[PATCH 3/3] vfs: check path permissions on target of LAST_BIND symlinks

Jeff Layton
2009-11-23T17:41:50Z

Because LAST_BIND symlinks aren't subject to the normal path walking
routines, they sidestep all of the permission checking that occurs while
resolving a path. Fix this by adding a routine to walk back up the
directory tree and check MAY_EXEC permission on the entire path back up to
the root.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
---
fs/namei.c | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
1 files changed, 56 insertions(+), 1 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 738b257..d4c1279 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -504,6 +504,57 @@ ok:
}

/*
+ * We have a cached struct path from a LAST_BIND symlink. The path is valid
+ * but it's possible that one of the components leading to this point
+ * might not be accessible. This check walks back up the tree to the root
+ * of the namespace and checks whether each component is accessible. The
+ * supplied path is put on error.
+ */
+static int
+check_path_accessible(struct path *path)
+{
+ int err;
+ struct dentry *parent, *tdentry = dget(path->dentry);
+ struct vfsmount *vfsmnt = mntget(path->mnt);
+ struct path root = current->fs->root;
+
+ path_get(&root);
+ for(;;) {
+ parent = dget_parent(tdentry);
+ err = inode_permission(parent->d_inode, MAY_EXEC);
+ if (err < 0) {
+ dput(parent);
+ break;
+ }
+ dput(tdentry);
+ tdentry = parent;
+
+ /* keep going if not to root of mnt */
+ if (!IS_ROOT(tdentry))
+ continue;
+
+ /* are we at global root or root of namespace? */
+ if ((tdentry == root.dentry && vfsmnt == root.mnt) ||
+ vfsmnt->mnt_parent == vfsmnt)
+ break;
+
+ /* cross to parent mount and keep walking */
+ mntput(vfsmnt);
+ vfsmnt = mntget(vfsmnt->mnt_parent);
+ tdentry = dget(vfsmnt->mnt_mountpoint);
+ dput(parent);
+ }
+ dput(tdentry);
+ mntput(vfsmnt);
+ path_put(&root);
+
+ if (err)
+ path_put(path);
+
+ return err;
+}
+
+/*
* This is called when everything else fails, and we actually have
* to go to the low-level filesystem to find out what we should do..
*
@@ -679,8 +730,12 @@ static __always_inline int __do_follow_link(struct path *path, struct nameidata
error = 0;
if (s)
error = __vfs_follow_link(nd, s);
- else if (nd->last_type == LAST_BIND)
+ else if (nd->last_type == LAST_BIND) {
error = force_reval_path(&nd->path, nd);
+ if (!error)
+ error = check_path_accessible(&nd->path);
+ }
+
if (dentry->d_inode->i_op->put_link)
dentry->d_inode->i_op->put_link(dentry, nd, cookie);
}
--
1.5.5.6