From: Jean Delvare Date: Thu, 5 Apr 2007 06:52:46 +0000 (-0700) Subject: [APPLETALK]: Fix a remotely triggerable crash X-Git-Tag: v2.6.21-rc6~3 X-Git-Url: http://git.kernel.org/?p=linux%2Fkernel%2Fgit%2Ftorvalds%2Flinux-2.6.git;a=commitdiff_plain;h=75559c167bddc1254db5bcff032ad5eed8bd6f4a [APPLETALK]: Fix a remotely triggerable crash When we receive an AppleTalk frame shorter than what its header says, we still attempt to verify its checksum, and trip on the BUG_ON() at the end of function atalk_sum_skb() because of the length mismatch. This has security implications because this can be triggered by simply sending a specially crafted ethernet frame to a target victim, effectively crashing that host. Thus this qualifies, I think, as a remote DoS. Here is the frame I used to trigger the crash, in npg format: { # Ethernet header ----- XX XX XX XX XX XX # Destination MAC 00 00 00 00 00 00 # Source MAC 00 1D # Length # LLC header ----- AA AA 03 08 00 07 80 9B # Appletalk # Appletalk header ----- 00 1B # Packet length (invalid) 00 01 # Fake checksum 00 00 00 00 # Destination and source networks 00 00 00 00 # Destination and source nodes and ports # Payload ----- 0C 0D 0E 0F 10 11 12 13 14 } The destination MAC address must be set to those of the victim. The severity is mitigated by two requirements: * The target host must have the appletalk kernel module loaded. I suspect this isn't so frequent. * AppleTalk frames are non-IP, thus I guess they can only travel on local networks. I am no network expert though, maybe it is possible to somehow encapsulate AppleTalk packets over IP. The bug has been reported back in June 2004: http://bugzilla.kernel.org/show_bug.cgi?id=2979 But it wasn't investigated, and was closed in July 2006 as both reporters had vanished meanwhile. This code was new in kernel 2.6.0-test5: http://git.kernel.org/?p=linux/kernel/git/tglx/history.git;a=commitdiff;h=7ab442d7e0a76402c12553ee256f756097cae2d2 And not modified since then, so we can assume that vanilla kernels 2.6.0-test5 and later, and distribution kernels based thereon, are affected. Note that I still do not know for sure what triggered the bug in the real-world cases. The frame could have been corrupted by the kernel if we have a bug hiding somewhere. But more likely, we are receiving the faulty frame from the network. Signed-off-by: Jean Delvare Signed-off-by: David S. Miller --- diff --git a/net/appletalk/ddp.c b/net/appletalk/ddp.c index 113c175..c8b7dc2 100644 --- a/net/appletalk/ddp.c +++ b/net/appletalk/ddp.c @@ -1417,10 +1417,13 @@ static int atalk_rcv(struct sk_buff *skb, struct net_device *dev, /* * Size check to see if ddp->deh_len was crap * (Otherwise we'll detonate most spectacularly - * in the middle of recvmsg()). + * in the middle of atalk_checksum() or recvmsg()). */ - if (skb->len < sizeof(*ddp)) + if (skb->len < sizeof(*ddp) || skb->len < (len_hops & 1023)) { + pr_debug("AppleTalk: dropping corrupted frame (deh_len=%u, " + "skb->len=%u)\n", len_hops & 1023, skb->len); goto freeit; + } /* * Any checksums. Note we don't do htons() on this == is assumed to be