[PATCH 5.10 055/120] ovl: avoid deadlock on directory ioctl

Greg Kroah-Hartman
2021-02-08T17:09:16Z

From: Miklos Szeredi <mszeredi@redhat.com>

commit b854cc659dcb80f172cb35dbedc15d39d49c383f upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/overlayfs/readdir.c | 23 +++++++----------------
1 file changed, 7 insertions(+), 16 deletions(-)

--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -865,7 +865,7 @@ struct file *ovl_dir_real_file(const str

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 str
* 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);
}
}