[PATCH v6 3/6] namei: LOOKUP_IN_ROOT: chroot-like path resolution

[Aleksa Sarai <cyphar@cyphar.com>]

The primary motivation for the need for this flag is container runtimes
which have to interact with malicious root filesystems in the host
namespaces. One of the first requirements for a container runtime to be
secure against a malicious rootfs is that they correctly scope symlinks
(that is, they should be scoped as though they are chroot(2)ed into the
container's rootfs) and ".."-style paths[*]. The already-existing O_XDEV
and O_NOMAGICLINKS[**] help defend against other potential attacks in a
malicious rootfs scenario.

Currently most container runtimes try to do this resolution in
userspace[1], causing many potential race conditions. In addition, the
"obvious" alternative (actually performing a {ch,pivot_}root(2))
requires a fork+exec (for some runtimes) which is *very* costly if
necessary for every filesystem operation involving a container.

[*] At the moment, ".." and "magic link" jumping are disallowed for the
    same reason it is disabled for LOOKUP_BENEATH -- currently it is not
    safe to allow it. Future patches may enable it unconditionally once
    we have resolved the possible races (for "..") and semantics (for
    "magic link" jumping).

The most significant openat(2) semantic change with LOOKUP_THISROOT is
that absolute pathnames no longer cause dirfd to be ignored completely.
The rationale is that LOOKUP_THISROOT must necessarily chroot-scope
symlinks with absolute paths to dirfd, and so doing it for the base path
seems to be the most consistent behaviour (and also avoids foot-gunning
users who want to scope paths that are absolute).

[1]: https://github.com/cyphar/filepath-securejoin

Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Christian Brauner <christian@brauner.io>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 fs/namei.c            | 6 +++---
 include/linux/namei.h | 1 +
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index e13a02720a9d..3a3cba593b85 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1095,7 +1095,7 @@ const char *get_link(struct nameidata *nd)
 			if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS))
 				return ERR_PTR(-ELOOP);
 			/* Not currently safe. */
-			if (unlikely(nd->flags & LOOKUP_BENEATH))
+			if (unlikely(nd->flags & (LOOKUP_BENEATH | LOOKUP_IN_ROOT)))
 				return ERR_PTR(-EXDEV);
 		}
 		if (IS_ERR_OR_NULL(res))
@@ -1744,7 +1744,7 @@ static inline int handle_dots(struct nameidata *nd, int type)
 		 * cause our parent to have moved outside of the root and us to skip
 		 * over it.
 		 */
-		if (unlikely(nd->flags & LOOKUP_BENEATH))
+		if (unlikely(nd->flags & (LOOKUP_BENEATH | LOOKUP_IN_ROOT)))
 			return -EXDEV;
 		if (!nd->root.mnt)
 			set_root(nd);
@@ -2295,7 +2295,7 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
 
 	nd->m_seq = read_seqbegin(&mount_lock);
 
-	if (unlikely(nd->flags & LOOKUP_BENEATH)) {
+	if (unlikely(nd->flags & (LOOKUP_BENEATH | LOOKUP_IN_ROOT))) {
 		error = dirfd_path_init(nd);
 		if (unlikely(error))
 			return ERR_PTR(error);
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 7bc819ad0cd3..4b1ee717cb14 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -56,6 +56,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 #define LOOKUP_NO_MAGICLINKS	0x040000 /* No /proc/$pid/fd/ "symlink" crossing. */
 #define LOOKUP_NO_SYMLINKS	0x080000 /* No symlink crossing *at all*.
 					    Implies LOOKUP_NO_MAGICLINKS. */
+#define LOOKUP_IN_ROOT		0x100000 /* Treat dirfd as %current->fs->root. */
 
 extern int path_pts(struct path *path);
 
-- 
2.21.0