[PATCH v2 55/57][B] debugfs: full_proxy_open(): free proxy on ->open() failure

[Seth Forshee]

From: Nicolai Stange <nicstange at gmail.com>

BugLink: https://bugs.launchpad.net/bugs/1884159

Debugfs' full_proxy_open(), the ->open() installed at all inodes created
through debugfs_create_file(),
- grabs a reference to the original struct file_operations instance passed
  to debugfs_create_file(),
- dynamically allocates a proxy struct file_operations instance wrapping
  the original
- and installs this at the file's ->f_op.

Afterwards, it calls the original ->open() and passes its return value back
to the VFS layer.

Now, if that return value indicates failure, the VFS layer won't ever call
->release() and thus, neither the reference to the original file_operations
nor the memory for the proxy file_operations will get released, i.e. both
are leaked.

Upon failure of the original fops' ->open(), undo the proxy installation.
That is:
- Set the struct file ->f_op to what it had been when full_proxy_open()
  was entered.
- Drop the reference to the original file_operations.
- Free the memory holding the proxy file_operations.

Fixes: 49d200deaa68 ("debugfs: prevent access to removed files' private
                      data")
Signed-off-by: Nicolai Stange <nicstange at gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
(cherry picked from commit b10e3e90485e32e4cea9e35d2295ee7bffaeff73)
Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
---
 fs/debugfs/file.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 9c1c9a01b7e5..d1ec80331414 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -262,8 +262,10 @@ static int full_proxy_open(struct inode *inode, struct file *filp)
 
 	if (real_fops->open) {
 		r = real_fops->open(inode, filp);
-
-		if (filp->f_op != proxy_fops) {
+		if (r) {
+			replace_fops(filp, d_inode(dentry)->i_fop);
+			goto free_proxy;
+		} else if (filp->f_op != proxy_fops) {
 			/* No protection against file removal anymore. */
 			WARN(1, "debugfs file owner replaced proxy fops: %pd",
 				dentry);
-- 
2.27.0