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authorEric W. Biederman <ebiederm@xmission.com>2015-08-15 20:27:13 -0500
committerAl Viro <viro@zeniv.linux.org.uk>2015-08-21 03:20:10 -0400
commit397d425dc26da728396e66d392d5dcb8dac30c37 (patch)
tree14a2ec6d000fa7b5b45d91bb62d2f89fc11d28b5
parenta03e283bf5c3d4851b4998122196ce9f849e6dfb (diff)
vfs: Test for and handle paths that are unreachable from their mnt_root
In rare cases a directory can be renamed out from under a bind mount. In those cases without special handling it becomes possible to walk up the directory tree to the root dentry of the filesystem and down from the root dentry to every other file or directory on the filesystem. Like division by zero .. from an unconnected path can not be given a useful semantic as there is no predicting at which path component the code will realize it is unconnected. We certainly can not match the current behavior as the current behavior is a security hole. Therefore when encounting .. when following an unconnected path return -ENOENT. - Add a function path_connected to verify path->dentry is reachable from path->mnt.mnt_root. AKA to validate that rename did not do something nasty to the bind mount. To avoid races path_connected must be called after following a path component to it's next path component. Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
-rw-r--r--fs/namei.c27
1 files changed, 25 insertions, 2 deletions
diff --git a/fs/namei.c b/fs/namei.c
index 1c2105ed20c5..29b927938b8c 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -560,6 +560,24 @@ static int __nd_alloc_stack(struct nameidata *nd)
return 0;
}
+/**
+ * path_connected - Verify that a path->dentry is below path->mnt.mnt_root
+ * @path: nameidate to verify
+ *
+ * Rename can sometimes move a file or directory outside of a bind
+ * mount, path_connected allows those cases to be detected.
+ */
+static bool path_connected(const struct path *path)
+{
+ struct vfsmount *mnt = path->mnt;
+
+ /* Only bind mounts can have disconnected paths */
+ if (mnt->mnt_root == mnt->mnt_sb->s_root)
+ return true;
+
+ return is_subdir(path->dentry, mnt->mnt_root);
+}
+
static inline int nd_alloc_stack(struct nameidata *nd)
{
if (likely(nd->depth != EMBEDDED_LEVELS))
@@ -1296,6 +1314,8 @@ static int follow_dotdot_rcu(struct nameidata *nd)
return -ECHILD;
nd->path.dentry = parent;
nd->seq = seq;
+ if (unlikely(!path_connected(&nd->path)))
+ return -ENOENT;
break;
} else {
struct mount *mnt = real_mount(nd->path.mnt);
@@ -1396,7 +1416,7 @@ static void follow_mount(struct path *path)
}
}
-static void follow_dotdot(struct nameidata *nd)
+static int follow_dotdot(struct nameidata *nd)
{
if (!nd->root.mnt)
set_root(nd);
@@ -1412,6 +1432,8 @@ static void follow_dotdot(struct nameidata *nd)
/* rare case of legitimate dget_parent()... */
nd->path.dentry = dget_parent(nd->path.dentry);
dput(old);
+ if (unlikely(!path_connected(&nd->path)))
+ return -ENOENT;
break;
}
if (!follow_up(&nd->path))
@@ -1419,6 +1441,7 @@ static void follow_dotdot(struct nameidata *nd)
}
follow_mount(&nd->path);
nd->inode = nd->path.dentry->d_inode;
+ return 0;
}
/*
@@ -1634,7 +1657,7 @@ static inline int handle_dots(struct nameidata *nd, int type)
if (nd->flags & LOOKUP_RCU) {
return follow_dotdot_rcu(nd);
} else
- follow_dotdot(nd);
+ return follow_dotdot(nd);
}
return 0;
}