netdev - Re: [PATCH net v2] net: correct zerocopy refcnt with udp MSG

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Message-Id: <20190530.155457.207974832129262644.davem@davemloft.net>
Date:   Thu, 30 May 2019 15:54:57 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     willemdebruijn.kernel@...il.com
Cc:     netdev@...r.kernel.org, edumazet@...gle.com, willemb@...gle.com,
        syzkaller@...glegroups.com
Subject: Re: [PATCH net v2] net: correct zerocopy refcnt with udp MSG_MORE

From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Date: Thu, 30 May 2019 18:01:21 -0400

> From: Willem de Bruijn <willemb@...gle.com>
> 
> TCP zerocopy takes a uarg reference for every skb, plus one for the
> tcp_sendmsg_locked datapath temporarily, to avoid reaching refcnt zero
> as it builds, sends and frees skbs inside its inner loop.
> 
> UDP and RAW zerocopy do not send inside the inner loop so do not need
> the extra sock_zerocopy_get + sock_zerocopy_put pair. Commit
> 52900d22288ed ("udp: elide zerocopy operation in hot path") introduced
> extra_uref to pass the initial reference taken in sock_zerocopy_alloc
> to the first generated skb.
> 
> But, sock_zerocopy_realloc takes this extra reference at the start of
> every call. With MSG_MORE, no new skb may be generated to attach the
> extra_uref to, so refcnt is incorrectly 2 with only one skb.
> 
> Do not take the extra ref if uarg && !tcp, which implies MSG_MORE.
> Update extra_uref accordingly.
> 
> This conditional assignment triggers a false positive may be used
> uninitialized warning, so have to initialize extra_uref at define.
> 
> Changes v1->v2: fix typo in Fixes SHA1
> 
> Fixes: 52900d22288e7 ("udp: elide zerocopy operation in hot path")
> Reported-by: syzbot <syzkaller@...glegroups.com>
> Diagnosed-by: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>

Applied and queued up for -stable, thanks.