+ kasan-unpoison-vms-addresses-with-a-common-tag.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: kasan: unpoison vms[area] addresses with a common tag
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     kasan-unpoison-vms-addresses-with-a-common-tag.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/kasan-unpoison-vms-addresses-with-a-common-tag.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Subject: kasan: unpoison vms[area] addresses with a common tag
Date: Thu, 04 Dec 2025 19:00:11 +0000

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.

Link: https://lkml.kernel.org/r/873821114a9f722ffb5d6702b94782e902883fdf.1764874575.git.m.wieczorretman@pm.me
Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Fixes: 1d96320f8d53 ("kasan, vmalloc: add vmalloc tagging for SW_TAGS")
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Cc: Kees Cook <kees@kernel.org>
Cc: Marco Elver <elver@google.com>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: <stable@vger.kernel.org>	[6.1+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/kasan/common.c |   23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

--- a/mm/kasan/common.c~kasan-unpoison-vms-addresses-with-a-common-tag
+++ a/mm/kasan/common.c
@@ -591,11 +591,28 @@ void __kasan_unpoison_vmap_areas(struct
 	unsigned long size;
 	void *addr;
 	int area;
+	u8 tag;
 
-	for (area = 0 ; area < nr_vms ; area++) {
+	/*
+	 * 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 = 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
_

Patches currently in -mm which might be from maciej.wieczor-retman@intel.com are

kasan-refactor-pcpu-kasan-vmalloc-unpoison.patch
kasan-unpoison-vms-addresses-with-a-common-tag.patch