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

From: Kirill Tkhai <ktkhai@virtuozzo.com>
To: <serge@hallyn.com>, <ebiederm@xmission.com>,
	<agruenba@redhat.com>, <gregkh@linuxfoundation.org>,
	<linux-kernel@vger.kernel.org>, <oleg@redhat.com>,
	<paul@paul-moore.com>, <ktkhai@virtuozzo.com>,
	<viro@zeniv.linux.org.uk>, <avagin@openvz.org>,
	<linux-api@vger.kernel.org>, <linux-fsdevel@vger.kernel.org>,
	<mtk.manpages@gmail.com>, <akpm@linux-foundation.org>,
	<luto@amacapital.net>, <gorcunov@openvz.org>, <mingo@kernel.org>,
	<keescook@chromium.org>
Subject: [PATCH] security: Use user_namespace::level to avoid redundant iterations in cap_capable()
Date: Tue, 2 May 2017 15:40:33 +0300	[thread overview]
Message-ID: <149372879732.11927.12565684278286604561.stgit@localhost.localdomain> (raw)

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 <ktkhai@virtuozzo.com>
---
 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;
 
 		/* 
