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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>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Eric Biederman <ebiederm@xmission.com>,
	Andy Lutomirski <luto@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Kees Cook <keescook@chromium.org>, Jann Horn <jannh@google.com>,
	Tycho Andersen <tycho@tycho.ws>,
	David Drysdale <drysdale@google.com>,
	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,
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	linux-s390@vger.kernel.org, linux-sh@vger.kernel.org,
	linux-xtensa@linux-xtensa.org, sparclinux@vger.kernel.org
Subject: [PATCH RESEND v17 05/13] namei: LOOKUP_NO_SYMLINKS: block symlink resolution
Date: Wed, 20 Nov 2019 16:06:23 +1100
Message-ID: <20191120050631.12816-6-cyphar@cyphar.com> (raw)
In-Reply-To: <20191120050631.12816-1-cyphar@cyphar.com>

/* Background. */
Userspace cannot easily resolve a path without resolving symlinks, and
would have to manually resolve each path component with O_PATH and
O_NOFOLLOW. This is clearly inefficient, and can be fairly easy to screw
up (resulting in possible security bugs). Linus has mentioned that Git
has a particular need for this kind of flag[1]. It also resolves a
fairly long-standing perceived deficiency in O_NOFOLLOw -- that it only
blocks the opening of trailing symlinks.

This is part of a refresh of Al's AT_NO_JUMPS patchset[2] (which was a
variation on David Drysdale's O_BENEATH patchset[3], which in turn was
based on the Capsicum project[4]).

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

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

With LOOKUP_NO_SYMLINKS, any symlink path component encountered during
path resolution will yield -ELOOP. If the trailing component is a
symlink (and no other components were symlinks), then O_PATH|O_NOFOLLOW
will not error out and will instead provide a handle to the trailing
symlink -- without resolving it.

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

[1]: https://lore.kernel.org/lkml/CA+55aFyOKM7DW7+0sdDFKdZFXgptb5r1id9=Wvhd8AgSP7qjwQ@mail.gmail.com/
[2]: https://lore.kernel.org/lkml/20170429220414.GT29622@ZenIV.linux.org.uk/
[3]: https://lore.kernel.org/lkml/1415094884-18349-1-git-send-email-drysdale@google.com/
[4]: https://lore.kernel.org/lkml/1404124096-21445-1-git-send-email-drysdale@google.com/

Cc: Christian Brauner <christian.brauner@ubuntu.com>
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
 fs/namei.c            | 3 +++
 include/linux/namei.h | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index 74574a69a614..303731935eb2 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1052,6 +1052,9 @@ const char *get_link(struct nameidata *nd)
 	int error;
 	const char *res;
+	if (unlikely(nd->flags & LOOKUP_NO_SYMLINKS))
+		return ERR_PTR(-ELOOP);
 	if (!(nd->flags & LOOKUP_RCU)) {
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 758e9b47db6f..0d86e75c04a7 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
 #define LOOKUP_ROOT		0x2000
 #define LOOKUP_ROOT_GRABBED	0x0008
+/* Scoping flags for lookup. */
+#define LOOKUP_NO_SYMLINKS	0x010000 /* No symlink crossing. */
 extern int path_pts(struct path *path);
 extern int user_path_at_empty(int, const char __user *, unsigned, struct path *, int *empty);

  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 ` Aleksa Sarai [this message]
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 ` [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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