Re: [PATCH net] net-timestamp: avoid use-after-free in ip_recv_error

[David Miller <davem@davemloft.net>]

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Wed, 12 Apr 2017 19:24:35 -0400

> From: Willem de Bruijn <willemb@google.com>
> 
> Syzkaller reported a use-after-free in ip_recv_error at line
> 
>     info->ipi_ifindex = skb->dev->ifindex;
> 
> This function is called on dequeue from the error queue, at which
> point the device pointer may no longer be valid.
> 
> Save ifindex on enqueue in __skb_complete_tx_timestamp, when the
> pointer is valid or NULL. Store it in temporary storage skb->cb.
> 
> It is safe to reference skb->dev here, as called from device drivers
> or dev_queue_xmit. The exception is when called from tcp_ack_tstamp;
> in that case it is NULL and ifindex is set to 0 (invalid).
> 
> Do not return a pktinfo cmsg if ifindex is 0. This maintains the
> current behavior of not returning a cmsg if skb->dev was NULL.
> 
> On dequeue, the ipv4 path will cast from sock_exterr_skb to
> in_pktinfo. Both have ifindex as their first element, so no explicit
> conversion is needed. This is by design, introduced in commit
> 0b922b7a829c ("net: original ingress device index in PKTINFO"). For
> ipv6 ip6_datagram_support_cmsg converts to in6_pktinfo.
> 
> Fixes: 829ae9d61165 ("net-timestamp: allow reading recv cmsg on errqueue with origin tstamp")
> 
> Reported-by: Andrey Konovalov <andreyknvl@google.com>
> Signed-off-by: Willem de Bruijn <willemb@google.com>

Applied and queued up for -stable, thanks.

In the future please don't insert empty lines between the Fixes: and
other tags.

Thanks.