fix open/umount race

nameidata_to_filp() drops nd->path or transfers it to opened
file.  In the former case it's a Bad Idea(tm) to do mnt_drop_write()
on nd->path.mnt, since we might race with umount and vfsmount in
question might be gone already.

Fix: don't drop it, then...  IOW, have nameidata_to_filp() grab nd->path
in case it transfers it to file and do path_drop() in callers.  After
they are through with accessing nd->path...

Reported-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

2 files changed, 5 insertions, 3 deletions

diff --git a/fs/namei.c b/fs/namei.c
index f7dbc06857ab..5362af9b7372 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1574,6 +1574,7 @@ static struct file *finish_open(struct nameidata *nd,
          */
         if (will_truncate)
                 mnt_drop_write(nd->path.mnt);
+        path_put(&nd->path);
         return filp;
 
 exit:
@@ -1675,6 +1676,7 @@ static struct file *do_last(struct nameidata *nd, struct path *path,
                 }
                 filp = nameidata_to_filp(nd);
                 mnt_drop_write(nd->path.mnt);
+                path_put(&nd->path);
                 if (!IS_ERR(filp)) {
                         error = ima_file_check(filp, acc_mode);
                         if (error) {
diff --git a/fs/open.c b/fs/open.c
index d74e1983e8dc..4197b9ed023d 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -786,11 +786,11 @@ struct file *nameidata_to_filp(struct nameidata *nd)
         /* Pick up the filp from the open intent */
         filp = nd->intent.open.file;
         /* Has the filesystem initialised the file for us? */
-        if (filp->f_path.dentry == NULL)
+        if (filp->f_path.dentry == NULL) {
+                path_get(&nd->path);
                 filp = __dentry_open(nd->path.dentry, nd->path.mnt, filp,
                                      NULL, cred);
-        else
-                path_put(&nd->path);
+        }
         return filp;
 }