Check fops_get() return value

Several subsystem open handlers dereference the fops_get() return value
without checking it for nullness.  This opens a race condition between the
open handler and module unloading.

A module can be marked as being unloaded (MODULE_STATE_GOING) before its
exit function is called and gets the chance to unregister the driver.
During that window open handlers can still be called, and fops_get() will
fail in try_module_get() and return a NULL pointer.

This change checks the fops_get() return value and returns -ENODEV if NULL.
Reported-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Acked-by: Takashi Iwai <tiwai@suse.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Dave Airlie <airlied@linux.ie>
Acked-by: Mauro Carvalho Chehab <mchehab@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/gpu/drm/drm_fops.c

@@ -183,6 +183,10 @@ int drm_stub_open(struct inode *inode, struct file *filp)
 
 	old_fops = filp->f_op;
 	filp->f_op = fops_get(&dev->driver->fops);
+	if (filp->f_op == NULL) {
+		filp->f_op = old_fops;
+		goto out;
+	}
 	if (filp->f_op->open && (err = filp->f_op->open(inode, filp))) {
 		fops_put(filp->f_op);
 		filp->f_op = fops_get(old_fops);

drivers/media/dvb/dvb-core/dvbdev.c

@@ -79,6 +79,10 @@ static int dvb_device_open(struct inode *inode, struct file *file)
 		file->private_data = dvbdev;
 		old_fops = file->f_op;
 		file->f_op = fops_get(dvbdev->fops);
+		if (file->f_op == NULL) {
+			file->f_op = old_fops;
+			goto fail;
+		}
 		if(file->f_op->open)
 			err = file->f_op->open(inode,file);
 		if (err) {
@@ -90,6 +94,7 @@ static int dvb_device_open(struct inode *inode, struct file *file)
 		unlock_kernel();
 		return err;
 	}
+fail:
 	up_read(&minor_rwsem);
 	unlock_kernel();
 	return -ENODEV;

sound/core/sound.c

@@ -152,6 +152,10 @@ static int __snd_open(struct inode *inode, struct file *file)
 	}
 	old_fops = file->f_op;
 	file->f_op = fops_get(mptr->f_ops);
+	if (file->f_op == NULL) {
+		file->f_op = old_fops;
+		return -ENODEV;
+	}
 	if (file->f_op->open)
 		err = file->f_op->open(inode, file);
 	if (err) {