namei: LOOKUP_NO_XDEV: block mountpoint crossing

/* Background. */
The need to contain path operations within a mountpoint has been a
long-standing usecase that userspace has historically implemented
manually with liberal usage of stat(). find, rsync, tar and
many other programs implement these semantics -- but it'd be much
simpler to have a fool-proof way of refusing to open a path if it
crosses a mountpoint.

This is part of a refresh of Al's AT_NO_JUMPS patchset[1] (which was a
variation on David Drysdale's O_BENEATH patchset[2], which in turn was
based on the Capsicum project[3]).

/* Userspace API. */
LOOKUP_NO_XDEV will be exposed to userspace through openat2(2).

/* Semantics. */
Unlike most other LOOKUP flags (most notably LOOKUP_FOLLOW),
LOOKUP_NO_XDEV applies to all components of the path.

With LOOKUP_NO_XDEV, any path component which crosses a mount-point
during path resolution (including "..") will yield an -EXDEV. Absolute
paths, absolute symlinks, and magic-links will only yield an -EXDEV if
the jump involved changing mount-points.

/* Testing. */
LOOKUP_NO_XDEV is tested as part of the openat2(2) selftests.

[1]: https://lore.kernel.org/lkml/20170429220414.GT29622@ZenIV.linux.org.uk/
[2]: https://lore.kernel.org/lkml/1415094884-18349-1-git-send-email-drysdale@google.com/
[3]: https://lore.kernel.org/lkml/1404124096-21445-1-git-send-email-drysdale@google.com/

Cc: Christian Brauner <christian.brauner@ubuntu.com>
Suggested-by: David Drysdale <drysdale@google.com>
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Suggested-by: Andy Lutomirski <luto@kernel.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namei.c b/fs/namei.c
index 7fd801a..a9ca46e 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -838,6 +838,11 @@ static inline void path_to_nameidata(const struct path *path,
 
 static int nd_jump_root(struct nameidata *nd)
 {
+       if (unlikely(nd->flags & LOOKUP_NO_XDEV)) {
+               /* Absolute path arguments to path_init() are allowed. */
+               if (nd->path.mnt != NULL && nd->path.mnt != nd->root.mnt)
+                       return -EXDEV;
+       }
        if (!nd->root.mnt) {
                int error = set_root(nd);
                if (error)
@@ -873,6 +878,12 @@ int nd_jump_link(struct path *path)
        if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS))
                goto err;
 
+       error = -EXDEV;
+       if (unlikely(nd->flags & LOOKUP_NO_XDEV)) {
+               if (nd->path.mnt != path->mnt)
+                       goto err;
+       }
+
        path_put(&nd->path);
        nd->path = *path;
        nd->inode = nd->path.dentry->d_inode;
@@ -1284,10 +1295,14 @@ static int follow_managed(struct path *path, struct nameidata *nd)
                break;
        }
 
-       if (need_mntput && path->mnt == mnt)
-               mntput(path->mnt);
-       if (need_mntput)
-               nd->flags |= LOOKUP_JUMPED;
+       if (need_mntput) {
+               if (path->mnt == mnt)
+                       mntput(path->mnt);
+               if (unlikely(nd->flags & LOOKUP_NO_XDEV))
+                       ret = -EXDEV;
+               else
+                       nd->flags |= LOOKUP_JUMPED;
+       }
        if (ret == -EISDIR || !ret)
                ret = 1;
        if (ret > 0 && unlikely(d_flags_negative(flags)))
@@ -1348,6 +1363,8 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
                mounted = __lookup_mnt(path->mnt, path->dentry);
                if (!mounted)
                        break;
+               if (unlikely(nd->flags & LOOKUP_NO_XDEV))
+                       return false;
                path->mnt = &mounted->mnt;
                path->dentry = mounted->mnt.mnt_root;
                nd->flags |= LOOKUP_JUMPED;
@@ -1394,6 +1411,8 @@ static int follow_dotdot_rcu(struct nameidata *nd)
                                return -ECHILD;
                        if (&mparent->mnt == nd->path.mnt)
                                break;
+                       if (unlikely(nd->flags & LOOKUP_NO_XDEV))
+                               return -ECHILD;
                        /* we know that mountpoint was pinned */
                        nd->path.dentry = mountpoint;
                        nd->path.mnt = &mparent->mnt;
@@ -1408,6 +1427,8 @@ static int follow_dotdot_rcu(struct nameidata *nd)
                        return -ECHILD;
                if (!mounted)
                        break;
+               if (unlikely(nd->flags & LOOKUP_NO_XDEV))
+                       return -ECHILD;
                nd->path.mnt = &mounted->mnt;
                nd->path.dentry = mounted->mnt.mnt_root;
                inode = nd->path.dentry->d_inode;
@@ -1506,6 +1527,8 @@ static int follow_dotdot(struct nameidata *nd)
                }
                if (!follow_up(&nd->path))
                        break;
+               if (unlikely(nd->flags & LOOKUP_NO_XDEV))
+                       return -EXDEV;
        }
        follow_mount(&nd->path);
        nd->inode = nd->path.dentry->d_inode;

diff --git a/include/linux/namei.h b/include/linux/namei.h
index f8acec8..edb4562 100644 (file)
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -42,6 +42,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 /* Scoping flags for lookup. */
 #define LOOKUP_NO_SYMLINKS     0x010000 /* No symlink crossing. */
 #define LOOKUP_NO_MAGICLINKS   0x020000 /* No nd_jump_link() crossing. */
+#define LOOKUP_NO_XDEV         0x040000 /* No mountpoint crossing. */
 
 extern int path_pts(struct path *path);