linux-kernel - [PATCH v18 05/13] namei: LOOKUP_NO_SYMLINKS: block symlink resolution

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Message-Id: <20191206141338.23338-6-cyphar@cyphar.com>
Date:   Sat,  7 Dec 2019 01:13:30 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        Jonathan Corbet <corbet@....net>
Cc:     Aleksa Sarai <cyphar@...har.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Tycho Andersen <tycho@...ho.ws>,
        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Aleksa Sarai <asarai@...e.de>, dev@...ncontainers.org,
        containers@...ts.linux-foundation.org, bpf@...r.kernel.org,
        netdev@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-api@...r.kernel.org, libc-alpha@...rceware.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
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        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        sparclinux@...r.kernel.org
Subject: [PATCH v18 05/13] namei: LOOKUP_NO_SYMLINKS: block symlink resolution

/* 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@...ntu.com>
Suggested-by: Al Viro <viro@...iv.linux.org.uk>
Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Aleksa Sarai <cyphar@...har.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)) {
 		touch_atime(&last->link);
 		cond_resched();
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
@@ -39,6 +39,9 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 #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);
-- 
2.24.0