[v15,2/9] namei: LOOKUP_NO_MAGICLINKS: block magic-link resolution

Commit Message

Aleksa Sarai Nov. 5, 2019, 9:05 a.m. UTC

/* Background. */
There has always been a special class of symlink-like objects in procfs
(and a few other pseudo-filesystems) which allow for non-lexical
resolution of paths using nd_jump_link(). These "magic-links" do not
follow traditional mount namespace boundaries, and have been used
consistently in container escape attacks because they can be used to
trick unsuspecting privileged processes into resolving unexpected paths.

It is also non-trivial for userspace to unambiguously avoid resolving
magic-links, because they do not have a reliable indication that they
are a magic-link (in order to verify them you'd have to manually open
the path given by readlink(2) and then verify that the two file
descriptors reference the same underlying file, which is plagued with
possible race conditions or supplementary attack scenarios).

It would therefore be very helpful for userspace to be able to avoid
these symlinks easily, thus hopefully removing a tool from attackers'
toolboxes.

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_MAGICLINKS will be exposed to userspace through openat2(2).

/* Semantics. */
Unlike most other LOOKUP flags (most notably LOOKUP_FOLLOW),
LOOKUP_NO_MAGICLINKS applies to all components of the path.

With LOOKUP_NO_MAGICLINKS, any magic-link path component encountered
during path resolution will yield -ELOOP. The handling of ~LOOKUP_FOLLOW
for a trailing magic-link is identical to LOOKUP_NO_SYMLINKS.

LOOKUP_NO_SYMLINKS implies LOOKUP_NO_MAGICLINKS.

/* Testing. */
LOOKUP_NO_MAGICLINKS is tested as part of the openat2(2) selftests.

[1]: https://lore.kernel.org/lkml/[e-mail]/
[2]: https://lore.kernel.org/lkml/[e-mail]/
[3]: https://lore.kernel.org/lkml/[e-mail]/

Cc: Christian Brauner <[e-mail]>
Suggested-by: David Drysdale <[e-mail]>
Suggested-by: Al Viro <[e-mail]>
Suggested-by: Andy Lutomirski <[e-mail]>
Suggested-by: Linus Torvalds <[e-mail]>
Signed-off-by: Aleksa Sarai <[e-mail]>
---
 fs/namei.c            | 7 ++++++-
 include/linux/namei.h | 2 ++
 2 files changed, 8 insertions(+), 1 deletion(-)

Comments

Al Viro Nov. 13, 2019, 1:24 a.m. UTC | #1

On Tue, Nov 05, 2019 at 08:05:46PM +1100, Aleksa Sarai wrote:
> @@ -1078,6 +1079,10 @@ const char *get_link(struct nameidata *nd)
>  		} else {
>  			res = get(dentry, inode, &last->done);
>  		}
> +		if (nd->flags & LOOKUP_MAGICLINK_JUMPED) {
> +			if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS))
> +				return ERR_PTR(-ELOOP);
> +		}

Minor nit - the first check probably wants unlikely() more than the
second one; it's probably noise anyway, but most of the symlinks
traversed are not going to be procfs ones, so you get test + branch
taken most of the time.

OTOH, that just might compile into
	fetch nd->flags
	and with LOOKUP_MAGICLINK_JUMPED | LOOKUP_NO_MAGICLINKS
	compare with the same constant
	unlikely branch when equal

Anyway, that's no more than a minor nit and can be dealt with later (if
at all)

Patch

diff --git a/fs/namei.c b/fs/namei.c
index 4e85d6fa4048..1f0d871199e5 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -866,7 +866,7 @@  void nd_jump_link(struct path *path)
 
 	nd->path = *path;
 	nd->inode = nd->path.dentry->d_inode;
-	nd->flags |= LOOKUP_JUMPED;
+	nd->flags |= LOOKUP_JUMPED | LOOKUP_MAGICLINK_JUMPED;
 }
 
 static inline void put_link(struct nameidata *nd)
@@ -1063,6 +1063,7 @@  const char *get_link(struct nameidata *nd)
 		return ERR_PTR(error);
 
 	nd->last_type = LAST_BIND;
+	nd->flags &= ~LOOKUP_MAGICLINK_JUMPED;
 	res = READ_ONCE(inode->i_link);
 	if (!res) {
 		const char * (*get)(struct dentry *, struct inode *,
@@ -1078,6 +1079,10 @@  const char *get_link(struct nameidata *nd)
 		} else {
 			res = get(dentry, inode, &last->done);
 		}
+		if (nd->flags & LOOKUP_MAGICLINK_JUMPED) {
+			if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS))
+				return ERR_PTR(-ELOOP);
+		}
 		if (IS_ERR_OR_NULL(res))
 			return res;
 	}
diff --git a/include/linux/namei.h b/include/linux/namei.h
index ee2e35af387f..a8b3f93338da 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -38,9 +38,11 @@  enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 #define LOOKUP_JUMPED		0x1000
 #define LOOKUP_ROOT		0x2000
 #define LOOKUP_ROOT_GRABBED	0x0008
+#define LOOKUP_MAGICLINK_JUMPED	0x10000
 
 /* Scoping flags for lookup. */
 #define LOOKUP_NO_SYMLINKS	0x020000 /* No symlink crossing. */
+#define LOOKUP_NO_MAGICLINKS	0x040000 /* No /proc/$pid/fd/ "symlink" crossing. */
 
 extern int path_pts(struct path *path);