Re: [PATCH] drm/panthor: Fix TOCTOU race between GROUP_REGISTERED check and erase

[Boris Brezillon]

On Wed,  7 Oct 2026 20:12:19 +0200
Osama Abdelkader <osama.abdelkader@xxxxxxxxx> wrote:

> panthor_group_destroy() checks the GROUP_REGISTERED mark and then calls
> xa_erase() without holding the xarray lock across both. If the handle is
> destroyed by another thread in between, and a concurrent GROUP_CREATE
> reuses the freed ID, the first destroy erases and releases the new,
> still-initializing group, leading to a use-after-free in
> panthor_group_create().
> 
> Do the mark check and erase atomically under xa_lock().
> 
> Fixes: eec7e23d848d ("drm/panthor: Prevent potential UAF in group creation")

The fix looks legit, but I wonder how the bug was found. If this was
found with the help of a tool (AI or static code analyzer), this should
be reflected with an `Assisted-by` tag (see [1]).

With the relevant Assisted-by tag, this is

Reviewed-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>

[1]https://docs.kernel.org/process/coding-assistants.html

> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Osama Abdelkader <osama.abdelkader@xxxxxxxxx>
> ---
>  drivers/gpu/drm/panthor/panthor_sched.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_sched.c b/drivers/gpu/drm/panthor/panthor_sched.c
> index 0493858e55d7..a343c5180e01 100644
> --- a/drivers/gpu/drm/panthor/panthor_sched.c
> +++ b/drivers/gpu/drm/panthor/panthor_sched.c
> @@ -3770,12 +3770,18 @@ int panthor_group_destroy(struct panthor_file *pfile, u32 group_handle)
>  	struct panthor_group_pool *gpool = pfile->groups;
>  	struct panthor_device *ptdev = pfile->ptdev;
>  	struct panthor_scheduler *sched = ptdev->scheduler;
> -	struct panthor_group *group;
> +	struct panthor_group *group = NULL;
>  
> -	if (!xa_get_mark(&gpool->xa, group_handle, GROUP_REGISTERED))
> -		return -EINVAL;
> +	/*
> +	 * Check the mark and erase the entry atomically, so a concurrent
> +	 * destroy + create can't make us erase a group that's still being
> +	 * initialized and happens to reuse the same handle.
> +	 */
> +	xa_lock(&gpool->xa);
> +	if (xa_get_mark(&gpool->xa, group_handle, GROUP_REGISTERED))
> +		group = __xa_erase(&gpool->xa, group_handle);
> +	xa_unlock(&gpool->xa);
>  
> -	group = xa_erase(&gpool->xa, group_handle);
>  	if (!group)
>  		return -EINVAL;
>