@@ -1149,7 +1149,7 @@ const char *get_link(struct nameidata *nd, bool trailing)
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);
/*
* For trailing_symlink we check whether the symlink's
@@ -1833,7 +1833,7 @@ 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);
@@ -2384,7 +2384,7 @@ 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);
@@ -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);
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 LOOKUP_XDEV and LOOKUP_NO_MAGICLINKS 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 *at(2) semantic change with LOOKUP_IN_ROOT is that absolute pathnames no longer cause dirfd to be ignored completely. The rationale is that LOOKUP_IN_ROOT 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 Co-developed-by: Christian Brauner <christian@brauner.io> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> --- fs/namei.c | 6 +++--- include/linux/namei.h | 1 + 2 files changed, 4 insertions(+), 3 deletions(-)