* [PATCH] ARM: entry: use byte load for KASAN VMAP stack shadow
@ 2026-05-22 21:15 Karl Mehltretter
2026-05-23 22:08 ` Linus Walleij
0 siblings, 1 reply; 3+ messages in thread
From: Karl Mehltretter @ 2026-05-22 21:15 UTC (permalink / raw)
To: Russell King
Cc: Linus Walleij, Russell King (Oracle), linux-arm-kernel,
linux-kernel, stable, 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)
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] ARM: entry: use byte load for KASAN VMAP stack shadow
2026-05-22 21:15 [PATCH] ARM: entry: use byte load for KASAN VMAP stack shadow Karl Mehltretter
@ 2026-05-23 22:08 ` Linus Walleij
2026-05-24 7:49 ` Karl Mehltretter
0 siblings, 1 reply; 3+ messages in thread
From: Linus Walleij @ 2026-05-23 22:08 UTC (permalink / raw)
To: Karl Mehltretter
Cc: Russell King, Russell King (Oracle), linux-arm-kernel,
linux-kernel, stable
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
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] ARM: entry: use byte load for KASAN VMAP stack shadow
2026-05-23 22:08 ` Linus Walleij
@ 2026-05-24 7:49 ` Karl Mehltretter
0 siblings, 0 replies; 3+ messages in thread
From: Karl Mehltretter @ 2026-05-24 7:49 UTC (permalink / raw)
To: Linus Walleij
Cc: Russell King, Russell King (Oracle), linux-arm-kernel,
linux-kernel, stable
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
^ permalink raw reply [flat|nested] 3+ messages in thread
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2026-05-22 21:15 [PATCH] ARM: entry: use byte load for KASAN VMAP stack shadow Karl Mehltretter
2026-05-23 22:08 ` Linus Walleij
2026-05-24 7:49 ` Karl Mehltretter
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