firmware: fix theoretical UAF race with firmware cache and resume

This race was discovered when I carefully analyzed the code to locate
another firmware-related UAF issue. It can be triggered only when the
firmware load operation is executed during suspend. This possibility is
almost impossible because there are few firmware load and suspend actions
in the actual environment.

		CPU0			CPU1
__device_uncache_fw_images():		assign_fw():
					fw_cache_piggyback_on_request()
					<----- P0
	spin_lock(&fwc->name_lock);
	...
	list_del(&fce->list);
	spin_unlock(&fwc->name_lock);

	uncache_firmware(fce->name);
					<----- P1
					kref_get(&fw_priv->ref);

If CPU1 is interrupted at position P0, the new 'fce' has been added to the
list fwc->fw_names by the fw_cache_piggyback_on_request(). In this case,
CPU0 executes __device_uncache_fw_images() and will be able to see it when
it traverses list fwc->fw_names. Before CPU1 executes kref_get() at P1, if
CPU0 further executes uncache_firmware(), the count of fw_priv->ref may
decrease to 0, causing fw_priv to be released in advance.

Move kref_get() to the lock protection range of fwc->name_lock to fix it.

Fixes: ac39b3ea73aa ("firmware loader: let caching firmware piggyback on loading firmware")
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20210719064531.3733-2-thunder.leizhen@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 8 insertions, 12 deletions

diff --git a/drivers/base/firmware_loader/main.c b/drivers/base/firmware_loader/main.c
index 4fdb8219cd08..2f267b8d2fd9 100644
--- a/drivers/base/firmware_loader/main.c
+++ b/drivers/base/firmware_loader/main.c
@@ -165,7 +165,7 @@ static inline int fw_state_wait(struct fw_priv *fw_priv)
 	return __fw_state_wait_common(fw_priv, MAX_SCHEDULE_TIMEOUT);
 }
 
-static int fw_cache_piggyback_on_request(const char *name);
+static void fw_cache_piggyback_on_request(struct fw_priv *fw_priv);
 
 static struct fw_priv *__allocate_fw_priv(const char *fw_name,
 					  struct firmware_cache *fwc,
@@ -707,10 +707,8 @@ int assign_fw(struct firmware *fw, struct device *device)
 	 * on request firmware.
 	 */
 	if (!(fw_priv->opt_flags & FW_OPT_NOCACHE) &&
-	    fw_priv->fwc->state == FW_LOADER_START_CACHE) {
-		if (fw_cache_piggyback_on_request(fw_priv->fw_name))
-			kref_get(&fw_priv->ref);
-	}
+	    fw_priv->fwc->state == FW_LOADER_START_CACHE)
+		fw_cache_piggyback_on_request(fw_priv);
 
 	/* pass the pages buffer to driver at the last minute */
 	fw_set_page_data(fw_priv, fw);
@@ -1257,11 +1255,11 @@ static int __fw_entry_found(const char *name)
 	return 0;
 }
 
-static int fw_cache_piggyback_on_request(const char *name)
+static void fw_cache_piggyback_on_request(struct fw_priv *fw_priv)
 {
-	struct firmware_cache *fwc = &fw_cache;
+	const char *name = fw_priv->fw_name;
+	struct firmware_cache *fwc = fw_priv->fwc;
 	struct fw_cache_entry *fce;
-	int ret = 0;
 
 	spin_lock(&fwc->name_lock);
 	if (__fw_entry_found(name))
@@ -1269,13 +1267,12 @@ static int fw_cache_piggyback_on_request(const char *name)
 
 	fce = alloc_fw_cache_entry(name);
 	if (fce) {
-		ret = 1;
 		list_add(&fce->list, &fwc->fw_names);
+		kref_get(&fw_priv->ref);
 		pr_debug("%s: fw: %s\n", __func__, name);
 	}
 found:
 	spin_unlock(&fwc->name_lock);
-	return ret;
 }
 
 static void free_fw_cache_entry(struct fw_cache_entry *fce)
@@ -1506,9 +1503,8 @@ static inline void unregister_fw_pm_ops(void)
 	unregister_pm_notifier(&fw_cache.pm_notify);
 }
 #else
-static int fw_cache_piggyback_on_request(const char *name)
+static void fw_cache_piggyback_on_request(struct fw_priv *fw_priv)
 {
-	return 0;
 }
 static inline int register_fw_pm_ops(void)
 {