[PATCH net] ipmr: prevent info-leak in pmr_cache_report()

[Eric Dumazet <edumazet@google.com>]

Yiming Qian reported:

<quote>
 ipmr_cache_report()` allocates a report skb with `alloc_skb(128,
 GFP_ATOMIC)` and appends a `struct igmphdr` using `skb_put()`. In the
 non-`IGMPMSG_WHOLEPKT` path it initializes only:

 - `igmp->type`
 - `igmp->code`

 but does not initialize:

 - `igmp->csum`
 - `igmp->group`

 Later, `igmpmsg_netlink_event()` copies the bytes after `sizeof(struct
 igmpmsg)` into the `IPMRA_CREPORT_PKT` netlink attribute and emits
 `RTM_NEWCACHEREPORT` on `RTNLGRP_IPV4_MROUTE_R`.

 As a result, 6 bytes of stale heap data from the skb head are
 disclosed to userspace.
</quote>

Let's use skb_put_zero() instead of skb_put() to fix this bug.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Yiming Qian <yimingqian591@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/ipv4/ipmr.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index 2058ca860294b01385063555d0354b7a9a736118..05fb6eefe0beb3c45c7ec485692460b84cb332c4 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -1112,11 +1112,12 @@ static int ipmr_cache_report(const struct mr_table *mrt,
 		msg->im_vif_hi = vifi >> 8;
 		ipv4_pktinfo_prepare(mroute_sk, pkt, false);
 		memcpy(skb->cb, pkt->cb, sizeof(skb->cb));
-		/* Add our header */
-		igmp = skb_put(skb, sizeof(struct igmphdr));
+		/* Add our header.
+		 * Note that code, csum and group fields are cleared.
+		 */
+		igmp = skb_put_zero(skb, sizeof(struct igmphdr));
 		igmp->type = assert;
 		msg->im_msgtype = assert;
-		igmp->code = 0;
 		ip_hdr(skb)->tot_len = htons(skb->len);	/* Fix the length */
 		skb->transport_header = skb->network_header;
 	}
-- 
2.54.0.545.g6539524ca2-goog