Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Pavel Skripkin <paskripkin@gmail.com>
To:     steffen.klassert@secunet.com, herbert@gondor.apana.org.au,
        davem@davemloft.net, kuba@kernel.org
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Pavel Skripkin <paskripkin@gmail.com>,
        syzbot+9cd5837a045bbee5b810@syzkaller.appspotmail.com
Subject: [PATCH next] net: xfrm: fix shift-out-of-bounce
Date:   Wed, 28 Jul 2021 19:38:18 +0300
Message-Id: <20210728163818.10744-1-paskripkin@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

We need to check up->dirmask to avoid shift-out-of-bounce bug,
since up->dirmask comes from userspace.

Also, added XFRM_USERPOLICY_DIRMASK_MAX constant to uapi to inform
user-space that up->dirmask has maximum possible value

Fixes: 2d151d39073a ("xfrm: Add possibility to set the default to block if we have no policy")
Reported-and-tested-by: syzbot+9cd5837a045bbee5b810@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
---
 include/uapi/linux/xfrm.h | 1 +
 net/xfrm/xfrm_user.c      | 7 ++++++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/xfrm.h b/include/uapi/linux/xfrm.h
index 6e8095106192..b96c1ea7166d 100644
--- a/include/uapi/linux/xfrm.h
+++ b/include/uapi/linux/xfrm.h
@@ -514,6 +514,7 @@ struct xfrm_user_offload {
 #define XFRM_OFFLOAD_INBOUND	2
 
 struct xfrm_userpolicy_default {
+#define XFRM_USERPOLICY_DIRMASK_MAX	(sizeof(__u8) * 8)
 	__u8				dirmask;
 	__u8				action;
 };
diff --git a/net/xfrm/xfrm_user.c b/net/xfrm/xfrm_user.c
index acc3a0dab331..03b66d154b2b 100644
--- a/net/xfrm/xfrm_user.c
+++ b/net/xfrm/xfrm_user.c
@@ -1966,9 +1966,14 @@ static int xfrm_set_default(struct sk_buff *skb, struct nlmsghdr *nlh,
 {
 	struct net *net = sock_net(skb->sk);
 	struct xfrm_userpolicy_default *up = nlmsg_data(nlh);
-	u8 dirmask = (1 << up->dirmask) & XFRM_POL_DEFAULT_MASK;
+	u8 dirmask;
 	u8 old_default = net->xfrm.policy_default;
 
+	if (up->dirmask >= XFRM_USERPOLICY_DIRMASK_MAX)
+		return -EINVAL;
+
+	dirmask = (1 << up->dirmask) & XFRM_POL_DEFAULT_MASK;
+
 	net->xfrm.policy_default = (old_default & (0xff ^ dirmask))
 				    | (up->action << up->dirmask);
 
-- 
2.32.0