ovl: avoid deadlock on directory ioctl

The function ovl_dir_real_file() currently uses the inode lock to serialize
writes to the od->upperfile field.

However, this function will get called by ovl_ioctl_set_flags(), which
utilizes the inode lock too.  In this case ovl_dir_real_file() will try to
claim a lock that is owned by a function in its call stack, which won't get
released before ovl_dir_real_file() returns.

Fix by replacing the open coded compare and exchange by an explicit atomic
op.

Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR ioctls for directories")
Cc: stable@vger.kernel.org # v5.10
Reported-by: Icenowy Zheng <icenowy@aosc.io>
Tested-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 01620ebae1bd4e0f1a03a9bd05e8f28c9c5afb80..60d751f28fea5cacefecbb5d93575db2f003a735 100644 (file)
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -865,7 +865,7 @@ struct file *ovl_dir_real_file(const struct file *file, bool want_upper)
 
        struct ovl_dir_file *od = file->private_data;
        struct dentry *dentry = file->f_path.dentry;
-       struct file *realfile = od->realfile;
+       struct file *old, *realfile = od->realfile;
 
        if (!OVL_TYPE_UPPER(ovl_path_type(dentry)))
                return want_upper ? NULL : realfile;
@@ -874,29 +874,20 @@ struct file *ovl_dir_real_file(const struct file *file, bool want_upper)
         * Need to check if we started out being a lower dir, but got copied up
         */
        if (!od->is_upper) {
-               struct inode *inode = file_inode(file);
-
                realfile = READ_ONCE(od->upperfile);
                if (!realfile) {
                        struct path upperpath;
 
                        ovl_path_upper(dentry, &upperpath);
                        realfile = ovl_dir_open_realfile(file, &upperpath);
+                       if (IS_ERR(realfile))
+                               return realfile;
 
-                       inode_lock(inode);
-                       if (!od->upperfile) {
-                               if (IS_ERR(realfile)) {
-                                       inode_unlock(inode);
-                                       return realfile;
-                               }
-                               smp_store_release(&od->upperfile, realfile);
-                       } else {
-                               /* somebody has beaten us to it */
-                               if (!IS_ERR(realfile))
-                                       fput(realfile);
-                               realfile = od->upperfile;
+                       old = cmpxchg_release(&od->upperfile, NULL, realfile);
+                       if (old) {
+                               fput(realfile);
+                               realfile = old;
                        }
-                       inode_unlock(inode);
                }
        }