[PATCH v3 4/4] ARM: Implement PAN for LPAE by TTBR0 page table walks disablement

[Ard Biesheuvel]

On Tue, 14 May 2024 at 09:46, Linus Walleij <linus.walleij at linaro.org> wrote:
>
> On Tue, May 14, 2024 at 8:41 AM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
>
> > I sent you a small initramfs by PM.
>
> Booted this just fine on Vexpress QEMU:
>
> Run /init as init process
> sysctl: error: 'kernel.hotplug' is an unknown key
>
>
> boot (Linux 6.9.0-rc1+, BusyBox v1.16.0.git, kexec-tools 2.0.1-git)
> / # mount -t debugfs none /sys/kernel/debug
> / # echo "ACCESS_USERSPACE" | cat >/sys/kernel/debug/provoke-crash/DIRECT
> lkdtm: Performing direct entry ACCESS_USERSPACE
> lkdtm: attempting bad read at 76fea000
> 8<--- cut here ---
> Unable to handle kernel paging request at virtual address 76fea000 when read
> [76fea000] *pgd=82c93003, *pmd=82c94003, *pte=a00000811e2f5f
> Internal error: Oops: 206 [#1] SMP ARM
> CPU: 1 PID: 86 Comm: cat Not tainted 6.9.0-rc1+ #46
> Hardware name: ARM-Versatile Express
> PC is at lkdtm_ACCESS_USERSPACE+0xc0/0x138
> LR is at lkdtm_ACCESS_USERSPACE+0xc0/0x138
>
> I'm starting to think it is something about different LPAE implementations here.
>

I have built multi_v7_defconfig with the following enabled

CONFIG_ARM_LPAE=y
CONFIG_CPU_TTBR0_PAN=y
CONFIG_LKDTM=y

and the resulting kernel boots happily as a 32-bit VM running under a
Rpi4 KVM host.

Could someone post an actual .config that reproduces this? Rpi4 is
A72, which both works and doesn't work in Florian's testing, so I'd be
highly surprised if this is not a config issue.