@@ -641,6 +641,14 @@ static bool legitimize_links(struct nameidata *nd)
static bool legitimize_root(struct nameidata *nd)
{
+ /*
+ * For scoped-lookups (where nd->root has been zeroed), we need to
+ * restart the whole lookup from scratch -- because set_root() is wrong
+ * for these lookups (nd->dfd is the root, not the filesystem root).
+ */
+ if (!nd->root.mnt && (nd->flags & LOOKUP_IS_SCOPED))
+ return false;
+ /* Nothing to do if nd->root is zero or is managed by the VFS user. */
if (!nd->root.mnt || (nd->flags & LOOKUP_ROOT))
return true;
nd->flags |= LOOKUP_ROOT_GRABBED;
@@ -776,12 +784,27 @@ static int complete_walk(struct nameidata *nd)
int status;
if (nd->flags & LOOKUP_RCU) {
- if (!(nd->flags & LOOKUP_ROOT))
+ /*
+ * We don't want to zero nd->root for scoped-lookups or
+ * externally-managed nd->root.
+ */
+ if (!(nd->flags & (LOOKUP_ROOT | LOOKUP_IS_SCOPED)))
nd->root.mnt = NULL;
if (unlikely(unlazy_walk(nd)))
return -ECHILD;
}
+ if (unlikely(nd->flags & LOOKUP_IS_SCOPED)) {
+ /*
+ * Do a final check to ensure that the path didn't escape. Note
+ * that this should already be guaranteed by all of the other
+ * LOOKUP_IS_SCOPED checks (and delaying this check this late
+ * does open the door to some possible timing-based attacks).
+ */
+ if (WARN_ON(!path_is_under(&nd->path, &nd->root)))
+ return -EXDEV;
+ }
+
if (likely(!(nd->flags & LOOKUP_JUMPED)))
return 0;
@@ -802,6 +825,14 @@ static int set_root(struct nameidata *nd)
{
struct fs_struct *fs = current->fs;
+ /*
+ * Jumping to the real root in a scoped-lookup is a BUG in namei, but we
+ * still have to ensure it doesn't happen because it will cause a breakout
+ * from the dirfd.
+ */
+ if (WARN_ON(nd->flags & LOOKUP_IS_SCOPED))
+ return -ENOTRECOVERABLE;
+
if (nd->flags & LOOKUP_RCU) {
unsigned seq;
@@ -838,6 +869,8 @@ static inline void path_to_nameidata(const struct path *path,
static int nd_jump_root(struct nameidata *nd)
{
+ if (unlikely(nd->flags & LOOKUP_BENEATH))
+ return -EXDEV;
if (unlikely(nd->flags & LOOKUP_NO_XDEV)) {
/* Absolute path arguments to path_init() are allowed. */
if (nd->path.mnt != NULL && nd->path.mnt != nd->root.mnt)
@@ -883,6 +916,9 @@ int nd_jump_link(struct path *path)
if (nd->path.mnt != path->mnt)
goto err;
}
+ /* Not currently safe for scoped-lookups. */
+ if (unlikely(nd->flags & LOOKUP_IS_SCOPED))
+ goto err;
path_put(&nd->path);
nd->path = *path;
@@ -1379,8 +1415,11 @@ static int follow_dotdot_rcu(struct nameidata *nd)
struct inode *inode = nd->inode;
while (1) {
- if (path_equal(&nd->path, &nd->root))
+ if (path_equal(&nd->path, &nd->root)) {
+ if (unlikely(nd->flags & LOOKUP_BENEATH))
+ return -ECHILD;
break;
+ }
if (nd->path.dentry != nd->path.mnt->mnt_root) {
struct dentry *old = nd->path.dentry;
struct dentry *parent = old->d_parent;
@@ -1510,9 +1549,12 @@ static int path_parent_directory(struct path *path)
static int follow_dotdot(struct nameidata *nd)
{
- while(1) {
- if (path_equal(&nd->path, &nd->root))
+ while (1) {
+ if (path_equal(&nd->path, &nd->root)) {
+ if (unlikely(nd->flags & LOOKUP_BENEATH))
+ return -EXDEV;
break;
+ }
if (nd->path.dentry != nd->path.mnt->mnt_root) {
int ret = path_parent_directory(&nd->path);
if (ret)
@@ -1739,6 +1781,13 @@ static inline int handle_dots(struct nameidata *nd, int type)
if (type == LAST_DOTDOT) {
int error = 0;
+ /*
+ * Scoped-lookup flags resolving ".." is not currently safe --
+ * races can cause our parent to have moved outside of the root
+ * and us to skip over it.
