Re: [net-next v2] tcp: Explicitly mark reserved field in tcp_zerocopy_receive args.

[David Ahern <dsahern@gmail.com>]

On 2/7/21 10:49 AM, David Ahern wrote:
> On 2/6/21 1:36 PM, Arjun Roy wrote:
>> From: Arjun Roy <arjunroy@google.com>
>>
>> Explicitly define reserved field and require it to be 0-valued.
>>
>> Fixes: 7eeba1706eba ("tcp: Add receive timestamp support for receive zerocopy.")
>> Signed-off-by: Arjun Roy <arjunroy@google.com>
>> Signed-off-by: Eric Dumazet <edumazet@google.com>
>> Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
>> Suggested-by: David Ahern <dsahern@gmail.com>
>> Suggested-by: Leon Romanovsky <leon@kernel.org>
>> Suggested-by: Jakub Kicinski <kuba@kernel.org>
>> ---
>>  include/uapi/linux/tcp.h | 2 +-
>>  net/ipv4/tcp.c           | 2 ++
>>  2 files changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
>> index 42fc5a640df4..8fc09e8638b3 100644
>> --- a/include/uapi/linux/tcp.h
>> +++ b/include/uapi/linux/tcp.h
>> @@ -357,6 +357,6 @@ struct tcp_zerocopy_receive {
>>  	__u64 msg_control; /* ancillary data */
>>  	__u64 msg_controllen;
>>  	__u32 msg_flags;
>> -	/* __u32 hole;  Next we must add >1 u32 otherwise length checks fail. */
>> +	__u32 reserved; /* set to 0 for now */
>>  };
>>  #endif /* _UAPI_LINUX_TCP_H */
>> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
>> index e1a17c6b473c..c8469c579ed8 100644
>> --- a/net/ipv4/tcp.c
>> +++ b/net/ipv4/tcp.c
>> @@ -4159,6 +4159,8 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
>>  		}
>>  		if (copy_from_user(&zc, optval, len))
>>  			return -EFAULT;
>> +		if (zc.reserved)
>> +			return -EINVAL;
>>  		lock_sock(sk);
>>  		err = tcp_zerocopy_receive(sk, &zc, &tss);
>>  		release_sock(sk);
>>
> 
> 
> The 'switch (len)' statement needs to be updated now that 'len' is not
> going to end on the 'msg_flags' boundary? But then, how did that work
> before if there was 4 byte padding?
> 
> Maybe I am missing something here. You currently have:
> 
> 	switch (len) {
> 	case offsetofend(struct tcp_zerocopy_receive, msg_flags):
> 

Ah, I missed the lines before it:

                if (len >= offsetofend(struct tcp_zerocopy_receive,
msg_flags))
                        goto zerocopy_rcv_cmsg;


Also, I see a check on zc.msg_flags for a specific flag option being
set. What about other invalid bits in msg_flags? I do not see a check like:

#define TCP_VALID_ZC_MSG_FLAGS   (TCP_CMSG_TS)

	if (zc.msg_flags & ~(TCP_VALID_ZC_MSG_FLAGS))
		return -EINVAL;