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Subject[PATCH v15 3/9] namei: LOOKUP_NO_XDEV: block mountpoint crossing
Date
/* 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@google.com/
[3]: https://lore.kernel.org/lkml/1404124096-21445-1-git-send-email-drysdale@google.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 | 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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