linux-kernel - Re: [PATCH net v3] rxrpc: Fix use of Don't Fragment flag

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Message-ID: <fc91d5c566639a0b94fb15e8c76851d3c62fbb62.camel@redhat.com>
Date: Thu, 11 Jan 2024 12:58:43 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: David Howells <dhowells@...hat.com>, Marc Dionne
 <marc.dionne@...istor.com>
Cc: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>,  Jakub Kicinski <kuba@...nel.org>,
 linux-afs@...ts.infradead.org, netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v3] rxrpc: Fix use of Don't Fragment flag

On Tue, 2024-01-09 at 15:10 +0000, David Howells wrote:
>     
> rxrpc normally has the Don't Fragment flag set on the UDP packets it
> transmits, except when it has decided that DATA packets aren't getting
> through - in which case it turns it off just for the DATA transmissions.
> This can be a problem, however, for RESPONSE packets that convey
> authentication and crypto data from the client to the server as ticket may
> be larger than can fit in the MTU.
> 
> In such a case, rxrpc gets itself into an infinite loop as the sendmsg
> returns an error (EMSGSIZE), which causes rxkad_send_response() to return
> -EAGAIN - and the CHALLENGE packet is put back on the Rx queue to retry,
> leading to the I/O thread endlessly attempting to perform the transmission.
> 
> Fix this by disabling DF on RESPONSE packets for now.  The use of DF and
> best data MTU determination needs reconsidering at some point in the
> future.
> 
> Fixes: 17926a79320a ("[AF_RXRPC]: Provide secure RxRPC sockets for use by userspace and kernel both")
> Reported-by: Marc Dionne <marc.dionne@...istor.com>
> Signed-off-by: David Howells <dhowells@...hat.com>
> cc: "David S. Miller" <davem@...emloft.net>
> cc: Eric Dumazet <edumazet@...gle.com>
> cc: Jakub Kicinski <kuba@...nel.org>
> cc: Paolo Abeni <pabeni@...hat.com>
> cc: linux-afs@...ts.infradead.org
> cc: netdev@...r.kernel.org

Acked-by: Paolo Abeni <pabeni@...hat.com>