[PATCH] ARM: entry: use byte load for KASAN VMAP stack shadow

[PATCH] ARM: entry: use byte load for KASAN VMAP stack shadow

[Karl Mehltretter]

Commit 44e9a3bb76e5 ("ARM: 9430/1: entry: Do a dummy read from
VMAP shadow") added a dummy read from the KASAN VMAP stack shadow in
__switch_to(). The read uses ldr, but KASAN shadow memory is
byte-granular and the computed shadow address is not guaranteed to be
word aligned.

Booting the QEMU versatilepb machine with an ARM926EJ-S CPU and
CONFIG_KASAN=y, CONFIG_KASAN_VMALLOC=y and CONFIG_VMAP_STACK=y faults
before init:

  Unhandled fault: alignment exception (0x001) at 0xb91037f6
  PC is at __switch_to+0x64/0x88

Use ldrb for the dummy shadow access. The code only needs to fault if
the shadow mapping is missing, so a byte load is sufficient and matches
the granularity of KASAN shadow memory.

Fixes: 44e9a3bb76e5 ("ARM: 9430/1: entry: Do a dummy read from VMAP shadow")
Cc: stable@vger.kernel.org # v6.13+
Assisted-by: Codex:gpt-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 arch/arm/kernel/entry-armv.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index ef6a657c8d13..a3d050ce9b79 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -567,7 +567,7 @@ ENTRY(__switch_to)
 	@ are using KASAN
 	mov_l	r2, KASAN_SHADOW_OFFSET
 	add	r2, r2, ip, lsr #KASAN_SHADOW_SCALE_SHIFT
-	ldr	r2, [r2]
+	ldrb	r2, [r2]
 #endif
 #endif
 
-- 
2.39.5 (Apple Git-154)

Re: [PATCH] ARM: entry: use byte load for KASAN VMAP stack shadow

[Linus Walleij]

On Fri, May 22, 2026 at 11:15 PM Karl Mehltretter
<kmehltretter@gmail.com> wrote:

> Commit 44e9a3bb76e5 ("ARM: 9430/1: entry: Do a dummy read from
> VMAP shadow") added a dummy read from the KASAN VMAP stack shadow in
> __switch_to(). The read uses ldr, but KASAN shadow memory is
> byte-granular and the computed shadow address is not guaranteed to be
> word aligned.
>
> Booting the QEMU versatilepb machine with an ARM926EJ-S CPU and
> CONFIG_KASAN=y, CONFIG_KASAN_VMALLOC=y and CONFIG_VMAP_STACK=y faults
> before init:
>
>   Unhandled fault: alignment exception (0x001) at 0xb91037f6
>   PC is at __switch_to+0x64/0x88
>
> Use ldrb for the dummy shadow access. The code only needs to fault if
> the shadow mapping is missing, so a byte load is sufficient and matches
> the granularity of KASAN shadow memory.
>
> Fixes: 44e9a3bb76e5 ("ARM: 9430/1: entry: Do a dummy read from VMAP shadow")
> Cc: stable@vger.kernel.org # v6.13+
> Assisted-by: Codex:gpt-5
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>

Good catch!
Reviewed-by: Linus Walleij <linusw@kernel.org>

Please put this patch into Russell's patch tracker.

Yours,
Linus Walleij

Re: [PATCH] ARM: entry: use byte load for KASAN VMAP stack shadow

[Karl Mehltretter]

On Sun, May 24, 2026 at 12:08:57AM +0100, Linus Walleij wrote:
> Please put this patch into Russell's patch tracker.

Done: https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9475/1

Thanks,
Karl