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

[Yeoreum Yun <yeoreum.yun@arm.com>]

Hi Ryan,

> 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@arm.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);
> 	}

Thanks to clarify me to know.
I've misread the address of arm64_use_ng_mapping :( so my brain works wrong.

Thanks again!

--
Sincerely,
Yeoreum Yun