[PATCH v2] firmware_loader: fix use-after-free in firmware_fallback_sysfs

[Anirudh Rayabharam <mail@anirudhrb.com>]

The use-after-free happens when a fw_priv object has been freed but
hasn't been removed from the pending list (pending_fw_head). The next
time fw_load_sysfs_fallback tries to insert into the list, it ends up
accessing the pending_list member of the previoiusly freed fw_priv.

In commit bcfbd3523f3c ("firmware: fix a double abort case with
fw_load_sysfs_fallback"), fw_load_abort() is skipped if
fw_sysfs_wait_timeout() returns -ENOENT. This causes the fw_priv to
not be removed from the pending list.

To fix this, delete the fw_priv from the pending list when retval
is -ENOENT instead of skipping the entire block.

Fixes: bcfbd3523f3c ("firmware: fix a double abort case with fw_load_sysfs_fallback")
Reported-by: syzbot+de271708674e2093097b@syzkaller.appspotmail.com
Tested-by: syzbot+de271708674e2093097b@syzkaller.appspotmail.com
Signed-off-by: Anirudh Rayabharam <mail@anirudhrb.com>
---

Changes in v2:
1. Fixed 1 error and 1 warning (in the commit message) reported by
   checkpatch.pl. The error was regarding the format for referring to
   another commit "commit <sha> ("oneline")". The warning was for line
   longer than 75 chars.

---
 drivers/base/firmware_loader/fallback.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/base/firmware_loader/fallback.c b/drivers/base/firmware_loader/fallback.c
index 91899d185e31..56ae4ab3199d 100644
--- a/drivers/base/firmware_loader/fallback.c
+++ b/drivers/base/firmware_loader/fallback.c
@@ -526,9 +526,14 @@ static int fw_load_sysfs_fallback(struct fw_sysfs *fw_sysfs, long timeout)
 	}
 
 	retval = fw_sysfs_wait_timeout(fw_priv, timeout);
-	if (retval < 0 && retval != -ENOENT) {
+	if (retval < 0) {
 		mutex_lock(&fw_lock);
-		fw_load_abort(fw_sysfs);
+
+		if (retval != -ENOENT)
+			fw_load_abort(fw_sysfs);
+		else
+			list_del_init(&fw_priv->pending_list);
+
 		mutex_unlock(&fw_lock);
 	}
 
-- 
2.26.2