linux-kernel - [PATCH v8 2/2] firmware_loader: fix use-after-free in firmware_fallback

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Message-Id: <20210728085107.4141-3-mail@anirudhrb.com>
Date:   Wed, 28 Jul 2021 14:21:07 +0530
From:   Anirudh Rayabharam <mail@...rudhrb.com>
To:     mcgrof@...nel.org, gregkh@...uxfoundation.org, rafael@...nel.org,
        skhan@...uxfoundation.org
Cc:     Anirudh Rayabharam <mail@...rudhrb.com>,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+de271708674e2093097b@...kaller.appspotmail.com
Subject: [PATCH v8 2/2] firmware_loader: fix use-after-free in firmware_fallback_sysfs

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 previously 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 pending
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@...kaller.appspotmail.com
Tested-by: syzbot+de271708674e2093097b@...kaller.appspotmail.com
Acked-by: Luis Chamberlain <mcgrof@...nel.org>
Signed-off-by: Anirudh Rayabharam <mail@...rudhrb.com>
---
 drivers/base/firmware_loader/fallback.c | 12 ++++++++----
 drivers/base/firmware_loader/firmware.h | 10 +++++++++-
 drivers/base/firmware_loader/main.c     |  2 ++
 3 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/drivers/base/firmware_loader/fallback.c b/drivers/base/firmware_loader/fallback.c
index 1a48be0a030e..d7d63c1aa993 100644
--- a/drivers/base/firmware_loader/fallback.c
+++ b/drivers/base/firmware_loader/fallback.c
@@ -89,12 +89,11 @@ 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'
+	 * between loading done/aborted 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 +279,6 @@ 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 +511,11 @@ 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);
 
@@ -538,6 +541,7 @@ static int fw_load_sysfs_fallback(struct fw_sysfs *fw_sysfs, long timeout)
 	} 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..a3014e9e2c85 100644
--- a/drivers/base/firmware_loader/firmware.h
+++ b/drivers/base/firmware_loader/firmware.h
@@ -117,8 +117,16 @@ 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
+		/*
+		 * Doing this here ensures that the fw_priv is deleted from
+		 * the pending list in all abort/done paths.
+		 */
+		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