Commit 72ba2929 authored by Aleksa Sarai's avatar Aleksa Sarai Committed by Al Viro
Browse files

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: default avatarDavid Drysdale <drysdale@google.com>
Suggested-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
Suggested-by: default avatarAndy Lutomirski <luto@kernel.org>
Suggested-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarAleksa Sarai <cyphar@cyphar.com>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 4b99d499

fs/namei.c

+27 −4
Original line number Diff line number Diff line
@@ -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)

	if (need_mntput) {

		if (path->mnt == mnt)

			mntput(path->mnt);

	if (need_mntput)

		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;

include/linux/namei.h

+1 −0
Original line number Diff line number Diff line
@@ -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);

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