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

Hillf Danton hdanton at sina.com
Wed May 19 09:10:47 UTC 2021


On  Tue, 18 May 2021 21:29:20 +0530 Anirudh Rayabharam wrote:
>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 at syzkaller.appspotmail.com
>Tested-by: syzbot+de271708674e2093097b at syzkaller.appspotmail.com
>Signed-off-by: Anirudh Rayabharam <mail at 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);

This looks like prepare_to_wait().

> 	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);
		  __fw_load_abort();
		    fw_state_aborted();
		      __fw_state_set();

Is this your finish_wait() part? See what you add below.

> 		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);
>+	}

Fine, apart from what you are fixing, you are adding something like
finish_wait() into the waker's backyard. Why are you calling
complete_all() on the waiter side?

> }
> 
> 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


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