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

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Message-ID: <dbx8a52vuy82.fsf@ynaffit-andsys.c.googlers.com>
Date: Mon, 15 Sep 2025 15:50:37 -0700
From: Tiffany Yang <ynaffit@...gle.com>
To: Carlos Llamas <cmllamas@...gle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Arve Hjønnevåg" <arve@...roid.com>, Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, 
	Joel Fernandes <joelagnelf@...dia.com>, Christian Brauner <brauner@...nel.org>, 
	Suren Baghdasaryan <surenb@...gle.com>, Alice Ryhl <aliceryhl@...gle.com>, kernel-team@...roid.com, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] binder: fix double-free in dbitmap

Carlos Llamas <cmllamas@...gle.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@...r.kernel.org
> Fixes: 15d9da3f818c ("binder: use bitmap for faster descriptor lookup")
> Signed-off-by: Carlos Llamas <cmllamas@...gle.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@...gle.com>

-- 
Tiffany Y. Yang