[PATCH] mm/slub: reset KASAN tag in defer_free() before accessing freed memory

[PATCH] mm/slub: reset KASAN tag in defer_free() before accessing freed memory

[Deepanshu Kartikey]

When CONFIG_SLUB_TINY is enabled, kfree_nolock() calls kasan_slab_free()
before defer_free(). On ARM64 with MTE (Memory Tagging Extension),
kasan_slab_free() poisons the memory and changes the tag from the
original (e.g., 0xf3) to a poison tag (0xfe).

When defer_free() then tries to write to the freed object to build the
deferred free list via llist_add(), the pointer still has the old tag,
causing a tag mismatch and triggering a KASAN use-after-free report:

  BUG: KASAN: slab-use-after-free in defer_free+0x3c/0xbc mm/slub.c:6537
  Write at addr f3f000000854f020 by task kworker/u8:6/983
  Pointer tag: [f3], memory tag: [fe]

Fix this by calling kasan_reset_tag() before accessing the freed memory.
This is safe because defer_free() is part of the allocator itself and is
expected to manipulate freed memory for bookkeeping purposes.

Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Reported-by: syzbot+7a25305a76d872abcfa1@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7a25305a76d872abcfa1
Tested-by: syzbot+7a25305a76d872abcfa1@syzkaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 mm/slub.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/slub.c b/mm/slub.c
index e6a330e24145..46959c6da2cf 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -6534,6 +6534,8 @@ static void defer_free(struct kmem_cache *s, void *head)
 
 	guard(preempt)();
 
+	head = kasan_reset_tag(head);
+
 	df = this_cpu_ptr(&defer_free_objects);
 	if (llist_add(head + s->offset, &df->objects))
 		irq_work_queue(&df->work);
-- 
2.43.0

Re: [PATCH] mm/slub: reset KASAN tag in defer_free() before accessing freed memory

[Alexei Starovoitov]

On Wed, Dec 10, 2025 at 11:20 AM Deepanshu Kartikey
<kartikey406@gmail.com> wrote:
>
> When CONFIG_SLUB_TINY is enabled, kfree_nolock() calls kasan_slab_free()
> before defer_free(). On ARM64 with MTE (Memory Tagging Extension),
> kasan_slab_free() poisons the memory and changes the tag from the
> original (e.g., 0xf3) to a poison tag (0xfe).
>
> When defer_free() then tries to write to the freed object to build the
> deferred free list via llist_add(), the pointer still has the old tag,
> causing a tag mismatch and triggering a KASAN use-after-free report:
>
>   BUG: KASAN: slab-use-after-free in defer_free+0x3c/0xbc mm/slub.c:6537
>   Write at addr f3f000000854f020 by task kworker/u8:6/983
>   Pointer tag: [f3], memory tag: [fe]
>
> Fix this by calling kasan_reset_tag() before accessing the freed memory.
> This is safe because defer_free() is part of the allocator itself and is
> expected to manipulate freed memory for bookkeeping purposes.
>
> Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
> Reported-by: syzbot+7a25305a76d872abcfa1@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=7a25305a76d872abcfa1
> Tested-by: syzbot+7a25305a76d872abcfa1@syzkaller.appspotmail.com
> Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
> ---
>  mm/slub.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index e6a330e24145..46959c6da2cf 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -6534,6 +6534,8 @@ static void defer_free(struct kmem_cache *s, void *head)
>
>         guard(preempt)();
>
> +       head = kasan_reset_tag(head);
> +
>         df = this_cpu_ptr(&defer_free_objects);
>         if (llist_add(head + s->offset, &df->objects))
>                 irq_work_queue(&df->work);

makes sense to me.
Acked-by: Alexei Starovoitov <ast@kernel.org>

Re: [PATCH] mm/slub: reset KASAN tag in defer_free() before accessing freed memory

[Vlastimil Babka]

On 12/10/25 03:20, Deepanshu Kartikey wrote:
> When CONFIG_SLUB_TINY is enabled, kfree_nolock() calls kasan_slab_free()
> before defer_free(). On ARM64 with MTE (Memory Tagging Extension),
> kasan_slab_free() poisons the memory and changes the tag from the
> original (e.g., 0xf3) to a poison tag (0xfe).
> 
> When defer_free() then tries to write to the freed object to build the
> deferred free list via llist_add(), the pointer still has the old tag,
> causing a tag mismatch and triggering a KASAN use-after-free report:
> 
>   BUG: KASAN: slab-use-after-free in defer_free+0x3c/0xbc mm/slub.c:6537
>   Write at addr f3f000000854f020 by task kworker/u8:6/983
>   Pointer tag: [f3], memory tag: [fe]
> 
> Fix this by calling kasan_reset_tag() before accessing the freed memory.
> This is safe because defer_free() is part of the allocator itself and is
> expected to manipulate freed memory for bookkeeping purposes.
> 
> Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
> Reported-by: syzbot+7a25305a76d872abcfa1@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=7a25305a76d872abcfa1
> Tested-by: syzbot+7a25305a76d872abcfa1@syzkaller.appspotmail.com
> Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>

Thanks, added cc: stable and pushed to slab/for-next-fixes

> ---
>  mm/slub.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index e6a330e24145..46959c6da2cf 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -6534,6 +6534,8 @@ static void defer_free(struct kmem_cache *s, void *head)
>  
>  	guard(preempt)();
>  
> +	head = kasan_reset_tag(head);
> +
>  	df = this_cpu_ptr(&defer_free_objects);
>  	if (llist_add(head + s->offset, &df->objects))
>  		irq_work_queue(&df->work);