binder: fix double-free in dbitmap

[PATCH] binder: fix double-free in dbitmap

Posted by Carlos Llamas 2 weeks, 2 days ago

A process might fail to allocate a new bitmap when trying to expand its
proc->dmap. In that case, dbitmap_grow() fails and frees the old bitmap
via dbitmap_free(). However, the driver calls dbitmap_free() again when
the same process terminates, leading to a double-free error:

  ==================================================================
  BUG: KASAN: double-free in binder_proc_dec_tmpref+0x2e0/0x55c
  Free of addr ffff00000b7c1420 by task kworker/9:1/209

  CPU: 9 UID: 0 PID: 209 Comm: kworker/9:1 Not tainted 6.17.0-rc6-dirty #5 PREEMPT
  Hardware name: linux,dummy-virt (DT)
  Workqueue: events binder_deferred_func
  Call trace:
   kfree+0x164/0x31c
   binder_proc_dec_tmpref+0x2e0/0x55c
   binder_deferred_func+0xc24/0x1120
   process_one_work+0x520/0xba4
  [...]

  Allocated by task 448:
   __kmalloc_noprof+0x178/0x3c0
   bitmap_zalloc+0x24/0x30
   binder_open+0x14c/0xc10
  [...]

  Freed by task 449:
   kfree+0x184/0x31c
   binder_inc_ref_for_node+0xb44/0xe44
   binder_transaction+0x29b4/0x7fbc
   binder_thread_write+0x1708/0x442c
   binder_ioctl+0x1b50/0x2900
  [...]
  ==================================================================

Fix this issue by marking proc->map NULL in dbitmap_free().

Cc: stable@vger.kernel.org
Fixes: 15d9da3f818c ("binder: use bitmap for faster descriptor lookup")
Signed-off-by: Carlos Llamas <cmllamas@google.com>
---
 drivers/android/dbitmap.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/android/dbitmap.h b/drivers/android/dbitmap.h
index 956f1bd087d1..c7299ce8b374 100644
--- a/drivers/android/dbitmap.h
+++ b/drivers/android/dbitmap.h
@@ -37,6 +37,7 @@ static inline void dbitmap_free(struct dbitmap *dmap)
 {
 	dmap->nbits = 0;
 	kfree(dmap->map);
+	dmap->map = NULL;
 }
 
 /* Returns the nbits that a dbitmap can shrink to, 0 if not possible. */
-- 
2.51.0.384.g4c02a37b29-goog

Re: [PATCH] binder: fix double-free in dbitmap

Posted by Alice Ryhl 2 weeks, 1 day ago

On Tue, Sep 16, 2025 at 12:12 AM Carlos Llamas <cmllamas@google.com> wrote:
>
> A process might fail to allocate a new bitmap when trying to expand its
> proc->dmap. In that case, dbitmap_grow() fails and frees the old bitmap
> via dbitmap_free(). However, the driver calls dbitmap_free() again when
> the same process terminates, leading to a double-free error:
>
>   ==================================================================
>   BUG: KASAN: double-free in binder_proc_dec_tmpref+0x2e0/0x55c
>   Free of addr ffff00000b7c1420 by task kworker/9:1/209
>
>   CPU: 9 UID: 0 PID: 209 Comm: kworker/9:1 Not tainted 6.17.0-rc6-dirty #5 PREEMPT
>   Hardware name: linux,dummy-virt (DT)
>   Workqueue: events binder_deferred_func
>   Call trace:
>    kfree+0x164/0x31c
>    binder_proc_dec_tmpref+0x2e0/0x55c
>    binder_deferred_func+0xc24/0x1120
>    process_one_work+0x520/0xba4
>   [...]
>
>   Allocated by task 448:
>    __kmalloc_noprof+0x178/0x3c0
>    bitmap_zalloc+0x24/0x30
>    binder_open+0x14c/0xc10
>   [...]
>
>   Freed by task 449:
>    kfree+0x184/0x31c
>    binder_inc_ref_for_node+0xb44/0xe44
>    binder_transaction+0x29b4/0x7fbc
>    binder_thread_write+0x1708/0x442c
>    binder_ioctl+0x1b50/0x2900
>   [...]
>   ==================================================================
>
> Fix this issue by marking proc->map NULL in dbitmap_free().
>
> Cc: stable@vger.kernel.org
> Fixes: 15d9da3f818c ("binder: use bitmap for faster descriptor lookup")
> Signed-off-by: Carlos Llamas <cmllamas@google.com>

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

Re: [PATCH] binder: fix double-free in dbitmap

Posted by Tiffany Yang 2 weeks, 2 days ago

Carlos Llamas <cmllamas@google.com> writes:

> A process might fail to allocate a new bitmap when trying to expand its
> proc->dmap. In that case, dbitmap_grow() fails and frees the old bitmap
> via dbitmap_free(). However, the driver calls dbitmap_free() again when
> the same process terminates, leading to a double-free error:

>    ==================================================================
>    BUG: KASAN: double-free in binder_proc_dec_tmpref+0x2e0/0x55c
>    Free of addr ffff00000b7c1420 by task kworker/9:1/209

>    CPU: 9 UID: 0 PID: 209 Comm: kworker/9:1 Not tainted 6.17.0-rc6-dirty  
> #5 PREEMPT
>    Hardware name: linux,dummy-virt (DT)
>    Workqueue: events binder_deferred_func
>    Call trace:
>     kfree+0x164/0x31c
>     binder_proc_dec_tmpref+0x2e0/0x55c
>     binder_deferred_func+0xc24/0x1120
>     process_one_work+0x520/0xba4
>    [...]

>    Allocated by task 448:
>     __kmalloc_noprof+0x178/0x3c0
>     bitmap_zalloc+0x24/0x30
>     binder_open+0x14c/0xc10
>    [...]

>    Freed by task 449:
>     kfree+0x184/0x31c
>     binder_inc_ref_for_node+0xb44/0xe44
>     binder_transaction+0x29b4/0x7fbc
>     binder_thread_write+0x1708/0x442c
>     binder_ioctl+0x1b50/0x2900
>    [...]
>    ==================================================================

> Fix this issue by marking proc->map NULL in dbitmap_free().

> Cc: stable@vger.kernel.org
> Fixes: 15d9da3f818c ("binder: use bitmap for faster descriptor lookup")
> Signed-off-by: Carlos Llamas <cmllamas@google.com>
> ---
>   drivers/android/dbitmap.h | 1 +
>   1 file changed, 1 insertion(+)

> diff --git a/drivers/android/dbitmap.h b/drivers/android/dbitmap.h
> index 956f1bd087d1..c7299ce8b374 100644
> --- a/drivers/android/dbitmap.h
> +++ b/drivers/android/dbitmap.h
> @@ -37,6 +37,7 @@ static inline void dbitmap_free(struct dbitmap *dmap)
>   {
>   	dmap->nbits = 0;
>   	kfree(dmap->map);
> +	dmap->map = NULL;
>   }

>   /* Returns the nbits that a dbitmap can shrink to, 0 if not possible. */

Reviewed-by: Tiffany Yang <ynaffit@google.com>

-- 
Tiffany Y. Yang