Re: [PATCH] net/dccp: fix use after free in tw_timer_handler()

[Dmitry Vyukov <dvyukov@google.com>]

On Tue, Feb 21, 2017 at 2:27 PM, Andrey Ryabinin
<aryabinin@virtuozzo.com> wrote:
> DCCP doesn't purge timewait sockets on network namespace shutdown.
> So, after net namespace destroyed we could still have an active timer
> which will trigger use after free in tw_timer_handler():
>
>     BUG: KASAN: use-after-free in tw_timer_handler+0x4a/0xa0 at addr ffff88010e0d1e10
>     Read of size 8 by task swapper/1/0
>     Call Trace:
>      __asan_load8+0x54/0x90
>      tw_timer_handler+0x4a/0xa0
>      call_timer_fn+0x127/0x480
>      expire_timers+0x1db/0x2e0
>      run_timer_softirq+0x12f/0x2a0
>      __do_softirq+0x105/0x5b4
>      irq_exit+0xdd/0xf0
>      smp_apic_timer_interrupt+0x57/0x70
>      apic_timer_interrupt+0x90/0xa0
>
>     Object at ffff88010e0d1bc0, in cache net_namespace size: 6848
>     Allocated:
>      save_stack_trace+0x1b/0x20
>      kasan_kmalloc+0xee/0x180
>      kasan_slab_alloc+0x12/0x20
>      kmem_cache_alloc+0x134/0x310
>      copy_net_ns+0x8d/0x280
>      create_new_namespaces+0x23f/0x340
>      unshare_nsproxy_namespaces+0x75/0xf0
>      SyS_unshare+0x299/0x4f0
>      entry_SYSCALL_64_fastpath+0x18/0xad
>     Freed:
>      save_stack_trace+0x1b/0x20
>      kasan_slab_free+0xae/0x180
>      kmem_cache_free+0xb4/0x350
>      net_drop_ns+0x3f/0x50
>      cleanup_net+0x3df/0x450
>      process_one_work+0x419/0xbb0
>      worker_thread+0x92/0x850
>      kthread+0x192/0x1e0
>      ret_from_fork+0x2e/0x40
>
> Add .exit_batch hook to dccp_v4_ops()/dccp_v6_ops() which will purge
> timewait sockets on net namespace destruction and prevent above issue.


Aha! Thanks for tracking this down!

Queued the patch on syzkaller bots, should have the verdict in several hours.



> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
> ---
>  net/dccp/ipv4.c | 6 ++++++
>  net/dccp/ipv6.c | 6 ++++++
>  2 files changed, 12 insertions(+)
>
> diff --git a/net/dccp/ipv4.c b/net/dccp/ipv4.c
> index d859a5c..da7cb16 100644
> --- a/net/dccp/ipv4.c
> +++ b/net/dccp/ipv4.c
> @@ -1018,9 +1018,15 @@ static void __net_exit dccp_v4_exit_net(struct net *net)
>         inet_ctl_sock_destroy(net->dccp.v4_ctl_sk);
>  }
>
> +static void __net_exit dccp_v4_exit_batch(struct list_head *net_exit_list)
> +{
> +       inet_twsk_purge(&dccp_hashinfo, &dccp_death_row, AF_INET);
> +}
> +
>  static struct pernet_operations dccp_v4_ops = {
>         .init   = dccp_v4_init_net,
>         .exit   = dccp_v4_exit_net,
> +       .exit_batch = dccp_v4_exit_batch,
>  };
>
>  static int __init dccp_v4_init(void)
> diff --git a/net/dccp/ipv6.c b/net/dccp/ipv6.c
> index c4e879c..f3d8f92 100644
> --- a/net/dccp/ipv6.c
> +++ b/net/dccp/ipv6.c
> @@ -1077,9 +1077,15 @@ static void __net_exit dccp_v6_exit_net(struct net *net)
>         inet_ctl_sock_destroy(net->dccp.v6_ctl_sk);
>  }
>
> +static void __net_exit dccp_v6_exit_batch(struct list_head *net_exit_list)
> +{
> +       inet_twsk_purge(&dccp_hashinfo, &dccp_death_row, AF_INET6);
> +}
> +
>  static struct pernet_operations dccp_v6_ops = {
>         .init   = dccp_v6_init_net,
>         .exit   = dccp_v6_exit_net,
> +       .exit_batch = dccp_v6_exit_batch,
>  };
>
>  static int __init dccp_v6_init(void)
> --
> 2.10.2
>