Re: [PATCH v2 2/2] ARM: entry: Do a dummy read from VMAP shadow

[Clement LE GOFFIC <clement.legoffic@foss.st.com>]

On 10/16/24 21:15, Linus Walleij wrote:
> When switching task, in addition to a dummy read from the new
> VMAP stack, also do a dummy read from the VMAP stack's
> corresponding KASAN shadow memory to sync things up in
> the new MM context.
> 
> Cc: stable@vger.kernel.org
> Fixes: a1c510d0adc6 ("ARM: implement support for vmap'ed stacks")
> Link: https://lore.kernel.org/linux-arm-kernel/a1a1d062-f3a2-4d05-9836-3b098de9db6d@foss.st.com/
> Reported-by: Clement LE GOFFIC <clement.legoffic@foss.st.com>
> Suggested-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
> ---
>   arch/arm/kernel/entry-armv.S | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> index 1dfae1af8e31..12a4040a04ff 100644
> --- a/arch/arm/kernel/entry-armv.S
> +++ b/arch/arm/kernel/entry-armv.S
> @@ -25,6 +25,7 @@
>   #include <asm/tls.h>
>   #include <asm/system_info.h>
>   #include <asm/uaccess-asm.h>
> +#include <asm/kasan_def.h>
>   
>   #include "entry-header.S"
>   #include <asm/probes.h>
> @@ -561,6 +562,13 @@ ENTRY(__switch_to)
>   	@ entries covering the vmalloc region.
>   	@
>   	ldr	r2, [ip]
> +#ifdef CONFIG_KASAN_VMALLOC
> +	@ Also dummy read from the KASAN shadow memory for the new stack if we
> +	@ are using KASAN
> +	mov_l	r2, KASAN_SHADOW_OFFSET
> +	add	r2, ip, lsr #KASAN_SHADOW_SCALE_SHIFT

Hello Linus,

While ARM TRM says that if Rd is the same of Rn then it can be omitted, 
such syntax causes error on my build.
Looking around for such syntax in the kernel, this line should be :
add	r2, r2, ip, lsr #KASAN_SHADOW_SCALE_SHIFT

Regards,

Clement