[PATCH v1] kasan: Fix false-positive wild-memory-access on x86 under 5-level paging

[Ihor Solodrai]

On x86_64 with 5-level paging (LA57) and inline generic KASAN, the
following flaky splat may be observed on boot:

BUG: KASAN: wild-memory-access in do_raw_spin_lock+0xcf/0x260
Write of size 4 at addr ff110001000c90b8 by task swapper/0/0

CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 7.1.0-rc5-gcba33e0b2907 #1 PREEMPT(full)
Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
<IRQ>
dump_stack_lvl+0x54/0x70
kasan_report+0x117/0x150
? do_raw_spin_lock+0xcf/0x260
kasan_check_range+0x264/0x2c0
do_raw_spin_lock+0xcf/0x260
handle_edge_irq+0x35/0x770
? do_raw_spin_unlock+0x51/0x2a0
__common_interrupt+0xae/0x120
common_interrupt+0x7c/0x90
</IRQ>
<TASK>
asm_common_interrupt+0x26/0x40
RIP: 0010:identify_cpu+0x2b2/0x3460
Code: 00 41 c7 07 00 00 00 00 4d 89 e6 49 c1 ee 03 43 0f b6 04 06 84 c0 0f 85 a3 1c 00 00 41 c7 04 24 00 00 00 00 31 c0 31 c9 0f a2 <89> c7 42 0f b6 44 05 00 84 c0 0f 85 ad 1c 00 00 41 89 3f 48 8b 44
RSP: 0000:ffffffff97807df0 EFLAGS: 00000246
RAX: 0000000000000020 RBX: 00000000756e6547 RCX: 000000006c65746e
RDX: 0000000049656e69 RSI: 0000000000000000 RDI: ffffffff98632fd8
RBP: 1ffffffff30c65fc R08: dffffc0000000000 R09: 0000000000000004
R10: ffffffff98632fc4 R11: fffffbfff30c65fb R12: ffffffff98633050
R13: ffffffff98633048 R14: 1ffffffff30c660a R15: ffffffff98632fe0
identify_boot_cpu+0xd/0xd0
arch_cpu_finalize_init+0x24/0x1f0
start_kernel+0x31e/0x3e0
x86_64_start_reservations+0x24/0x30
x86_64_start_kernel+0x13a/0x140
common_startup_64+0x12c/0x137
</TASK>

It fires very early in boot. If kasan_multi_shot is set, the reports
are non-fatal and keep repeating, and the boot CPU wedges before
userspace is reached. The accessed addresses are valid 5-level kernel
pointers, so the report is a false positive.

The root cause is in generic KASAN not seeing
cpu_feature_enabled(X86_FEATURE_LA57) set, because the bit is cleared
in identify_cpu() when the offending interrupt happens [1]:

memset(&c->x86_capability, 0, ...); /* clears X86_FEATURE_LA57 */
...
get_cpu_cap(c); /* re-reads CPUID, restores it */

addr_has_metadata() then uses the 4-level threshold, and 5-level
kernel addresses fall below it, so kasan_check_range() reports them as
wild-memory-access.

Define USE_EARLY_PGTABLE_L5 in mm/kasan/generic.c so
addr_has_metadata() uses the stable variable, as
arch/x86/mm/kasan_init_64.c already does.

Some context on how this was noticed and reproduced below.

We started seeing flaky splats as above [2][3] on BPF CI runs after
runner hardware has been upgraded. Specifically, new x86 runners are
c7i.metal-24xl AWS EC2 instances, which are Intel Sapphire Rapids
machines that support LA57 feature, and have it enabled.

The splats can be reproduced with qemu on any x86_64 host with
-cpu max -accel tcg

Build a kernel with:
CONFIG_KASAN=y
CONFIG_KASAN_GENERIC=y
CONFIG_KASAN_INLINE=y

Boot it with kasan_multi_shot. The fault fires fast before userspace,
so no rootfs is required. For example:

qemu-system-x86_64 -display none -serial stdio -no-reboot \
-smp 4 -m 5G -cpu max -accel tcg \
-kernel arch/x86/boot/bzImage \
-append "console=ttyS0,115200 earlyprintk=serial,0,115200 panic=-1 kasan_multi_shot nokaslr"

It's a timing race, so a single boot hits it only sometimes.
However running several qemu instances in parallel on the same host
significantly increases the hitrate.

