[PATCH v3 1/3] mm/kasan: Fix incorrect unpoisoning in vrealloc for KASAN

[Maciej Wieczor-Retman <m.wieczorretman@pm.me>]

From: Jiayuan Chen <jiayuan.chen@linux.dev>

Syzkaller reported a memory out-of-bounds bug [1]. This patch fixes two
issues:

1. In vrealloc the KASAN_VMALLOC_VM_ALLOC flag is missing when
   unpoisoning the extended region. This flag is required to correctly
   associate the allocation with KASAN's vmalloc tracking.

   Note: In contrast, vzalloc (via __vmalloc_node_range_noprof) explicitly
   sets KASAN_VMALLOC_VM_ALLOC and calls kasan_unpoison_vmalloc() with it.
   vrealloc must behave consistently — especially when reusing existing
   vmalloc regions — to ensure KASAN can track allocations correctly.

2. When vrealloc reuses an existing vmalloc region (without allocating
   new pages) KASAN generates a new tag, which breaks tag-based memory
   access tracking.

Introduce KASAN_VMALLOC_KEEP_TAG, a new KASAN flag that allows reusing
the tag already attached to the pointer, ensuring consistent tag
behavior during reallocation.

Pass KASAN_VMALLOC_KEEP_TAG and KASAN_VMALLOC_VM_ALLOC to the
kasan_unpoison_vmalloc inside vrealloc_node_align_noprof().

[1]: https://syzkaller.appspot.com/bug?extid=997752115a851cb0cf36

Fixes: a0309faf1cb0 ("mm: vmalloc: support more granular vrealloc() sizing")
Reported-by: syzbot+997752115a851cb0cf36@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/68e243a2.050a0220.1696c6.007d.GAE@google.com/T/
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Co-developed-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
---
 include/linux/kasan.h | 1 +
 mm/kasan/hw_tags.c    | 2 +-
 mm/kasan/shadow.c     | 4 +++-
 mm/vmalloc.c          | 4 +++-
 4 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index d12e1a5f5a9a..6d7972bb390c 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -28,6 +28,7 @@ typedef unsigned int __bitwise kasan_vmalloc_flags_t;
 #define KASAN_VMALLOC_INIT		((__force kasan_vmalloc_flags_t)0x01u)
 #define KASAN_VMALLOC_VM_ALLOC		((__force kasan_vmalloc_flags_t)0x02u)
 #define KASAN_VMALLOC_PROT_NORMAL	((__force kasan_vmalloc_flags_t)0x04u)
+#define KASAN_VMALLOC_KEEP_TAG		((__force kasan_vmalloc_flags_t)0x08u)
 
 #define KASAN_VMALLOC_PAGE_RANGE 0x1 /* Apply exsiting page range */
 #define KASAN_VMALLOC_TLB_FLUSH  0x2 /* TLB flush */
diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
index 1c373cc4b3fa..cbef5e450954 100644
--- a/mm/kasan/hw_tags.c
+++ b/mm/kasan/hw_tags.c
@@ -361,7 +361,7 @@ void *__kasan_unpoison_vmalloc(const void *start, unsigned long size,
 		return (void *)start;
 	}
 
-	tag = kasan_random_tag();
+	tag = (flags & KASAN_VMALLOC_KEEP_TAG) ? get_tag(start) : kasan_random_tag();
 	start = set_tag(start, tag);
 
 	/* Unpoison and initialize memory up to size. */
diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
index 5d2a876035d6..5e47ae7fdd59 100644
--- a/mm/kasan/shadow.c
+++ b/mm/kasan/shadow.c
@@ -648,7 +648,9 @@ void *__kasan_unpoison_vmalloc(const void *start, unsigned long size,
 	    !(flags & KASAN_VMALLOC_PROT_NORMAL))
 		return (void *)start;
 
-	start = set_tag(start, kasan_random_tag());
+	if (unlikely(!(flags & KASAN_VMALLOC_KEEP_TAG)))
+		start = set_tag(start, kasan_random_tag());
+
 	kasan_unpoison(start, size, false);
 	return (void *)start;
 }
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 798b2ed21e46..22a73a087135 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4176,7 +4176,9 @@ void *vrealloc_node_align_noprof(const void *p, size_t size, unsigned long align
 	 */
 	if (size <= alloced_size) {
 		kasan_unpoison_vmalloc(p + old_size, size - old_size,
-				       KASAN_VMALLOC_PROT_NORMAL);
+				       KASAN_VMALLOC_PROT_NORMAL |
+				       KASAN_VMALLOC_VM_ALLOC |
+				       KASAN_VMALLOC_KEEP_TAG);
 		/*
 		 * No need to zero memory here, as unused memory will have
 		 * already been zeroed at initial allocation time or during
-- 
2.52.0