patch "USB: gadgetfs: Fix race between mounting and unmounting" added to usb-linus

This is a note to let you know that I've just added the patch titled

USB: gadgetfs: Fix race between mounting and unmounting

to my usb git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git
in the usb-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From d18dcfe9860e842f394e37ba01ca9440ab2178f4 Mon Sep 17 00:00:00 2001
From: Alan Stern <[email protected]>
Date: Fri, 23 Dec 2022 09:59:09 -0500
Subject: USB: gadgetfs: Fix race between mounting and unmounting

The syzbot fuzzer and Gerald Lee have identified a use-after-free bug in the gadgetfs driver, involving processes concurrently mounting and unmounting the gadgetfs filesystem. In particular, gadgetfs_fill_super() can race with gadgetfs_kill_sb(), causing the latter to deallocate the_device while the former is using it. The output from KASAN says, in part:

BUG: KASAN: use-after-free in instrument_atomic_read_write include/linux/instrumented.h:102 [inline]
BUG: KASAN: use-after-free in atomic_fetch_sub_release include/linux/atomic/atomic-instrumented.h:176 [inline]
BUG: KASAN: use-after-free in __refcount_sub_and_test include/linux/refcount.h:272 [inline]
BUG: KASAN: use-after-free in __refcount_dec_and_test include/linux/refcount.h:315 [inline]
BUG: KASAN: use-after-free in refcount_dec_and_test include/linux/refcount.h:333 [inline]
BUG: KASAN: use-after-free in put_dev drivers/usb/gadget/legacy/inode.c:159 [inline]
BUG: KASAN: use-after-free in gadgetfs_kill_sb+0x33/0x100 drivers/usb/gadget/legacy/inode.c:2086
Write of size 4 at addr ffff8880276d7840 by task syz-executor126/18689

CPU: 0 PID: 18689 Comm: syz-executor126 Not tainted 6.1.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022
Call Trace:
 <TASK>
...
 atomic_fetch_sub_release include/linux/atomic/atomic-instrumented.h:176 [inline]
 __refcount_sub_and_test include/linux/refcount.h:272 [inline]
 __refcount_dec_and_test include/linux/refcount.h:315 [inline]
 refcount_dec_and_test include/linux/refcount.h:333 [inline]
 put_dev drivers/usb/gadget/legacy/inode.c:159 [inline]
 gadgetfs_kill_sb+0x33/0x100 drivers/usb/gadget/legacy/inode.c:2086
 deactivate_locked_super+0xa7/0xf0 fs/super.c:332
 vfs_get_super fs/super.c:1190 [inline]
 get_tree_single+0xd0/0x160 fs/super.c:1207
 vfs_get_tree+0x88/0x270 fs/super.c:1531
 vfs_fsconfig_locked fs/fsopen.c:232 [inline]

The simplest solution is to ensure that gadgetfs_fill_super() and gadgetfs_kill_sb() are serialized by making them both acquire a new mutex.

---
 drivers/usb/gadget/legacy/inode.c | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

index 01c3ead7d1b4..d605bc2e7e8f 100644
@@ -229,6 +229,7 @@ static void put_ep (struct ep_data *data)
  */
 
 static const char *CHIP;
+static DEFINE_MUTEX(sb_mutex);		/* Serialize superblock operations */
 
 /*----------------------------------------------------------------------*/
 

@@ -2010,13 +2011,20 @@ gadgetfs_fill_super (struct super_block *sb, struct fs_context *fc)
 {
 	struct inode	*inode;
 	struct dev_data	*dev;
+	int		rc;
 
-	if (the_device)
-		return -ESRCH;
+	mutex_lock(&sb_mutex);
+
+	if (the_device) {
+		rc = -ESRCH;
+		goto Done;
+	}
 
 	CHIP = usb_get_gadget_udc_name();
-	if (!CHIP)
-		return -ENODEV;
+	if (!CHIP) {
+		rc = -ENODEV;
+		goto Done;
+	}
 
 	/* superblock */
 	sb->s_blocksize = PAGE_SIZE;

@@ -2053,13 +2061,17 @@ gadgetfs_fill_super (struct super_block *sb, struct fs_context *fc)
 	 * from binding to a controller.
 	 */
 	the_device = dev;
-	return 0;
+	rc = 0;
+	goto Done;
 
-Enomem:
+ Enomem:
 	kfree(CHIP);
 	CHIP = NULL;
+	rc = -ENOMEM;
 
-	return -ENOMEM;
+ Done:
+	mutex_unlock(&sb_mutex);
+	return rc;
 }
 
 /* "mount -t gadgetfs path /dev/gadget" ends up here */

@@ -2081,6 +2093,7 @@ static int gadgetfs_init_fs_context(struct fs_context *fc)
 static void
 gadgetfs_kill_sb (struct super_block *sb)
 {
+	mutex_lock(&sb_mutex);
 	kill_litter_super (sb);
 	if (the_device) {
 		put_dev (the_device);

@@ -2088,6 +2101,7 @@ gadgetfs_kill_sb (struct super_block *sb)
 	}
 	kfree(CHIP);
 	CHIP = NULL;
+	mutex_unlock(&sb_mutex);
 }
 
 /*----------------------------------------------------------------------*/

2.39.0