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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>,
	Eric Biederman <ebiederm@xmission.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Kees Cook <keescook@chromium.org>, Jann Horn <jannh@google.com>,
	Tycho Andersen <tycho@tycho.ws>, Chanho Min <chanho.min@lge.com>,
	Oleg Nesterov <oleg@redhat.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Christian Brauner <christian@brauner.io>,
	Aleksa Sarai <asarai@suse.de>,
	dev@opencontainers.org, containers@lists.linux-foundation.org,
	bpf@vger.kernel.org, netdev@vger.kernel.org,
	linux-alpha@vger.kernel.org, linux-api@vger.kernel.org,
	libc-alpha@sourceware.org, linux-arch@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-ia64@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org,
	linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-s390@vger.kernel.org, linux-sh@vger.kernel.org,
	linux-xtensa@linux-xtensa.org, sparclinux@vger.kernel.org
Subject: [PATCH RESEND v17 06/13] namei: LOOKUP_NO_MAGICLINKS: block magic-link resolution
Date: Wed, 20 Nov 2019 16:06:24 +1100
Message-ID: <20191120050631.12816-7-cyphar@cyphar.com> (raw)
In-Reply-To: <20191120050631.12816-1-cyphar@cyphar.com>

/* Background. */
There has always been a special class of symlink-like objects in procfs
(and a few other pseudo-filesystems) which allow for non-lexical
resolution of paths using nd_jump_link(). These "magic-links" do not
follow traditional mount namespace boundaries, and have been used
consistently in container escape attacks because they can be used to
trick unsuspecting privileged processes into resolving unexpected paths.

It is also non-trivial for userspace to unambiguously avoid resolving
magic-links, because they do not have a reliable indication that they
are a magic-link (in order to verify them you'd have to manually open
the path given by readlink(2) and then verify that the two file
descriptors reference the same underlying file, which is plagued with
possible race conditions or supplementary attack scenarios).

It would therefore be very helpful for userspace to be able to avoid
these symlinks easily, thus hopefully removing a tool from attackers'

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_MAGICLINKS will be exposed to userspace through openat2(2).

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

With LOOKUP_NO_MAGICLINKS, any magic-link path component encountered
during path resolution will yield -ELOOP. The handling of ~LOOKUP_FOLLOW
for a trailing magic-link is identical to LOOKUP_NO_SYMLINKS.


/* Testing. */
LOOKUP_NO_MAGICLINKS 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            | 10 +++++++++-
 include/linux/namei.h |  1 +
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index 303731935eb2..415a897729c8 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -867,13 +867,21 @@ static int nd_jump_root(struct nameidata *nd)
 int nd_jump_link(struct path *path)
+	int error = -ELOOP;
 	struct nameidata *nd = current->nameidata;
-	path_put(&nd->path);
+	if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS))
+		goto err;
+	path_put(&nd->path);
 	nd->path = *path;
 	nd->inode = nd->path.dentry->d_inode;
 	nd->flags |= LOOKUP_JUMPED;
 	return 0;
+	path_put(path);
+	return error;
 static inline void put_link(struct nameidata *nd)
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 0d86e75c04a7..1573b8493d98 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
 /* Scoping flags for lookup. */
 #define LOOKUP_NO_SYMLINKS	0x010000 /* No symlink crossing. */
+#define LOOKUP_NO_MAGICLINKS	0x020000 /* No nd_jump_link() crossing. */
 extern int path_pts(struct path *path);

  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 ` Aleksa Sarai [this message]
2019-11-20  5:06 ` [PATCH RESEND v17 07/13] namei: LOOKUP_NO_XDEV: block mountpoint crossing Aleksa Sarai
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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