[merged mm-hotfixes-stable] kasan-unpoison-task-stack-below-watermark-only-in-generic-mode.patch removed from -mm tree

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

The quilt patch titled
     Subject: kasan: unpoison task stack below watermark only in generic mode
has been removed from the -mm tree.  Its filename was
     kasan-unpoison-task-stack-below-watermark-only-in-generic-mode.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Subject: kasan: unpoison task stack below watermark only in generic mode
Date: Wed, 16 Sep 2026 19:51:13 +0200

CPU resume and BPF exception handling can discard stack frames without
running their compiler-generated epilogues.  Generic KASAN needs
kasan_unpoison_task_stack_below() to clear the redzones left behind by
those frames before the stack is reused.

CONFIG_KASAN_STACK also enables this helper for SW_TAGS.  The helper
derives the stack base from an untagged stack pointer, so kasan_unpoison()
writes KASAN_TAG_KERNEL (0xff) into the shadow for [base, watermark).  For
a vmapped task stack, vm_area->addr still carries the original random
allocation tag.  This creates a tag mismatch at the stack base even if
that memory has never held an instrumented stack object.

When the task exits and its stack is not cached, thread_stack_free_rcu()
passes vm_area->addr to vfree().  In RCU callback context this reaches
vfree_atomic(), whose llist_add() writes to the allocation base through
the tagged pointer and triggers a false KASAN invalid-access report:

  BUG: KASAN: invalid-access in vfree_atomic+0x90/0x150
  Write of size 8 at addr c2ffffc0a8f70000 by task rcuop/7/75
  Pointer tag: [c2], memory tag: [ff]
  Call trace:
   __hwasan_store8_noabort+0xe8/0xf8
   vfree_atomic+0x90/0x150
   vfree+0x220/0x298
   thread_stack_free_rcu+0x3c/0x4c
   rcu_do_batch+0x308/0xaf0
   rcu_nocb_cb_kthread+0x33c/0x708

The deferred-free path started using the tagged vm_area->addr in commit
449e0b4ed5a1 ("fork: clean-up naming of vm_stack/vm_struct variables in
vmap stacks code").  Previously it freed the untagged pointer derived from
tsk->stack, so the write was never tag-checked.

SW_TAGS does not need the blanket shadow reset to make subsequent stack
accesses valid: untagged kernel pointers have the match-all 0xff tag, and
the compiler initializes the shadow tags of instrumented stack objects
before use.

Return early unless CONFIG_KASAN_GENERIC is enabled.  Keep the mode check
in the common helper so it covers both resume and bpf_throw().

Link: https://lore.kernel.org/20260916175113.1327454-1-ryabinin.a.a@gmail.com
Fixes: 449e0b4ed5a1 ("fork: clean-up naming of vm_stack/vm_struct variables in vmap stacks code")
Signed-off-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Shaobo Huang <huangshaobo3@xiaomi.com>
Closes: https://lore.kernel.org/all/20260806123020.90869-1-huangshaobo3@xiaomi.com/
Assisted-by: LLM
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: <stable@vger.kernel.org>
---

 mm/kasan/common.c |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

--- a/mm/kasan/common.c~kasan-unpoison-task-stack-below-watermark-only-in-generic-mode
+++ a/mm/kasan/common.c
@@ -114,12 +114,23 @@ void kasan_unpoison_task_stack(struct ta
 /* Unpoison the stack for the current task beyond a watermark sp value. */
 asmlinkage void kasan_unpoison_task_stack_below(const void *watermark)
 {
+	void *base;
+
+	/*
+	 * Only the generic mode needs this. In the tag-based mode the
+	 * compiler fully initializes the shadow of stack variables on
+	 * function entry, so stale tags left below the watermark are
+	 * harmless.
+	 */
+	if (!IS_ENABLED(CONFIG_KASAN_GENERIC))
+		return;
+
 	/*
 	 * Calculate the task stack base address.  Avoid using 'current'
 	 * because this function is called by early resume code which hasn't
 	 * yet set up the percpu register (%gs).
 	 */
-	void *base = (void *)((unsigned long)watermark & ~(THREAD_SIZE - 1));
+	base = (void *)((unsigned long)watermark & ~(THREAD_SIZE - 1));
 
 	kasan_unpoison(base, watermark - base, false);
 }
_

Patches currently in -mm which might be from ryabinin.a.a@gmail.com are