linux-kernel - [PATCH] security: Use user_namespace::level to avoid redundant iterations in cap

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Message-ID: <149372879732.11927.12565684278286604561.stgit@localhost.localdomain>
Date:   Tue, 2 May 2017 15:40:33 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     <serge@...lyn.com>, <ebiederm@...ssion.com>, <agruenba@...hat.com>,
        <gregkh@...uxfoundation.org>, <linux-kernel@...r.kernel.org>,
        <oleg@...hat.com>, <paul@...l-moore.com>, <ktkhai@...tuozzo.com>,
        <viro@...iv.linux.org.uk>, <avagin@...nvz.org>,
        <linux-api@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
        <mtk.manpages@...il.com>, <akpm@...ux-foundation.org>,
        <luto@...capital.net>, <gorcunov@...nvz.org>, <mingo@...nel.org>,
        <keescook@...omium.org>
Subject: [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 <ktkhai@...tuozzo.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;
 
 		/*