu-user, fex, ...) we may want to
>> allow calling syscalls and ioctls for foreign ABIs in a native
>> task, and at that point having a mechanism to control this
>> capability globally or per task would be useful as well.
>> 
>> The compat mode (arm32 on arm64) is the easiest case here, but the
>> same thing could be done for emulating the very subtle architecture
>> differences (x86-64 on arm64, arm64 on x86_64, arm32 on x86-compat,
>> or any of the above on riscv or  loongarch).
>
> Really interesting, Since it's more related to emulation needs (my patch
> has another focus due to the fact that A64 can execute A32 natively),
> I'll take a look at this separately.

A64 mode (unlike some other architectures, notably mips64) cannot
execute A32 or T32 instructions without a mode switch, the three are
entirely incompatible on the binary level.

Many ARMv8-CPUs support both Aarch64 mode and Aarch32 (A32/T32),
but a lot of the newer ones (e.g. Apple M1/M2, Cortex-R82 or
Cortex-A715) only do Aarch64 and need user-space emulation to run
32-bit binaries.

    Arnd