I confirmed the proposed fix eliminates the splats.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/cpu/common.c?h=v7.1-rc7#n2001
[2] https://github.com/kernel-patches/bpf/actions/runs/27271262414/job/80542509369
[3] https://github.com/kernel-patches/bpf/actions/runs/27260143782/job/80505353689

Signed-off-by: Ihor Solodrai <ihor.s...@linux.dev>
---
mm/kasan/generic.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
index 2b8e73f5f6a7..b5f430f2dbb6 100644
--- a/mm/kasan/generic.c
+++ b/mm/kasan/generic.c
@@ -9,6 +9,13 @@
* Andrey Konovalov <andre...@gmail.com>
*/

+/*
+ * check_region_inline() and addr_has_metadata() can run very early.
+ * For example, in an interrupt taken while identify_cpu() has the CPU
+ * capability bits temporarily cleared.
+ */
+#define USE_EARLY_PGTABLE_L5
+
#include <linux/export.h>
#include <linux/interrupt.h>
#include <linux/init.h>
--
2.54.0

[Andrey Konovalov]

It took me some digging to find which stable variable you refer to
here; I think this needs to be better explained.

There are other users of addr_has_metadata(), besides ones in generic.c.

Thus, should we place this definition into mm/kasan/kasan.h? And
likely guard it with ifdef KASAN_GENERIC || KASAN_SW_TAGS (HW_TAGS
should not be affected by this AFAIU).

And let's also extend the comment to explain better what's going on. I
like the way the issue is explained here:

https://elixir.bootlin.com/linux/v7.0.12/source/arch/x86/boot/startup/sme.c#L36

> +
> #include <linux/export.h>
> #include <linux/interrupt.h>
> #include <linux/init.h>
> --
> 2.54.0
>

> --
> You received this message because you are subscribed to the Google Groups "kasan-dev" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to kasan-dev+...@googlegroups.com.
> To view this discussion visit https://groups.google.com/d/msgid/kasan-dev/20260610175651.647515-1-ihor.solodrai%40linux.dev.

[Ihor Solodrai]

Hi Andrey, thank you for the quick review.

Yeah, it's:

unsigned int __section(".data") __pgtable_l5_enabled;

I'll add more details in next revision.

It appears setting USE_EARLY_PGTABLE_L5 in kasan.h will not work.

USE_EARLY_PGTABLE_L5 needs to be set before
arch/x86/include/asm/pgtable_64_types.h is included, which is very early.

For example, it arrives to kasan/generic.c through a long path starting
at linux/interrupt.h:

In file included from ./arch/x86/include/asm/pgtable_types.h:277,
from ./arch/x86/include/asm/processor.h:21,
from ./arch/x86/include/asm/cpufeature.h:5,
from ./arch/x86/include/asm/thread_info.h:59,
from ./include/linux/thread_info.h:62,
from ./include/linux/smp.h:119,
from ./include/linux/alloc_tag.h:14,
from ./include/linux/percpu.h:5,
from ./include/linux/context_tracking_state.h:5,
from ./include/linux/hardirq.h:5,
from ./include/linux/interrupt.h:11,
from mm/kasan/generic.c:13:

So if we want to cover all users of addr_has_metadata(), we'll have to
add #define USE_EARLY_PGTABLE_L5 in multiple files: generic.c, report.c,
report_generic.c, maybe others?

Any other ideas?

I determined the include path like follows:

cd linux # at 9716c086c8e8 torvalds/master
make defconfig
./scripts/config -e KASAN -e KASAN_GENERIC -e KASAN_INLINE
make olddefconfig
make mm/kasan/generic.i

# then modify pgtable_64_types.h

diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
index 7eb61ef6a185..c6b754143fb4 100644
--- a/arch/x86/include/asm/pgtable_64_types.h
+++ b/arch/x86/include/asm/pgtable_64_types.h
@@ -1,6 +1,7 @@
/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _ASM_X86_PGTABLE_64_DEFS_H
#define _ASM_X86_PGTABLE_64_DEFS_H
+#error TRACE

#include <asm/sparsemem.h>

And then:
eval "$(grep '^savedcmd_' mm/kasan/.generic.i.cmd | head -1 | sed 's/^savedcmd_[^ ]* := //')"

>
> And let's also extend the comment to explain better what's going on. I
> like the way the issue is explained here:
>
> https://elixir.bootlin.com/linux/v7.0.12/source/arch/x86/boot/startup/sme.c#L36

ok