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

[Catalin Marinas <catalin.marinas@arm.com>]

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.

-- 
Catalin