[net] netfilter: xt_TCPMSS: add more sanity tests on tcph->doff

Message ID 1491242111.10124.32.camel@edumazet-glaptop3.roam.corp.google.com
State Accepted
Delegated to: Pablo Neira
Headers show

Commit Message

Eric Dumazet April 3, 2017, 5:55 p.m.
From: Eric Dumazet <edumazet@google.com>

Denys provided an awesome KASAN report pointing to an use
after free in xt_TCPMSS

I have provided three patches to fix this issue, either in xt_TCPMSS or
in xt_tcpudp.c. It seems xt_TCPMSS patch has the smallest possible
impact.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Denys Fedoryshchenko <nuclearcat@nuclearcat.com>
---
 net/netfilter/xt_TCPMSS.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)



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Comments

Pablo Neira Ayuso April 8, 2017, 8:24 p.m. | #1
On Mon, Apr 03, 2017 at 10:55:11AM -0700, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@google.com>
> 
> Denys provided an awesome KASAN report pointing to an use
> after free in xt_TCPMSS
> 
> I have provided three patches to fix this issue, either in xt_TCPMSS or
> in xt_tcpudp.c. It seems xt_TCPMSS patch has the smallest possible
> impact.

Applied to nf.git, thanks!
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Denys Fedoryshchenko April 20, 2017, 6:14 p.m. | #2
On 2017-04-08 23:24, Pablo Neira Ayuso wrote:
> On Mon, Apr 03, 2017 at 10:55:11AM -0700, Eric Dumazet wrote:
>> From: Eric Dumazet <edumazet@google.com>
>> 
>> Denys provided an awesome KASAN report pointing to an use
>> after free in xt_TCPMSS
>> 
>> I have provided three patches to fix this issue, either in xt_TCPMSS 
>> or
>> in xt_tcpudp.c. It seems xt_TCPMSS patch has the smallest possible
>> impact.
> 
> Applied to nf.git, thanks!
Any plans to queue it to stable trees?
It seems affected kernel for years.
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Patch

diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
index 27241a767f17b4b27d24095a31e5e9a2d3e29ce4..c64aca611ac5c5f81ad7c925652bbb90554763ac 100644
--- a/net/netfilter/xt_TCPMSS.c
+++ b/net/netfilter/xt_TCPMSS.c
@@ -104,7 +104,7 @@  tcpmss_mangle_packet(struct sk_buff *skb,
 	tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
 	tcp_hdrlen = tcph->doff * 4;
 
-	if (len < tcp_hdrlen)
+	if (len < tcp_hdrlen || tcp_hdrlen < sizeof(struct tcphdr))
 		return -1;
 
 	if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
@@ -152,6 +152,10 @@  tcpmss_mangle_packet(struct sk_buff *skb,
 	if (len > tcp_hdrlen)
 		return 0;
 
+	/* tcph->doff has 4 bits, do not wrap it to 0 */
+	if (tcp_hdrlen >= 15 * 4)
+		return 0;
+
 	/*
 	 * MSS Option not found ?! add it..
 	 */