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From: Aleksa Sarai <cyphar@cyphar.com>
To: Al Viro <viro@zeniv.linux.org.uk>,
	Jeff Layton <jlayton@kernel.org>,
	"J. Bruce Fields" <bfields@fieldses.org>,
	Arnd Bergmann <arnd@arndb.de>,
	David Howells <dhowells@redhat.com>,
	Shuah Khan <shuah@kernel.org>,
	Shuah Khan <skhan@linuxfoundation.org>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>, Andrii Nakryiko <andriin@fb.com>,
	Jonathan Corbet <corbet@lwn.net>
Cc: Aleksa Sarai <cyphar@cyphar.com>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	David Drysdale <drysdale@google.com>,
	Andy Lutomirski <luto@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
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	Andrew Morton <akpm@linux-foundation.org>,
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	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Christian Brauner <christian@brauner.io>,
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Subject: [PATCH RESEND v17 07/13] namei: LOOKUP_NO_XDEV: block mountpoint crossing
Date: Wed, 20 Nov 2019 16:06:25 +1100
Message-ID: <20191120050631.12816-8-cyphar@cyphar.com> (raw)
In-Reply-To: <20191120050631.12816-1-cyphar@cyphar.com>

/* 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>
---
 fs/namei.c            | 31 +++++++++++++++++++++++++++----
 include/linux/namei.h |  1 +
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 415a897729c8..321c8ad5d6b3 100644
--- 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;
@@ -1280,12 +1291,16 @@ static int follow_managed(struct path *path, struct nameidata *nd)
 		break;
 	}
 
-	if (need_mntput && path->mnt == mnt)
-		mntput(path->mnt);
+	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 (need_mntput)
-		nd->flags |= LOOKUP_JUMPED;
 	if (unlikely(ret < 0))
 		path_put_conditional(path, nd);
 	return ret;
@@ -1342,6 +1357,8 @@ 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;
@@ -1388,6 +1405,8 @@ 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;
@@ -1402,6 +1421,8 @@ 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;
@@ -1500,6 +1521,8 @@ 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 1573b8493d98..25ee88c4acb1 100644
--- 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);
 
-- 
2.24.0


  parent reply index

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-20  5:06 [PATCH RESEND v17 00/13] open: introduce openat2(2) syscall Aleksa Sarai
2019-11-20  5:06 ` [PATCH RESEND v17 01/13] namei: only return -ECHILD from follow_dotdot_rcu() Aleksa Sarai
2019-11-20  5:06 ` [PATCH RESEND v17 02/13] nsfs: clean-up ns_get_path() signature to return int Aleksa Sarai
2019-11-20  5:06 ` [PATCH RESEND v17 03/13] namei: allow nd_jump_link() to produce errors Aleksa Sarai
2019-11-20  5:06 ` [PATCH RESEND v17 04/13] namei: allow set_root() " Aleksa Sarai
2019-11-20  5:06 ` [PATCH RESEND v17 05/13] namei: LOOKUP_NO_SYMLINKS: block symlink resolution Aleksa Sarai
2019-11-20  5:06 ` [PATCH RESEND v17 06/13] namei: LOOKUP_NO_MAGICLINKS: block magic-link resolution Aleksa Sarai
2019-11-20  5:06 ` Aleksa Sarai [this message]
2019-11-20  5:06 ` [PATCH RESEND v17 08/13] namei: LOOKUP_BENEATH: O_BENEATH-like scoped resolution Aleksa Sarai
2019-11-20  5:06 ` [PATCH RESEND v17 09/13] namei: LOOKUP_IN_ROOT: chroot-like " Aleksa Sarai
2019-11-20  5:06 ` [PATCH RESEND v17 10/13] namei: LOOKUP_{IN_ROOT,BENEATH}: permit limited ".." resolution Aleksa Sarai
2019-11-20  5:06 ` [PATCH RESEND v17 11/13] open: introduce openat2(2) syscall Aleksa Sarai
2019-11-20  5:06 ` [PATCH RESEND v17 12/13] selftests: add openat2(2) selftests Aleksa Sarai
2019-11-20  5:06 ` [PATCH RESEND v17 13/13] Documentation: path-lookup: include new LOOKUP flags Aleksa Sarai

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