/* Background. */ The need to contain path operations within a mountpoint has been a long-standing usecase that userspace has historically implemented manually with liberal usage of stat(). find, rsync, tar and many other programs implement these semantics -- but it'd be much simpler to have a fool-proof way of refusing to open a path if it crosses a mountpoint.
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_XDEV will be exposed to userspace through openat2(2).
/* Semantics. */ Unlike most other LOOKUP flags (most notably LOOKUP_FOLLOW), LOOKUP_NO_XDEV applies to all components of the path.
With LOOKUP_NO_XDEV, any path component which crosses a mount-point during path resolution (including "..") will yield an -EXDEV. Absolute paths, absolute symlinks, and magic-links will only yield an -EXDEV if the jump involved changing mount-points.
/* Testing. */ LOOKUP_NO_XDEV 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 | 29 +++++++++++++++++++++++++---- include/linux/namei.h | 1 + 2 files changed, 26 insertions(+), 4 deletions(-)
diff --git a/fs/namei.c b/fs/namei.c index a97facc232af..854a1cbbe7b0 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -838,6 +838,11 @@ static inline void path_to_nameidata(const struct path *path,
static int nd_jump_root(struct nameidata *nd) { + 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) + return -EXDEV; + } if (!nd->root.mnt) { int error = set_root(nd); if (error) @@ -871,6 +876,10 @@ int nd_jump_link(struct path *path)
if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS)) return -ELOOP; + if (unlikely(nd->flags & LOOKUP_NO_XDEV)) { + if (nd->path.mnt != path->mnt) + return -EXDEV; + }
path_put(&nd->path); nd->path = *path; @@ -1275,12 +1284,16 @@ static int follow_managed(struct path *path, struct nameidata *nd) break; }
- if (need_mntput && path->mnt == mnt) - mntput(path->mnt); + if (need_mntput) { + if (path->mnt == mnt) + mntput(path->mnt); + if (unlikely(nd->flags & LOOKUP_NO_XDEV)) + ret = -EXDEV; + else + nd->flags |= LOOKUP_JUMPED; + } if (ret == -EISDIR || !ret) ret = 1; - if (need_mntput) - nd->flags |= LOOKUP_JUMPED; if (unlikely(ret < 0)) path_put_conditional(path, nd); return ret; @@ -1337,6 +1350,8 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path, mounted = __lookup_mnt(path->mnt, path->dentry); if (!mounted) break; + if (unlikely(nd->flags & LOOKUP_NO_XDEV)) + return false; path->mnt = &mounted->mnt; path->dentry = mounted->mnt.mnt_root; nd->flags |= LOOKUP_JUMPED; @@ -1383,6 +1398,8 @@ static int follow_dotdot_rcu(struct nameidata *nd) return -ECHILD; if (&mparent->mnt == nd->path.mnt) break; + if (unlikely(nd->flags & LOOKUP_NO_XDEV)) + return -EXDEV; /* we know that mountpoint was pinned */ nd->path.dentry = mountpoint; nd->path.mnt = &mparent->mnt; @@ -1397,6 +1414,8 @@ static int follow_dotdot_rcu(struct nameidata *nd) return -ECHILD; if (!mounted) break; + if (unlikely(nd->flags & LOOKUP_NO_XDEV)) + return -EXDEV; nd->path.mnt = &mounted->mnt; nd->path.dentry = mounted->mnt.mnt_root; inode = nd->path.dentry->d_inode; @@ -1495,6 +1514,8 @@ static int follow_dotdot(struct nameidata *nd) } if (!follow_up(&nd->path)) break; + if (unlikely(nd->flags & LOOKUP_NO_XDEV)) + return -EXDEV; } follow_mount(&nd->path); nd->inode = nd->path.dentry->d_inode; diff --git a/include/linux/namei.h b/include/linux/namei.h index 1573b8493d98..25ee88c4acb1 100644 --- a/include/linux/namei.h +++ b/include/linux/namei.h @@ -42,6 +42,7 @@ 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. */ +#define LOOKUP_NO_XDEV 0x040000 /* No mountpoint crossing. */
extern int path_pts(struct path *path);