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.

Signed-off-by: Alan Stern <[e-mail]>
Reported-and-tested-by: [e-mail]
Reported-and-tested-by: Gerald Lee <[e-mail]>
Link: https://lore.kernel.org/linux-usb/CAO3qeMVzXDP-JU6v1u5Ags6Q-bb35kg3=[e-mail]/
Fixes: e5d82a7360d1 ("vfs: Convert gadgetfs to use the new mount API")
CC: <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/gadget/legacy/inode.c

···
  */
 
 static const char *CHIP;
+static DEFINE_MUTEX(sb_mutex);		/* Serialize superblock operations */
 
 /*----------------------------------------------------------------------*/
 
···
 {
 	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;
···
 	 * 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 */
···
 static void
 gadgetfs_kill_sb (struct super_block *sb)
 {
+	mutex_lock(&sb_mutex);
 	kill_litter_super (sb);
 	if (the_device) {
 		put_dev (the_device);
···
 	}
 	kfree(CHIP);
 	CHIP = NULL;
+	mutex_unlock(&sb_mutex);
 }
 
 /*----------------------------------------------------------------------*/