[PATCH] ipv6: check return value of ipv6_skip_exthdr

The offset value is used in pointer math on skb->data.
Since ipv6_skip_exthdr may return -1 the pointer to uh and th
may not point to the actual udp and tcp headers and potentially
overwrite other stuff. This is why I think this should be checked.

Signed-off-by: Jordy Zomer <redacted>
---
 net/ipv6/esp6.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
index ed2f061b8768..dc4251655df9 100644
--- a/net/ipv6/esp6.c
+++ b/net/ipv6/esp6.c

@@ -808,6 +808,11 @@ int esp6_input_done2(struct sk_buff *skb, int err)
 		struct tcphdr *th;
 
 		offset = ipv6_skip_exthdr(skb, offset, &nexthdr, &frag_off);
+
+		if (offset < 0)
+			err = -EINVAL;
+			goto out;
+
 		uh = (void *)(skb->data + offset);
 		th = (void *)(skb->data + offset);
 		hdr_len += offset;

-- 
2.27.0

On Wed, Nov 17, 2021 at 07:16:10PM +0100, Jordy Zomer wrote:

Ew. Yeah, it seems like ipv6_skip_exthdr() needs to be checked in a lot
of places. If this is part of protocol decoding, I'm surprised fuzzers
haven't found this? Is this state reachable?

I assume so, as there have been similar fixes in the past:
https://git.kernel.org/linus/92c6058024e87087cf1b99b0389d67c0a886360e

 		uh = (void *)(skb->data + offset);
 		th = (void *)(skb->data + offset);
 		hdr_len += offset;
-- 
2.27.0

-- 
Kees Cook

On Wed, Nov 17, 2021 at 07:16:10PM +0100, Jordy Zomer wrote:

Oh, and, this needs { }s. The CI noticed too:
https://patchwork.kernel.org/project/netdevbpf/patch/20211117181610.2731938-1-jordy@pwning.systems/

 		uh = (void *)(skb->data + offset);
 		th = (void *)(skb->data + offset);
 		hdr_len += offset;
-- 
2.27.0

-- 
Kees Cook

The offset value is used in pointer math on skb->data.
Since ipv6_skip_exthdr may return -1 the pointer to uh and th
may not point to the actual udp and tcp headers and potentially
overwrite other stuff. This is why I think this should be checked.

EDIT:  added {}'s, thanks Kees

Signed-off-by: Jordy Zomer <redacted>
---
 net/ipv6/esp6.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
index ed2f061b8768..f0bac6f7ab6b 100644
--- a/net/ipv6/esp6.c
+++ b/net/ipv6/esp6.c

@@ -808,6 +808,12 @@ int esp6_input_done2(struct sk_buff *skb, int err)
 		struct tcphdr *th;
 
 		offset = ipv6_skip_exthdr(skb, offset, &nexthdr, &frag_off);
+
+		if (offset < 0) {
+			err = -EINVAL;
+			goto out;
+		}
+
 		uh = (void *)(skb->data + offset);
 		th = (void *)(skb->data + offset);
 		hdr_len += offset;

-- 
2.27.0

Hello:

This patch was applied to netdev/net.git (master)
by David S. Miller [off-list ref]:

On Wed, 17 Nov 2021 20:06:48 +0100 you wrote:

The offset value is used in pointer math on skb->data.
Since ipv6_skip_exthdr may return -1 the pointer to uh and th
may not point to the actual udp and tcp headers and potentially
overwrite other stuff. This is why I think this should be checked.

EDIT:  added {}'s, thanks Kees

[...]

Here is the summary with links:
  - [v2] ipv6: check return value of ipv6_skip_exthdr
    https://git.kernel.org/netdev/net/c/5f9c55c8066b

You are awesome, thank you!
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