+ */
+ if (unlikely(nd->flags & LOOKUP_IS_SCOPED))
+ return -EXDEV;
if (!nd->root.mnt) {
error = set_root(nd);
if (error)
@@ -2261,7 +2310,6 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
get_fs_pwd(current->fs, &nd->path);
nd->inode = nd->path.dentry->d_inode;
}
- return s;
} else {
/* Caller must check execute permissions on the starting path component */
struct fd f = fdget_raw(nd->dfd);
@@ -2286,8 +2334,18 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
nd->inode = nd->path.dentry->d_inode;
}
fdput(f);
- return s;
}
+ /* For scoped-lookups we need to set the root to the dirfd as well. */
+ if (flags & LOOKUP_IS_SCOPED) {
+ nd->root = nd->path;
+ if (flags & LOOKUP_RCU) {
+ nd->root_seq = nd->seq;
+ } else {
+ path_get(&nd->root);
+ nd->flags |= LOOKUP_ROOT_GRABBED;
+ }
+ }
+ return s;
}
static const char *trailing_symlink(struct nameidata *nd)
@@ -2,6 +2,7 @@
#ifndef _LINUX_NAMEI_H
#define _LINUX_NAMEI_H
+#include <linux/fs.h>
#include <linux/kernel.h>
#include <linux/path.h>
#include <linux/fcntl.h>
@@ -43,6 +44,9 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
#define LOOKUP_NO_SYMLINKS 0x010000 /* No symlink crossing. */
#define LOOKUP_NO_MAGICLINKS 0x020000 /* No nd_jump_link() crossing. */
#define LOOKUP_NO_XDEV 0x040000 /* No mountpoint crossing. */
+#define LOOKUP_BENEATH 0x080000 /* No escaping from starting point. */
+/* LOOKUP_* flags which do scope-related checks based on the dirfd. */
+#define LOOKUP_IS_SCOPED LOOKUP_BENEATH
extern int path_pts(struct path *path);
/* Background. */ There are many circumstances when userspace wants to resolve a path and ensure that it doesn't go outside of a particular root directory during resolution. Obvious examples include archive extraction tools, as well as other security-conscious userspace programs. FreeBSD spun out O_BENEATH from their Capsicum project[1,2], so it also seems reasonable to implement similar functionality for Linux. This is part of a refresh of Al's AT_NO_JUMPS patchset[3] (which was a variation on David Drysdale's O_BENEATH patchset[4], which in turn was based on the Capsicum project[5]). /* Userspace API. */ LOOKUP_BENEATH will be exposed to userspace through openat2(2). /* Semantics. */ Unlike most other LOOKUP flags (most notably LOOKUP_FOLLOW), LOOKUP_BENEATH applies to all components of the path. With LOOKUP_BENEATH, any path component which attempts to "escape" the starting point of the filesystem lookup (the dirfd passed to openat) will yield -EXDEV. Thus, all absolute paths and symlinks are disallowed. Due to a security concern brought up by Jann[6], any ".." path components are also blocked. This restriction will be lifted in a future patch, but requires more work to ensure that permitting ".." is done safely. Magic-link jumps are also blocked, because they can beam the path lookup across the starting point. It would be possible to detect and block only the "bad" crossings with path_is_under() checks, but it's unclear whether it makes sense to permit magic-links at all. However, userspace is recommended to pass LOOKUP_NO_MAGICLINKS if they want to ensure that magic-link crossing is entirely disabled. /* Testing. */ LOOKUP_BENEATH is tested as part of the openat2(2) selftests. [1]: https://reviews.freebsd.org/D2808 [2]: https://reviews.freebsd.org/D17547 [3]: https://lore.kernel.org/lkml/20170429220414.GT29622@ZenIV.linux.org.uk/ [4]: https://lore.kernel.org/lkml/1415094884-18349-1-git-send-email-drysdale@google.com/ [5]: https://lore.kernel.org/lkml/1404124096-21445-1-git-send-email-drysdale@google.com/ [6]: https://lore.kernel.org/lkml/CAG48ez1jzNvxB+bfOBnERFGp=oMM0vHWuLD6EULmne3R6xa53w@mail.gmail.com/ 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 | 70 +++++++++++++++++++++++++++++++++++++++---- include/linux/namei.h | 4 +++ 2 files changed, 68 insertions(+), 6 deletions(-)