[PATCH net v3] rxrpc: Fix use of Don't Fragment flag

[Paolo Abeni]

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 at auristor.com>
> Signed-off-by: David Howells <dhowells at redhat.com>
> cc: "David S. Miller" <davem at davemloft.net>
> cc: Eric Dumazet <edumazet at google.com>
> cc: Jakub Kicinski <kuba at kernel.org>
> cc: Paolo Abeni <pabeni at redhat.com>
> cc: linux-afs at lists.infradead.org
> cc: netdev at vger.kernel.org

Acked-by: Paolo Abeni <pabeni at redhat.com>