[PATCH] nstree: check listing permission before taking a namespace reference

[Norbert Szetei]

legitimize_ns() takes a reference on the candidate namespace before
may_list_ns() has decided whether the caller may see it. The
__free(ns_put) cleanup on the denied path can drop the last reference to a
mount namespace while we still hold the rcu read lock, and put_mnt_ns()
may sleep there. This is the same problem commit 2ec2aff3c8e2 ("ns: make
sure reference are dropped outside of rcu lock") fixed for the put_user()
path. Neither ns_requested() nor may_list_ns() needs a reference, both
only look at the namespace type and at the caller's own namespaces, so do
the checks first and take the reference last.

Fixes: 76b6f5dfb3fd ("nstree: add listns()")
Signed-off-by: Norbert Szetei <norbert@xxxxxxxxxxxx>
---
A reproducer is available on request.

 kernel/nstree.c | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/kernel/nstree.c b/kernel/nstree.c
index 6d12e5900ac0..831f279d174a 100644
--- a/kernel/nstree.c
+++ b/kernel/nstree.c
@@ -533,19 +533,13 @@ DEFINE_FREE(ns_put, struct ns_common *, if (!IS_ERR_OR_NULL(_T)) ns_put(_T))
 static inline struct ns_common *__must_check legitimize_ns(const struct klistns *kls,
 							   struct ns_common *candidate)
 {
-	struct ns_common *ns __free(ns_put) = NULL;
-
 	if (!ns_requested(kls, candidate))
 		return NULL;
 
-	ns = ns_get_unless_inactive(candidate);
-	if (!ns)
-		return NULL;
-
-	if (!may_list_ns(kls, ns))
+	if (!may_list_ns(kls, candidate))
 		return NULL;
 
-	return no_free_ptr(ns);
+	return ns_get_unless_inactive(candidate);
 }
 
 static ssize_t do_listns_userns(struct klistns *kls)
-- 
2.55.0