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@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>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namei.c b/fs/namei.c
index 7def6aa9a53dd4498b53e44e992207b00e2034e6..b638c9cd6d4c2bc1815b304710c2b515febacc7c 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1056,6 +1056,9 @@ 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 b2479cc119c68b1d95e7ea599dab6ff2ca58a1d5..c42a1924ad672956ac5a2017cd26219034855f37 100644 (file)
--- 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);