linux-kernel - [PATCH v1 2/2] kasan: Unpoison vms[area] addresses with a common tag

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Message-ID: <cf8fe0ffcdbf54e06d9df26c8473b123c4065f02.1762267022.git.m.wieczorretman@pm.me>
Date: Tue, 04 Nov 2025 14:49:48 +0000
From: Maciej Wieczor-Retman <m.wieczorretman@...me>
To: Andrey Ryabinin <ryabinin.a.a@...il.com>, Alexander Potapenko <glider@...gle.com>, Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, Vincenzo Frascino <vincenzo.frascino@....com>, Andrew Morton <akpm@...ux-foundation.org>, Marco Elver <elver@...gle.com>
Cc: m.wieczorretman@...me, stable@...r.kernel.org, Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, Baoquan He <bhe@...hat.com>, kasan-dev@...glegroups.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH v1 2/2] kasan: Unpoison vms[area] addresses with a common tag

From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>

A KASAN tag mismatch, possibly causing a kernel panic, can be observed
on systems with a tag-based KASAN enabled and with multiple NUMA nodes.
It was reported on arm64 and reproduced on x86. It can be explained in
the following points:

	1. There can be more than one virtual memory chunk.
	2. Chunk's base address has a tag.
	3. The base address points at the first chunk and thus inherits
	   the tag of the first chunk.
	4. The subsequent chunks will be accessed with the tag from the
	   first chunk.
	5. Thus, the subsequent chunks need to have their tag set to
	   match that of the first chunk.

Unpoison all vm_structs after allocating them for the percpu allocator.
Use the same tag to resolve the pcpu chunk address mismatch.

Fixes: 1d96320f8d53 ("kasan, vmalloc: add vmalloc tagging for SW_TAGS")
Cc: <stable@...r.kernel.org> # 6.1+
Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
Tested-by: Baoquan He <bhe@...hat.com>
---
Changelog v1 (after splitting of from the KASAN series):
- Rewrite the patch message to point at the user impact of the issue.
- Move helper to common.c so it can be compiled in all KASAN modes.

 mm/kasan/common.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index c63544a98c24..a6bbc68984cd 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -584,12 +584,20 @@ bool __kasan_check_byte(const void *address, unsigned long ip)
 	return true;
 }
 
+/*
+ * A tag mismatch happens when calculating per-cpu chunk addresses, because
+ * they all inherit the tag from vms[0]->addr, even when nr_vms is bigger
+ * than 1. This is a problem because all the vms[]->addr come from separate
+ * allocations and have different tags so while the calculated address is
+ * correct the tag isn't.
+ */
 void __kasan_unpoison_vmap_areas(struct vm_struct **vms, int nr_vms)
 {
 	int area;
 
 	for (area = 0 ; area < nr_vms ; area++) {
 		kasan_poison(vms[area]->addr, vms[area]->size,
-			     arch_kasan_get_tag(vms[area]->addr), false);
+			     arch_kasan_get_tag(vms[0]->addr), false);
+		arch_kasan_set_tag(vms[area]->addr, arch_kasan_get_tag(vms[0]->addr));
 	}
 }
-- 
2.51.0