[PATCH v2] arm64/cpufeature: annotate arm64_use_ng_mappings with ro_after_init to prevent wrong idmap generation

[Ryan Roberts]

On 06/05/2025 10:41, Ard Biesheuvel wrote:
> On Tue, 6 May 2025 at 10:16, Ryan Roberts <ryan.roberts at arm.com> wrote:
>>
>> On 06/05/2025 09:09, Yeoreum Yun wrote:
>>> Hi Catalin,
>>>
>>>> On Sat, May 03, 2025 at 09:23:27PM +0100, Yeoreum Yun wrote:
>>>>> Hi Catalin,
>>>>>
>>>>>> 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.

Yeah, fair enough.