Re: [PATCH net 3/3] sch_cake: Fix out of bounds when parsing TCP options

Maxim Mikityanskiy [off-list ref] writes:

On 2021-06-10 00:51, Toke Høiland-Jørgensen wrote:

quoted

Maxim Mikityanskiy [off-list ref] writes:

quoted

The TCP option parser in cake qdisc (cake_get_tcpopt and
cake_tcph_may_drop) could read one byte out of bounds. When the length
is 1, the execution flow gets into the loop, reads one byte of the
opcode, and if the opcode is neither TCPOPT_EOL nor TCPOPT_NOP, it reads
one more byte, which exceeds the length of 1.

This fix is inspired by commit 9609dad263f8 ("ipv4: tcp_input: fix stack
out of bounds when parsing TCP options.").

Cc: Young Xiao <redacted>
Fixes: 8b7138814f29 ("sch_cake: Add optional ACK filter")
Signed-off-by: Maxim Mikityanskiy <redacted>

Thanks for fixing this!

Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>

Could you also review whether Florian's comment on patch 1 is relevant 
to this patch too? I have concerns about cake_get_tcphdr, which returns 
`skb_header_pointer(skb, offset, min(__tcp_hdrlen(tcph), bufsize), 
buf)`. Although I don't see a way for it to get out of bounds (it will 
read garbage instead of TCP header in the worst case), such code doesn't 
look robust.

It's not possible for it to get out of bounds, because there is a call 
to skb_header_pointer above with sizeof(_tcph), which ensures that the 
SKB has at least 20 bytes after the beginning of the TCP header, which 
means that the second skb_header_pointer will either point to SKB (where 
we have at least 20 bytes) or to buf (which is allocated by the caller, 
so the caller shouldn't overflow its own buffer).

On the other hand, parsing garbage doesn't look like a valid behavior 
compared to dropping/ignoring/whatever-cake-does-with-bad-packets, so we 
may want to handle it, for example:

          return skb_header_pointer(skb, offset,
-                                  min(__tcp_hdrlen(tcph), bufsize), buf);
+                                  min(max(sizeof(struct tcphdr), 
__tcp_hdrlen(tcph)), bufsize), buf);

What do you think? Or did I just miss some early check for doff?

No, I think your analysis is correct: It won't lead to any out-of-bounds
reads, but I suppose we could end up trying to parse garbage. However,
if we do get a packet that sets doff to an invalid value, and we try to
parse it, we're essentially parsing garbage anyway. So I think the fix
should rather be something like:

diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index 7d37638ee1c7..d312d75ab698 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c

@@ -943,7 +943,7 @@ static struct tcphdr *cake_get_tcphdr(const struct sk_buff *skb,
        }
 
        tcph = skb_header_pointer(skb, offset, sizeof(_tcph), &_tcph);
-       if (!tcph)
+       if (!tcph || tcph->doff < 5)
                return NULL;
 
        return skb_header_pointer(skb, offset,

(I realize it's egress path and the packets produced by the system 
itself are unlikely to have bad doff, but it's not impossible, for 
example, with AF_PACKET, BPF hooks in tc, etc.)

Most CAKE deployments primarily handles forwarded packets, and I suppose
malformed TCP packets could make it through the forwarding path as
well...

-Toke