linux-kernel - Re: [PATCH v2] arm64/cpufeature: annotate arm64_use_ng_mappings with ro_after

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Message-ID: <3cfcd0c5-79a2-45de-8497-fb95ef834dc1@arm.com>
Date: Tue, 6 May 2025 11:22:47 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Catalin Marinas <catalin.marinas@....com>,
 Ard Biesheuvel <ardb@...nel.org>
Cc: Yeoreum Yun <yeoreum.yun@....com>, will@...nel.org, nathan@...nel.org,
 nick.desaulniers+lkml@...il.com, morbo@...gle.com, justinstitt@...gle.com,
 broonie@...nel.org, maz@...nel.org, oliver.upton@...ux.dev,
 joey.gouly@....com, shameerali.kolothum.thodi@...wei.com,
 james.morse@....com, hardevsinh.palaniya@...iconsignals.io,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 llvm@...ts.linux.dev, stable@...r.kernel.org
Subject: Re: [PATCH v2] arm64/cpufeature: annotate arm64_use_ng_mappings with
 ro_after_init to prevent wrong idmap generation

On 06/05/2025 11:17, Catalin Marinas wrote:
> On Tue, May 06, 2025 at 11:41:05AM +0200, Ard Biesheuvel wrote:
>> On Tue, 6 May 2025 at 10:16, Ryan Roberts <ryan.roberts@....com> wrote:
>>> On 06/05/2025 09:09, Yeoreum Yun wrote:
>>>>> On Sat, May 03, 2025 at 09:23:27PM +0100, Yeoreum Yun wrote:
>>>>>>> On Sat, May 03, 2025 at 11:16:12AM +0100, Catalin Marinas wrote:
>>>>>>>> On Fri, 02 May 2025 19:04:12 +0100, Yeoreum Yun wrote:
>>>>>>>>> create_init_idmap() could be called before .bss section initialization
>>>>>>>>> which is done in early_map_kernel().
>>>>>>>>> Therefore, data/test_prot could be set incorrectly by PTE_MAYBE_NG macro.
>>>>>>>>>
>>>>>>>>> PTE_MAYBE_NG macro set NG bit according to value of "arm64_use_ng_mappings".
>>>>>>>>> and this variable places in .bss section.
>>>>>>>>>
>>>>>>>>> [...]
>>>>>>>>
>>>>>>>> Applied to arm64 (for-next/fixes), with some slight tweaking of the
>>>>>>>> comment, thanks!
>>>>>>>>
>>>>>>>> [1/1] arm64/cpufeature: annotate arm64_use_ng_mappings with ro_after_init to prevent wrong idmap generation
>>>>>>>>       https://git.kernel.org/arm64/c/12657bcd1835
>>>>>>>
>>>>>>> I'm going to drop this for now. The kernel compiled with a clang 19.1.5
>>>>>>> version I have around (Debian sid) fails to boot, gets stuck early on:
>>>>>>>
>>>>>>> $ clang --version
>>>>>>> Debian clang version 19.1.5 (1)
>>>>>>> Target: aarch64-unknown-linux-gnu
>>>>>>> Thread model: posix
>>>>>>> InstalledDir: /usr/lib/llvm-19/bin
>>>>>>>
>>>>>>> I didn't have time to investigate, disassemble etc. I'll have a look
>>>>>>> next week.
>>>>>>
>>>>>> Just for your information.
>>>>>> When I see the debian package, clang 19.1.5-1 doesn't supply anymore:
>>>>>>  - https://ftp.debian.org/debian/pool/main/l/llvm-toolchain-19/
>>>>>>
>>>>>> and the default version for sid is below:
>>>>>>
>>>>>> $ clang-19 --version
>>>>>> Debian clang version 19.1.7 (3)
>>>>>> Target: aarch64-unknown-linux-gnu
>>>>>> Thread model: posix
>>>>>> InstalledDir: /usr/lib/llvm-19/bin
>>>>>>
>>>>>> When I tested with above version with arm64-linux's for-next/fixes
>>>>>> including this patch. it works well.
>>>>>
>>>>> It doesn't seem to be toolchain related. It fails with gcc as well from
>>>>> Debian stable but you'd need some older CPU (even if emulated, e.g.
>>>>> qemu). It fails with Cortex-A72 (guest on Raspberry Pi 4) but not
>>>>> Neoverse-N2. Also changing the annotation from __ro_after_init to
>>>>> __read_mostly also works.
>>>
>>> I think this is likely because __ro_after_init is also "ro before init" - i.e.
>>> if you try to write to it in the PI code an exception is generated due to it
>>> being mapped RO. Looks like early_map_kernel() is writiing to it.
>>
>> Indeed.
>>
>>> I've noticed a similar problem in the past and it would be nice to fix it so
>>> that PI code maps __ro_after_init RW.
>>
>> The issue is that the store occurs via the ID map, which only consists
>> of one R-X and one RW- section. I'm not convinced that it's worth the
>> hassle to relax this.
>>
>> If moving the variable to .data works, then let's just do that.
> 
> Good to know there's no other more serious issue. I'll move this
> variable to __read_mostly.
> 
> It seems to fail in early_map_kernel() if RANDOMIZE_BASE is enabled.

Ahh that explains why Yeoreum Yun can't see the issue:

	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
		u64 kaslr_seed = kaslr_early_init(fdt, chosen);

		if (kaslr_seed && kaslr_requires_kpti())
			arm64_use_ng_mappings = true;

		kaslr_offset |= kaslr_seed & ~(MIN_KIMG_ALIGN - 1);
	}