usb: f_fs: Fix use-after-free for epfile

Consider a case where ffs_func_eps_disable is called from
ffs_func_disable as part of composition switch and at the
same time ffs_epfile_release get called from userspace.
ffs_epfile_release will free up the read buffer and call
ffs_data_closed which in turn destroys ffs->epfiles and
mark it as NULL. While this was happening the driver has
already initialized the local epfile in ffs_func_eps_disable
which is now freed and waiting to acquire the spinlock. Once
spinlock is acquired the driver proceeds with the stale value
of epfile and tries to free the already freed read buffer
causing use-after-free.

Following is the illustration of the race:

CPU1 CPU2

ffs_func_eps_disable
epfiles (local copy)
ffs_epfile_release
ffs_data_closed
if (last file closed)
ffs_data_reset
ffs_data_clear
ffs_epfiles_destroy
spin_lock
dereference epfiles

Fix this races by taking epfiles local copy & assigning it under
spinlock and if epfiles(local) is null then update it in ffs->epfiles
then finally destroy it.
Extending the scope further from the race, protecting the ep related
structures, and concurrent accesses.

Fixes: a9e6f83c2df1 ("usb: gadget: f_fs: stop sleeping in ffs_func_eps_disable")
Co-developed-by: Udipto Goswami <[e-mail]>
Reviewed-by: John Keeping <[e-mail]>
Signed-off-by: Pratham Pratap <[e-mail]>
Signed-off-by: Udipto Goswami <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/gadget/function/f_fs.c

···
 
 static void ffs_data_closed(struct ffs_data *ffs)
 {
+	struct ffs_epfile *epfiles;
+	unsigned long flags;
+
 	ENTER();
 
 	if (atomic_dec_and_test(&ffs->opened)) {
 		if (ffs->no_disconnect) {
 			ffs->state = FFS_DEACTIVATED;
-			if (ffs->epfiles) {
-				ffs_epfiles_destroy(ffs->epfiles,
-						   ffs->eps_count);
-				ffs->epfiles = NULL;
-			}
+			spin_lock_irqsave(&ffs->eps_lock, flags);
+			epfiles = ffs->epfiles;
+			ffs->epfiles = NULL;
+			spin_unlock_irqrestore(&ffs->eps_lock,
+							flags);
+
+			if (epfiles)
+				ffs_epfiles_destroy(epfiles,
+						 ffs->eps_count);
+
 			if (ffs->setup_state == FFS_SETUP_PENDING)
 				__ffs_ep0_stall(ffs);
 		} else {
···
 
 static void ffs_data_clear(struct ffs_data *ffs)
 {
+	struct ffs_epfile *epfiles;
+	unsigned long flags;
+
 	ENTER();
 
 	ffs_closed(ffs);
 
 	BUG_ON(ffs->gadget);
 
-	if (ffs->epfiles) {
-		ffs_epfiles_destroy(ffs->epfiles, ffs->eps_count);
+	spin_lock_irqsave(&ffs->eps_lock, flags);
+	epfiles = ffs->epfiles;
+	ffs->epfiles = NULL;
+	spin_unlock_irqrestore(&ffs->eps_lock, flags);
+
+	/*
+	 * potential race possible between ffs_func_eps_disable
+	 * & ffs_epfile_release therefore maintaining a local
+	 * copy of epfile will save us from use-after-free.
+	 */
+	if (epfiles) {
+		ffs_epfiles_destroy(epfiles, ffs->eps_count);
 		ffs->epfiles = NULL;
 	}
 
···
 
 static void ffs_func_eps_disable(struct ffs_function *func)
 {
-	struct ffs_ep *ep         = func->eps;
-	struct ffs_epfile *epfile = func->ffs->epfiles;
-	unsigned count            = func->ffs->eps_count;
+	struct ffs_ep *ep;
+	struct ffs_epfile *epfile;
+	unsigned short count;
 	unsigned long flags;
 
 	spin_lock_irqsave(&func->ffs->eps_lock, flags);
+	count = func->ffs->eps_count;
+	epfile = func->ffs->epfiles;
+	ep = func->eps;
 	while (count--) {
 		/* pending requests get nuked */
 		if (ep->ep)
···
 
 static int ffs_func_eps_enable(struct ffs_function *func)
 {
-	struct ffs_data *ffs      = func->ffs;
-	struct ffs_ep *ep         = func->eps;
-	struct ffs_epfile *epfile = ffs->epfiles;
-	unsigned count            = ffs->eps_count;
+	struct ffs_data *ffs;
+	struct ffs_ep *ep;
+	struct ffs_epfile *epfile;
+	unsigned short count;
 	unsigned long flags;
 	int ret = 0;
 
 	spin_lock_irqsave(&func->ffs->eps_lock, flags);
+	ffs = func->ffs;
+	ep = func->eps;
+	epfile = ffs->epfiles;
+	count = ffs->eps_count;
 	while(count--) {
 		ep->ep->driver_data = ep;