[PATCH v9 4/4] fuse: Restrict allow_other to the superblock's namespace or a descendant

[Eric W. Biederman]

From: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>

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. Also
export current_in_userns() for use by fuse when built as a
module.

Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Serge Hallyn <serge@xxxxxxxxxx>
Cc: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Acked-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx>
Reviewed-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
Signed-off-by: Dongsu Park <dongsu@xxxxxxxxxx>
Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
---
 fs/fuse/dir.c           | 2 +-
 kernel/user_namespace.c | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index ad1cfac1942f..d41559a0aa6b 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -1030,7 +1030,7 @@ int fuse_allow_current_process(struct fuse_conn *fc)
 	const struct cred *cred;
 
 	if (fc->allow_other)
-		return 1;
+		return current_in_userns(fc->user_ns);
 
 	cred = current_cred();
 	if (uid_eq(cred->euid, fc->user_id) &&
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 246d4d4ce5c7..492c255e6c5a 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -1235,6 +1235,7 @@ bool current_in_userns(const struct user_namespace *target_ns)
 {
 	return in_userns(target_ns, current_user_ns());
 }
+EXPORT_SYMBOL(current_in_userns);
 
 static inline struct user_namespace *to_user_ns(struct ns_common *ns)
 {
-- 
2.14.1