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

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.

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

diff --git a/security/commoncap.c b/security/commoncap.c
index 78b37838a2d3..f6ef78208d2d 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 ns can't be a descendant of cred->user_ns, then it's
+                * needlessly to go up.
+                */
+               if (ns->level <= cred->user_ns->level)
                        return -EPERM;