From: Laurent Pinchart <laurent.pinchart@skynet.be>

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>
Cc: Mauro Carvalho Chehab <mchehab@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 drivers/gpu/drm/drm_fops.c          |    4 ++++
 drivers/media/dvb/dvb-core/dvbdev.c |    5 +++++
 sound/core/sound.c                  |    4 ++++
 3 files changed, 13 insertions(+)

diff -puN drivers/gpu/drm/drm_fops.c~check-fops_get-return-value drivers/gpu/drm/drm_fops.c
--- a/drivers/gpu/drm/drm_fops.c~check-fops_get-return-value
+++ a/drivers/gpu/drm/drm_fops.c
@@ -186,6 +186,10 @@ int drm_stub_open(struct inode *inode, s
 
 	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);
diff -puN drivers/media/dvb/dvb-core/dvbdev.c~check-fops_get-return-value drivers/media/dvb/dvb-core/dvbdev.c
--- a/drivers/media/dvb/dvb-core/dvbdev.c~check-fops_get-return-value
+++ a/drivers/media/dvb/dvb-core/dvbdev.c
@@ -85,6 +85,10 @@ static int dvb_device_open(struct inode 
 		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) {
@@ -95,6 +99,7 @@ static int dvb_device_open(struct inode 
 		unlock_kernel();
 		return err;
 	}
+fail:
 	unlock_kernel();
 	return -ENODEV;
 }
diff -puN sound/core/sound.c~check-fops_get-return-value sound/core/sound.c
--- a/sound/core/sound.c~check-fops_get-return-value
+++ a/sound/core/sound.c
@@ -152,6 +152,10 @@ static int __snd_open(struct inode *inod
 	}
 	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) {
_