Merge branch 'work.sane_pwd' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs

Pull misc vfs updates from Al Viro:
 "Making sure that something like a referral point won't end up as pwd
  or root.

  The main part is the last commit (fixing mntns_install()); that one
  fixes a hard-to-hit race. The fchdir() commit is making fchdir(2) a
  bit more robust - it should be impossible to get opened files (even
  O_PATH ones) for referral points in the first place, so the existing
  checks are OK, but checking the same thing as in chdir(2) is just as
  cheap.

  The path_init() commit removes a redundant check that shouldn't have
  been there in the first place"

* 'work.sane_pwd' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs:
  make sure that mntns_install() doesn't end up with referral for root
  path_init(): don't bother with checking MAY_EXEC for LOOKUP_ROOT
  make sure that fchdir() won't accept referral points, etc.

4 files changed, 55 insertions, 21 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 7286f87ce863..6571a5f5112e 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2142,7 +2142,6 @@ OK:
 
 static const char *path_init(struct nameidata *nd, unsigned flags)
 {
-        int retval = 0;
         const char *s = nd->name->name;
 
         if (!*s)
@@ -2154,13 +2153,8 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
         if (flags & LOOKUP_ROOT) {
                 struct dentry *root = nd->root.dentry;
                 struct inode *inode = root->d_inode;
-                if (*s) {
-                        if (!d_can_lookup(root))
-                                return ERR_PTR(-ENOTDIR);
-                        retval = inode_permission(inode, MAY_EXEC);
-                        if (retval)
-                                return ERR_PTR(retval);
-                }
+                if (*s && unlikely(!d_can_lookup(root)))
+                        return ERR_PTR(-ENOTDIR);
                 nd->path = nd->root;
                 nd->inode = inode;
                 if (flags & LOOKUP_RCU) {
@@ -2258,6 +2252,35 @@ static inline int lookup_last(struct nameidata *nd)
         return walk_component(nd, 0);
 }
 
+static int handle_lookup_down(struct nameidata *nd)
+{
+        struct path path = nd->path;
+        struct inode *inode = nd->inode;
+        unsigned seq = nd->seq;
+        int err;
+
+        if (nd->flags & LOOKUP_RCU) {
+                /*
+                 * don't bother with unlazy_walk on failure - we are
+                 * at the very beginning of walk, so we lose nothing
+                 * if we simply redo everything in non-RCU mode
+                 */
+                if (unlikely(!__follow_mount_rcu(nd, &path, &inode, &seq)))
+                        return -ECHILD;
+        } else {
+                dget(path.dentry);
+                err = follow_managed(&path, nd);
+                if (unlikely(err < 0))
+                        return err;
+                inode = d_backing_inode(path.dentry);
+                seq = 0;
+        }
+        path_to_nameidata(&path, nd);
+        nd->inode = inode;
+        nd->seq = seq;
+        return 0;
+}
+
 /* Returns 0 and nd will be valid on success; Retuns error, otherwise. */
 static int path_lookupat(struct nameidata *nd, unsigned flags, struct path *path)
 {
@@ -2266,6 +2289,15 @@ static int path_lookupat(struct nameidata *nd, unsigned flags, struct path *path
 
         if (IS_ERR(s))
                 return PTR_ERR(s);
+
+        if (unlikely(flags & LOOKUP_DOWN)) {
+                err = handle_lookup_down(nd);
+                if (unlikely(err < 0)) {
+                        terminate_walk(nd);
+                        return err;
+                }
+        }
+
         while (!(err = link_path_walk(s, nd))
                 && ((err = lookup_last(nd)) > 0)) {
                 s = trailing_symlink(nd);
diff --git a/fs/namespace.c b/fs/namespace.c
index b3b115bd4e1e..8bd3e4d448b9 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3462,8 +3462,9 @@ static void mntns_put(struct ns_common *ns)
 static int mntns_install(struct nsproxy *nsproxy, struct ns_common *ns)
 {
         struct fs_struct *fs = current->fs;
-        struct mnt_namespace *mnt_ns = to_mnt_ns(ns);
+        struct mnt_namespace *mnt_ns = to_mnt_ns(ns), *old_mnt_ns;
         struct path root;
+        int err;
 
         if (!ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN) ||
             !ns_capable(current_user_ns(), CAP_SYS_CHROOT) ||
@@ -3474,15 +3475,18 @@ static int mntns_install(struct nsproxy *nsproxy, struct ns_common *ns)
                 return -EINVAL;
 
         get_mnt_ns(mnt_ns);
-        put_mnt_ns(nsproxy->mnt_ns);
+        old_mnt_ns = nsproxy->mnt_ns;
         nsproxy->mnt_ns = mnt_ns;
 
         /* Find the root */
-        root.mnt    = &mnt_ns->root->mnt;
-        root.dentry = mnt_ns->root->mnt.mnt_root;
-        path_get(&root);
-        while(d_mountpoint(root.dentry) && follow_down_one(&root))
-                ;
+        err = vfs_path_lookup(mnt_ns->root->mnt.mnt_root, &mnt_ns->root->mnt,
+                                "/", LOOKUP_DOWN, &root);
+        if (err) {
+                /* revert to old namespace */
+                nsproxy->mnt_ns = old_mnt_ns;
+                put_mnt_ns(mnt_ns);
+                return err;
+        }
 
         /* Update the pwd and root */
         set_fs_pwd(fs, &root);
diff --git a/fs/open.c b/fs/open.c
index 6d2d2b33ac54..cd0c5be8d012 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -460,20 +460,17 @@ out:
 SYSCALL_DEFINE1(fchdir, unsigned int, fd)
 {
         struct fd f = fdget_raw(fd);
-        struct inode *inode;
-        int error = -EBADF;
+        int error;
 
         error = -EBADF;
         if (!f.file)
                 goto out;
 
-        inode = file_inode(f.file);
-
         error = -ENOTDIR;
-        if (!S_ISDIR(inode->i_mode))
+        if (!d_can_lookup(f.file->f_path.dentry))
                 goto out_putf;
 
-        error = inode_permission(inode, MAY_EXEC | MAY_CHDIR);
+        error = inode_permission(file_inode(f.file), MAY_EXEC | MAY_CHDIR);
         if (!error)
                 set_fs_pwd(current->fs, &f.file->f_path);
 out_putf:
diff --git a/include/linux/namei.h b/include/linux/namei.h
index f29abda31e6d..8b4794e83196 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -44,6 +44,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 #define LOOKUP_JUMPED           0x1000
 #define LOOKUP_ROOT             0x2000
 #define LOOKUP_EMPTY            0x4000
+#define LOOKUP_DOWN             0x8000
 
 extern int path_pts(struct path *path);