netdev - Re: [PATCH net-next v2] mptcp: be careful on MPTCP-level ack.

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Date:   Wed, 2 Dec 2020 14:18:53 +0100
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Cc:     Jakub Kicinski <kuba@...nel.org>, mptcp@...ts.01.org,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next v2] mptcp: be careful on MPTCP-level ack.



On 11/24/20 10:51 PM, Paolo Abeni wrote:
> We can enter the main mptcp_recvmsg() loop even when
> no subflows are connected. As note by Eric, that would
> result in a divide by zero oops on ack generation.
> 
> Address the issue by checking the subflow status before
> sending the ack.
> 
> Additionally protect mptcp_recvmsg() against invocation
> with weird socket states.
> 
> v1 -> v2:
>  - removed unneeded inline keyword - Jakub
> 
> Reported-and-suggested-by: Eric Dumazet <eric.dumazet@...il.com>
> Fixes: ea4ca586b16f ("mptcp: refine MPTCP-level ack scheduling")
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
>  net/mptcp/protocol.c | 67 ++++++++++++++++++++++++++++++++------------
>  1 file changed, 49 insertions(+), 18 deletions(-)
> 

Looking at mptcp recvmsg(), it seems that a read(fd, ..., 0) will
trigger an infinite loop if there is available data in receive queue ?

It seems the following is needed, commit ea4ca586b16f removed
a needed check to catch this condition.

Untested patch, I can submit it formally if you agree.

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 221f7cdd416bdb681968bf1b3ff2ed1b03cea3ce..57213ff60f784fae14c2a96f391ccdec6249c168 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1921,7 +1921,7 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
        len = min_t(size_t, len, INT_MAX);
        target = sock_rcvlowat(sk, flags & MSG_WAITALL, len);
 
-       for (;;) {
+       while (copied < len) {
                int bytes_read, old_space;
 
                bytes_read = __mptcp_recvmsg_mskq(msk, msg, len - copied);