[PATCH v4] firmware_loader: fix use-after-free in firmware_fallback_sysfs

[Anirudh Rayabharam <mail@anirudhrb.com>]

This 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.

The root cause here is that all code paths that abort the fw load
don't delete it from the pending list. For example:

	_request_firmware()
	  -> fw_abort_batch_reqs()
	      -> fw_state_aborted()

To fix this, delete the fw_priv from the list in __fw_set_state() if
the new state is DONE or ABORTED. This way, all aborts will remove
the fw_priv from the list. Accordingly, remove calls to list_del_init
that were being made before calling fw_state_(aborted|done)().

Also, in fw_load_sysfs_fallback, don't add the fw_priv to the list
if it is already aborted. Instead, just jump out and return early.

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 v4:
Documented the reasons behind the error codes returned from
fw_sysfs_wait_timeout() as suggested by Luis Chamberlain.

Changes in v3:
Modified the patch to incorporate suggestions by Luis Chamberlain in
order to fix the root cause instead of applying a "band-aid" kind of
fix.
https://lore.kernel.org/lkml/20210403013143.GV4332@42.do-not-panic.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 | 46 ++++++++++++++++++-------
 drivers/base/firmware_loader/firmware.h |  6 +++-
 drivers/base/firmware_loader/main.c     |  2 ++
 3 files changed, 40 insertions(+), 14 deletions(-)

diff --git a/drivers/base/firmware_loader/fallback.c b/drivers/base/firmware_loader/fallback.c
index 91899d185e31..f244c7b89ba5 100644
--- a/drivers/base/firmware_loader/fallback.c
+++ b/drivers/base/firmware_loader/fallback.c
@@ -70,7 +70,31 @@ static inline bool fw_sysfs_loading(struct fw_priv *fw_priv)
 
 static inline int fw_sysfs_wait_timeout(struct fw_priv *fw_priv,  long timeout)
 {
-	return __fw_state_wait_common(fw_priv, timeout);
+	int ret = __fw_state_wait_common(fw_priv, timeout);
+
+	/*
+	 * A signal could be sent to abort a wait. Consider Android's init
+	 * gettting a SIGCHLD, which in turn was the same process issuing the
+	 * sysfs store call for the fallback. In such cases we want to be able
+	 * to tell apart in userspace when a signal caused a failure on the
+	 * wait. In such cases we'd get -ERESTARTSYS.
+	 *
+	 * Likewise though another race can happen and abort the load earlier.
+	 *
+	 * In either case the situation is interrupted so we just inform
+	 * userspace of that and we end things right away.
+	 *
+	 * When we really time out just tell userspace it should try again,
+	 * perhaps later.
+	 */
+	if (ret == -ERESTARTSYS || fw_state_is_aborted(fw_priv))
+		ret = -EINTR;
+	else if (ret == -ETIMEDOUT)
+		ret = -EAGAIN;
+	else if (fw_priv->is_paged_buf && !fw_priv->data)
+		ret = -ENOMEM;
+
+	return ret;
 }
 
 struct fw_sysfs {
@@ -91,10 +115,9 @@ static void __fw_load_abort(struct fw_priv *fw_priv)
 	 * There is a small window in which user can write to 'loading'
 	 * between loading done and disappearance of 'loading'
 	 */
-	if (fw_sysfs_done(fw_priv))
+	if (fw_state_is_aborted(fw_priv) || fw_sysfs_done(fw_priv))
 		return;
 
-	list_del_init(&fw_priv->pending_list);
 	fw_state_aborted(fw_priv);
 }
 
@@ -280,7 +303,6 @@ static ssize_t firmware_loading_store(struct device *dev,
 			 * Same logic as fw_load_abort, only the DONE bit
 			 * is ignored and we set ABORT only on failure.
 			 */
-			list_del_init(&fw_priv->pending_list);
 			if (rc) {
 				fw_state_aborted(fw_priv);
 				written = rc;
@@ -513,6 +535,11 @@ static int fw_load_sysfs_fallback(struct fw_sysfs *fw_sysfs, long timeout)
 	}
 
 	mutex_lock(&fw_lock);
+	if (fw_state_is_aborted(fw_priv)) {
+		mutex_unlock(&fw_lock);
+		retval = -EINTR;
+		goto out;
+	}
 	list_add(&fw_priv->pending_list, &pending_fw_head);
 	mutex_unlock(&fw_lock);
 
@@ -526,20 +553,13 @@ 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);
 		mutex_unlock(&fw_lock);
 	}
 
-	if (fw_state_is_aborted(fw_priv)) {
-		if (retval == -ERESTARTSYS)
-			retval = -EINTR;
-		else
-			retval = -EAGAIN;
-	} else if (fw_priv->is_paged_buf && !fw_priv->data)
-		retval = -ENOMEM;
-
+out:
 	device_del(f_dev);
 err_put_dev:
 	put_device(f_dev);
diff --git a/drivers/base/firmware_loader/firmware.h b/drivers/base/firmware_loader/firmware.h
index 63bd29fdcb9c..36bdb413c998 100644
--- a/drivers/base/firmware_loader/firmware.h
+++ b/drivers/base/firmware_loader/firmware.h
@@ -117,8 +117,12 @@ static inline void __fw_state_set(struct fw_priv *fw_priv,
 
 	WRITE_ONCE(fw_st->status, status);
 
-	if (status == FW_STATUS_DONE || status == FW_STATUS_ABORTED)
+	if (status == FW_STATUS_DONE || status == FW_STATUS_ABORTED) {
+#ifdef CONFIG_FW_LOADER_USER_HELPER
+		list_del_init(&fw_priv->pending_list);
+#endif
 		complete_all(&fw_st->completion);
+	}
 }
 
 static inline void fw_state_aborted(struct fw_priv *fw_priv)
diff --git a/drivers/base/firmware_loader/main.c b/drivers/base/firmware_loader/main.c
index 4fdb8219cd08..68c549d71230 100644
--- a/drivers/base/firmware_loader/main.c
+++ b/drivers/base/firmware_loader/main.c
@@ -783,8 +783,10 @@ static void fw_abort_batch_reqs(struct firmware *fw)
 		return;
 
 	fw_priv = fw->priv;
+	mutex_lock(&fw_lock);
 	if (!fw_state_is_aborted(fw_priv))
 		fw_state_aborted(fw_priv);
+	mutex_unlock(&fw_lock);
 }
 
 /* called from request_firmware() and request_firmware_work_func() */
-- 
2.26.2