/* 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' toolboxes.
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.
LOOKUP_NO_SYMLINKS implies LOOKUP_NO_MAGICLINKS.
/* 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@goog... [3]: https://lore.kernel.org/lkml/1404124096-21445-1-git-send-email-drysdale@goog...
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 | 5 ++++- include/linux/namei.h | 1 + 2 files changed, 5 insertions(+), 1 deletion(-)
diff --git a/fs/namei.c b/fs/namei.c index 14d6d3afb9d3..a97facc232af 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -868,8 +868,11 @@ static int nd_jump_root(struct nameidata *nd) int nd_jump_link(struct path *path) { struct nameidata *nd = current->nameidata; - path_put(&nd->path);
+ if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS)) + return -ELOOP; + + path_put(&nd->path); nd->path = *path; nd->inode = nd->path.dentry->d_inode; nd->flags |= LOOKUP_JUMPED; 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 @@ -41,6 +41,7 @@ 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. */
extern int path_pts(struct path *path);