Re: [PATCH] net: ipv6: fix invalid alloclen in __ip6_append_data

[Tadeusz Struk <tadeusz.struk@linaro.org>]

On 3/10/22 09:32, Willem de Bruijn wrote:
> On Thu, Mar 10, 2022 at 11:06 AM Tadeusz Struk <tadeusz.struk@linaro.org> wrote:
>>
>> On 3/10/22 06:39, Willem de Bruijn wrote:
>>> On Wed, Mar 9, 2022 at 4:37 PM Tadeusz Struk <tadeusz.struk@linaro.org> wrote:
>>>>
>>>> On 3/8/22 21:01, David Ahern wrote:
>>>>> On 3/8/22 12:46 PM, Tadeusz Struk wrote:
>>>>>> That fails in the same way:
>>>>>>
>>>>>> skbuff: skb_over_panic: text:ffffffff83e7b48b len:65575 put:65575
>>>>>> head:ffff888101f8a000 data:ffff888101f8a088 tail:0x100af end:0x6c0
>>>>>> dev:<NULL>
>>>>>> ------------[ cut here ]------------
>>>>>> kernel BUG at net/core/skbuff.c:113!
>>>>>> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
>>>>>> CPU: 0 PID: 1852 Comm: repro Not tainted
>>>>>> 5.17.0-rc7-00020-gea4424be1688-dirty #19
>>>>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1.fc35
>>>>>> RIP: 0010:skb_panic+0x173/0x175
>>>>>>
>>>>>> I'm not sure how it supposed to help since it doesn't change the
>>>>>> alloclen at all.
>>>>>
>>>>> alloclen is a function of fraglen and fraglen is a function of datalen.
>>>>
>>>> Ok, but in this case it doesn't affect the alloclen and it still fails.
>>>
>>> This is some kind of non-standard packet that is being constructed. Do
>>> we understand how it is different?
>>>
>>> The .syz reproducer is generally a bit more readable than the .c
>>> equivalent. Though not as much as an strace of the binary, if you
>>> can share that.
>>>
>>> r0 = socket$inet6_icmp_raw(0xa, 0x3, 0x3a)
>>> connect$inet6(r0, &(0x7f0000000040)={0xa, 0x0, 0x0, @dev, 0x6}, 0x1c)
>>> setsockopt$inet6_IPV6_HOPOPTS(r0, 0x29, 0x36,
>>> &(0x7f0000000100)=ANY=[@ANYBLOB="52b3"], 0x5a0)
>>> sendmmsg$inet(r0, &(0x7f00000002c0)=[{{0x0, 0x0,
>>> &(0x7f0000000000)=[{&(0x7f00000000c0)="1d2d", 0xfa5f}], 0x1}}], 0x1,
>>> 0xfe80)
>>
>> Here it is:
>> https://termbin.com/krtr
>> It won't be of much help, I'm afraid, as the offending sendmmsg()
>> call isn't fully printed.
> 
> Thanks. It does offer some hints on the other two syscalls:
> 
> [pid   644] connect(3, {sa_family=AF_INET6, sin6_port=htons(0),
> sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "fe80::", &sin6_addr),
> sin6_scope_id=if_nametoindex("tunl0")}, 28) = 0
> [pid   644] setsockopt(3, SOL_IPV6, IPV6_HOPOPTS,
> "R\263\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 1440) = 0
> 
> IPV6_HOPOPTS is ns_capable CAP_NET_RAW.
> 
> So this adds 1440 bytes to opt_nflen, which is included in
> fragheaderlen, causing that to be exactly mtu. This means that the
> payload can never be sent, as each fragment header eats up the entire
> mtu? This is without any transport headers that would only be part of
> the first fragment (which go into opt_flen).
> 
> If you can maybe catch the error before the skb_put and just return
> EINVAL, we might see whether sendmmsg is relevant or a simple send
> would be equivalent. (not super important, that appears unrelated.)

Do you mean something like this:
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 622345af323e..6d45112322a0 100644
@@ -1656,6 +1649,16 @@ static int __ip6_append_data(struct sock *sk,
                         skb->protocol = htons(ETH_P_IPV6);
                         skb->ip_summed = csummode;
                         skb->csum = 0;
+
+                       /*
+                        *      Check if there is still room for the payload
+                        */
+                       if (fragheaderlen >= mtu) {
+                               err = -EMSGSIZE;
+                               kfree_skb(skb);
+                               goto error;
+                       }
+
                         /* reserve for fragmentation and ipsec header */
                         skb_reserve(skb, hh_len + sizeof(struct frag_hdr) +
                                     dst_exthdrlen);


That works as well.
-- 
Thanks,
Tadeusz