netdev - [PATCH net] ipv6: sr: fix out-of-bounds read when setting HMAC data.

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Message-Id: <20220902094506.89156-1-dav.lebrun@gmail.com>
Date:   Fri,  2 Sep 2022 10:45:06 +0100
From:   David Lebrun <dav.lebrun@...il.com>
To:     netdev@...r.kernel.org
Cc:     David Lebrun <dav.lebrun@...il.com>, edumazet@...gle.com,
        Lucas Leong <wmliang.tw@...il.com>,
        David Lebrun <dlebrun@...gle.com>
Subject: [PATCH net] ipv6: sr: fix out-of-bounds read when setting HMAC data.

From: David Lebrun <dlebrun@...gle.com>

The SRv6 layer allows defining HMAC data that can later be used to sign IPv6
Segment Routing Headers. This configuration is realised via netlink through
four attributes: SEG6_ATTR_HMACKEYID, SEG6_ATTR_SECRET, SEG6_ATTR_SECRETLEN and
SEG6_ATTR_ALGID. Because the SECRETLEN attribute is decoupled from the actual
length of the SECRET attribute, it is possible to provide invalid combinations
(e.g., secret = "", secretlen = 64). This case is not checked in the code and
with an appropriately crafted netlink message, an out-of-bounds read of up
to 64 bytes (max secret length) can occur past the skb end pointer and into
skb_shared_info:

Breakpoint 1, seg6_genl_sethmac (skb=<optimized out>, info=<optimized out>) at net/ipv6/seg6.c:208
208		memcpy(hinfo->secret, secret, slen);
(gdb) bt
 #0  seg6_genl_sethmac (skb=<optimized out>, info=<optimized out>) at net/ipv6/seg6.c:208
 #1  0xffffffff81e012e9 in genl_family_rcv_msg_doit (skb=skb@...ry=0xffff88800b1f9f00, nlh=nlh@...ry=0xffff88800b1b7600,
    extack=extack@...ry=0xffffc90000ba7af0, ops=ops@...ry=0xffffc90000ba7a80, hdrlen=4, net=0xffffffff84237580 <init_net>, family=<optimized out>,
    family=<optimized out>) at net/netlink/genetlink.c:731
 #2  0xffffffff81e01435 in genl_family_rcv_msg (extack=0xffffc90000ba7af0, nlh=0xffff88800b1b7600, skb=0xffff88800b1f9f00,
    family=0xffffffff82fef6c0 <seg6_genl_family>) at net/netlink/genetlink.c:775
 #3  genl_rcv_msg (skb=0xffff88800b1f9f00, nlh=0xffff88800b1b7600, extack=0xffffc90000ba7af0) at net/netlink/genetlink.c:792
 #4  0xffffffff81dfffc3 in netlink_rcv_skb (skb=skb@...ry=0xffff88800b1f9f00, cb=cb@...ry=0xffffffff81e01350 <genl_rcv_msg>)
    at net/netlink/af_netlink.c:2501
 #5  0xffffffff81e00919 in genl_rcv (skb=0xffff88800b1f9f00) at net/netlink/genetlink.c:803
 #6  0xffffffff81dff6ae in netlink_unicast_kernel (ssk=0xffff888010eec800, skb=0xffff88800b1f9f00, sk=0xffff888004aed000)
    at net/netlink/af_netlink.c:1319
 #7  netlink_unicast (ssk=ssk@...ry=0xffff888010eec800, skb=skb@...ry=0xffff88800b1f9f00, portid=portid@...ry=0, nonblock=<optimized out>)
    at net/netlink/af_netlink.c:1345
 #8  0xffffffff81dff9a4 in netlink_sendmsg (sock=<optimized out>, msg=0xffffc90000ba7e48, len=<optimized out>) at net/netlink/af_netlink.c:1921
...
(gdb) p/x ((struct sk_buff *)0xffff88800b1f9f00)->head + ((struct sk_buff *)0xffff88800b1f9f00)->end
$1 = 0xffff88800b1b76c0
(gdb) p/x secret
$2 = 0xffff88800b1b76c0
(gdb) p slen
$3 = 64 '@'

The OOB data can then be read back from userspace by dumping HMAC state. This
commit fixes this by ensuring SECRETLEN cannot exceed the actual length of
SECRET.

Reported-by: Lucas Leong <wmliang.tw@...il.com>
Tested: verified that EINVAL is correctly returned when secretlen > len(secret)
Fixes: 4f4853dc1c9c1 ("ipv6: sr: implement API to control SR HMAC structure")
Signed-off-by: David Lebrun <dlebrun@...gle.com>
---
 net/ipv6/seg6.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/ipv6/seg6.c b/net/ipv6/seg6.c
index 5421cc7c935f..29346a6eec9f 100644
--- a/net/ipv6/seg6.c
+++ b/net/ipv6/seg6.c
@@ -191,6 +191,11 @@ static int seg6_genl_sethmac(struct sk_buff *skb, struct genl_info *info)
 		goto out_unlock;
 	}
 
+	if (slen > nla_len(info->attrs[SEG6_ATTR_SECRET])) {
+		err = -EINVAL;
+		goto out_unlock;
+	}
+
 	if (hinfo) {
 		err = seg6_hmac_info_del(net, hmackeyid);
 		if (err)
-- 
2.25.1