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>

1 files changed, 5 insertions, 0 deletions

diff --git a/drivers/media/dvb/dvb-core/dvbdev.c b/drivers/media/dvb/dvb-core/dvbdev.c
index 65d69665f1fc..6a32680dbb1b 100644
--- a/drivers/media/dvb/dvb-core/dvbdev.c
+++ b/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;