[PATCH v2] security: Use user_namespace::level to avoid redundant iterations in cap_capable()

[Kirill Tkhai]

When ns->level is not larger then cred->user_ns->level,
then ns can't be cred->user_ns's descendant, and
there is no a sence to search in parents.

So, breake the cycle earlier and skip needless iterations.

v2: Change comment on suggested by Andy Lutomirski.

Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
---
 security/commoncap.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/security/commoncap.c b/security/commoncap.c
index 78b37838a2d3..a424aa0123af 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -82,8 +82,11 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
 		if (ns == cred->user_ns)
 			return cap_raised(cred->cap_effective, cap) ? 0 : -EPERM;
 
-		/* Have we tried all of the parent namespaces? */
-		if (ns == &init_user_ns)
+		/*
+		 * If we're already at a lower level than we're looking for, we're
+		 * done searching.
+		 */
+		if (ns->level <= cred->user_ns->level)
 			return -EPERM;
 
 		/*