linux-kernel - [PATCH v3 3/3] kasan: Unpoison vms[area] addresses with a common tag

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Message-ID: <873821114a9f722ffb5d6702b94782e902883fdf.1764874575.git.m.wieczorretman@pm.me>
Date: Thu, 04 Dec 2025 19:00:11 +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, jiayuan.chen@...ux.dev, stable@...r.kernel.org, Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, kasan-dev@...glegroups.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH v3 3/3] 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.

Use the new vmalloc flag that disables random tag assignment in
__kasan_unpoison_vmalloc() - pass the same random tag to all the
vm_structs by tagging the pointers before they go inside
__kasan_unpoison_vmalloc(). Assigning a common tag resolves 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>
---
Changelog v3:
- Redo the patch by using a flag instead of a new argument in
  __kasan_unpoison_vmalloc() (Andrey Konovalov)

Changelog v2:
- Revise the whole patch to match the fixed refactorization from the
  first patch.

Changelog v1:
- 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 | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 1ed6289d471a..496bb2c56911 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -591,11 +591,28 @@ void __kasan_unpoison_vmap_areas(struct vm_struct **vms, int nr_vms,
 	unsigned long size;
 	void *addr;
 	int area;
+	u8 tag;
+
+	/*
+	 * If KASAN_VMALLOC_KEEP_TAG was set at this point, all vms[] pointers
+	 * would be unpoisoned with the KASAN_TAG_KERNEL which would disable
+	 * KASAN checks down the line.
+	 */
+	if (flags & KASAN_VMALLOC_KEEP_TAG) {
+		pr_warn("KASAN_VMALLOC_KEEP_TAG flag shouldn't be already set!\n");
+		return;
+	}
+
+	size = vms[0]->size;
+	addr = vms[0]->addr;
+	vms[0]->addr = __kasan_unpoison_vmalloc(addr, size, flags);
+	tag = get_tag(vms[0]->addr);
 
-	for (area = 0 ; area < nr_vms ; area++) {
+	for (area = 1 ; area < nr_vms ; area++) {
 		size = vms[area]->size;
-		addr = vms[area]->addr;
-		vms[area]->addr = __kasan_unpoison_vmalloc(addr, size, flags);
+		addr = set_tag(vms[area]->addr, tag);
+		vms[area]->addr =
+			__kasan_unpoison_vmalloc(addr, size, flags | KASAN_VMALLOC_KEEP_TAG);
 	}
 }
 #endif
-- 
2.52.0