On Fri, Dec 04, 2015 at 02:05:41PM -0600, Serge E. Hallyn wrote:

Quoting Seth Forshee (seth.forshee@canonical.com):

quoted

Unprivileged users are normally restricted from mounting with the
allow_other option by system policy, but this could be bypassed
for a mount done with user namespace root permissions. In such
cases allow_other should not allow users outside the userns
to access the mount as doing so would give the unprivileged user
the ability to manipulate processes it would otherwise be unable
to manipulate. Restrict allow_other to apply to users in the same
userns used at mount or a descendant of that namespace.

Signed-off-by: Seth Forshee <redacted>
---
 fs/fuse/dir.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index f67f4dd86b36..5b8edb1203b8 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c

@@ -1018,8 +1018,14 @@ int fuse_allow_current_process(struct fuse_conn *fc)
 {
 	const struct cred *cred;
 
-	if (fc->flags & FUSE_ALLOW_OTHER)
-		return 1;
+	if (fc->flags & FUSE_ALLOW_OTHER) {
+		struct user_namespace *ns;
+		for (ns = current_user_ns(); ns; ns = ns->parent) {
+			if (ns == fc->user_ns)
+				return 1;
+		}

	use current_in_userns() ?

Yes, it should. I wrote this before I wrote the patch which adds that
function and never thought to go back to change it here.