Re: [PATCH 1/3] misc: fastrpc: Fix use-after-free and race in fastrpc_map_find

[Srinivas Kandagatla <srinivas.kandagatla@linaro.org>]

On 02/09/2022 16:14, Abel Vesa wrote:
> Currently, there is a race window between the point when the mutex is
> unlocked in fastrpc_map_lookup and the reference count increasing
> (fastrpc_map_get) in fastrpc_map_find, which can also lead to
> use-after-free.
> 
> So lets merge fastrpc_map_find into fastrpc_map_lookup which allows us
> to both protect the maps list by also taking the &fl->lock spinlock and
> the reference count, since the spinlock will be released only after.
> Add take_ref argument to make this suitable for all callers.
> 
> Fixes: 8f6c1d8c4f0c ("misc: fastrpc: Add fdlist implementation")
> Co-developed-by: Ola Jeppsson <ola@snap.com>
> Signed-off-by: Ola Jeppsson <ola@snap.com>
> Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
> ---

Reviewed-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>

>   drivers/misc/fastrpc.c | 41 +++++++++++++++++++++--------------------
>   1 file changed, 21 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index 93ebd174d848..0c816a11eeec 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -333,30 +333,31 @@ static void fastrpc_map_get(struct fastrpc_map *map)
>   
>   
>   static int fastrpc_map_lookup(struct fastrpc_user *fl, int fd,
> -			    struct fastrpc_map **ppmap)
> +			    struct fastrpc_map **ppmap, bool take_ref)
>   {
> +	struct fastrpc_session_ctx *sess = fl->sctx;
>   	struct fastrpc_map *map = NULL;
> +	int ret = -ENOENT;
>   
> -	mutex_lock(&fl->mutex);
> +	spin_lock(&fl->lock);
>   	list_for_each_entry(map, &fl->maps, node) {
> -		if (map->fd == fd) {
> -			*ppmap = map;
> -			mutex_unlock(&fl->mutex);
> -			return 0;
> -		}
> -	}
> -	mutex_unlock(&fl->mutex);
> -
> -	return -ENOENT;
> -}
> +		if (map->fd != fd)
> +			continue;
>   
> -static int fastrpc_map_find(struct fastrpc_user *fl, int fd,
> -			    struct fastrpc_map **ppmap)
> -{
> -	int ret = fastrpc_map_lookup(fl, fd, ppmap);
> +		if (take_ref) {
> +			ret = fastrpc_map_get(map);
> +			if (ret) {
> +				dev_dbg(sess->dev, "%s: Failed to get map fd=%d ret=%d\n",
> +					__func__, fd, ret);
> +				break;
> +			}
> +		}
>   
> -	if (!ret)
> -		fastrpc_map_get(*ppmap);
> +		*ppmap = map;
> +		ret = 0;
> +		break;
> +	}
> +	spin_unlock(&fl->lock);
>   
>   	return ret;
>   }
> @@ -703,7 +704,7 @@ static int fastrpc_map_create(struct fastrpc_user *fl, int fd,
>   	struct fastrpc_map *map = NULL;
>   	int err = 0;
>   
> -	if (!fastrpc_map_find(fl, fd, ppmap))
> +	if (!fastrpc_map_lookup(fl, fd, ppmap, true))
>   		return 0;
>   
>   	map = kzalloc(sizeof(*map), GFP_KERNEL);
> @@ -1026,7 +1027,7 @@ static int fastrpc_put_args(struct fastrpc_invoke_ctx *ctx,
>   	for (i = 0; i < FASTRPC_MAX_FDLIST; i++) {
>   		if (!fdlist[i])
>   			break;
> -		if (!fastrpc_map_lookup(fl, (int)fdlist[i], &mmap))
> +		if (!fastrpc_map_lookup(fl, (int)fdlist[i], &mmap, false))
>   			fastrpc_map_put(mmap);
>   	}
>