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, 4 insertions, 0 deletions

diff --git a/sound/core/sound.c b/sound/core/sound.c
index 44a69bb8d4f0..7872a02f6ca9 100644
--- a/sound/core/sound.c
+++ b/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) {