[PATCH v15 3/9] namei: LOOKUP_NO_XDEV: block mountpoint crossing
STALE2513d
From: Aleksa Sarai <hidden>
Date: 2019-11-05 09:08:03
Also in:
linux-alpha, linux-api, linux-arch, linux-fsdevel, linux-kselftest, linux-m68k, linux-mips, linux-s390, linux-sh, linuxppc-dev, lkml, sparclinux
Subsystem:
filesystems (vfs and infrastructure), the rest · Maintainers:
Alexander Viro, Christian Brauner, Linus Torvalds
Revision v15 of 7 in this series.
Revisions (7)
/* 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/ (local) [2]: https://lore.kernel.org/lkml/1415094884-18349-1-git-send-email-drysdale@google.com/ (local) [3]: https://lore.kernel.org/lkml/1404124096-21445-1-git-send-email-drysdale@google.com/ (local) Cc: Christian Brauner <redacted> Suggested-by: David Drysdale <redacted> 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 <redacted> --- fs/namei.c | 34 ++++++++++++++++++++++++++++++---- include/linux/namei.h | 1 + 2 files changed, 31 insertions(+), 4 deletions(-)
diff --git a/fs/namei.c b/fs/namei.c
index 1f0d871199e5..b73ee1601bd4 100644
--- a/fs/namei.c
+++ b/fs/namei.c@@ -504,6 +504,9 @@ struct nameidata { struct filename *name; struct nameidata *saved; struct inode *link_inode; + struct { + bool same_mnt; + } last_magiclink; unsigned root_seq; int dfd; } __randomize_layout;
@@ -837,6 +840,11 @@ 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->flags & LOOKUP_RCU) { struct dentry *d; nd->path = nd->root;
@@ -862,6 +870,8 @@ static int nd_jump_root(struct nameidata *nd) void nd_jump_link(struct path *path) { struct nameidata *nd = current->nameidata; + + nd->last_magiclink.same_mnt = (nd->path.mnt == path->mnt); path_put(&nd->path); nd->path = *path;
@@ -1082,6 +1092,10 @@ const char *get_link(struct nameidata *nd) if (nd->flags & LOOKUP_MAGICLINK_JUMPED) { if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS)) return ERR_PTR(-ELOOP); + if (unlikely(nd->flags & LOOKUP_NO_XDEV)) { + if (!nd->last_magiclink.same_mnt) + return ERR_PTR(-EXDEV); + } } if (IS_ERR_OR_NULL(res)) return res;
@@ -1271,12 +1285,16 @@ 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;
@@ -1333,6 +1351,8 @@ 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;
@@ -1379,6 +1399,8 @@ 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;
@@ -1393,6 +1415,8 @@ 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;
@@ -1491,6 +1515,8 @@ 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 a8b3f93338da..6105c8a59fc8 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h@@ -43,6 +43,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND}; /* Scoping flags for lookup. */ #define LOOKUP_NO_SYMLINKS 0x020000 /* No symlink crossing. */ #define LOOKUP_NO_MAGICLINKS 0x040000 /* No /proc/$pid/fd/ "symlink" crossing. */ +#define LOOKUP_NO_XDEV 0x080000 /* No mountpoint crossing. */ extern int path_pts(struct path *path);
--
2.23.0
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