[PATCH v5 3/4] fuse: Restrict allow_other to the superblock's namespace or a descendant

[Seth Forshee <seth.forshee@canonical.com>]

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. Therefore access with allow_other should be
restricted to users in the userns as the superblock or a
descendant of that namespace.

Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Serge H. Hallyn <serge.hallyn@ubuntu.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
---
 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 123db1e06c78..b23ec5c1ff18 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -1091,8 +1091,14 @@ 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;
+		}
+		return 0;
+	}
 
 	cred = current_cred();
 	if (uid_eq(cred->euid, fc->user_id) &&
-- 
1.9.1