misc: fastrpc: Fix use-after-free and race in fastrpc_map_find

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: 8f6c1d8 ("misc: fastrpc: Add fdlist implementation")
Cc: stable <stable@kernel.org>
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>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20221124174941.418450-2-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/fastrpc.c

@@ -351,30 +351,31 @@ 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;
 }
@@ -746,7 +747,7 @@ 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);
@@ -1070,7 +1071,7 @@ 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);
 	}