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SubjectRe: [PATCH] net: tun: Fix use-after-free in tun_detach()
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Hi Eric,

On Sat, 19 Nov 2022 10:31:38 -0800, Eric Dumazet wrote:
> On Fri, Nov 18, 2022 at 11:56 PM Shigeru Yoshida <syoshida@redhat.com> wrote:
>>
>> syzbot reported use-after-free in tun_detach() [1]. This causes call
>> trace like below:
>>
>> ==================================================================
>> BUG: KASAN: use-after-free in notifier_call_chain+0x1da/0x1e0
>> ...
>> Call Trace:
>
> Please include a symbolic stack trace, I think syzbot has them.

Thank you so much for your comment. I got it.

>> <TASK>
>> dump_stack_lvl+0x100/0x178
>> print_report+0x167/0x470
>> ? __virt_addr_valid+0x5e/0x2d0
>> ? __phys_addr+0xc6/0x140
>> ? notifier_call_chain+0x1da/0x1e0
>> ? notifier_call_chain+0x1da/0x1e0
>> kasan_report+0xbf/0x1e0
>> ? notifier_call_chain+0x1da/0x1e0
>> notifier_call_chain+0x1da/0x1e0
>> call_netdevice_notifiers_info+0x83/0x130
>> netdev_run_todo+0xc33/0x11b0
>> ? generic_xdp_install+0x490/0x490
>> ? __tun_detach+0x1500/0x1500
>> tun_chr_close+0xe2/0x190
>> __fput+0x26a/0xa40
>> task_work_run+0x14d/0x240
>> ? task_work_cancel+0x30/0x30
>> do_exit+0xb31/0x2a40
>> ? reacquire_held_locks+0x4a0/0x4a0
>> ? do_raw_spin_lock+0x12e/0x2b0
>> ? mm_update_next_owner+0x7c0/0x7c0
>> ? rwlock_bug.part.0+0x90/0x90
>> ? lockdep_hardirqs_on_prepare+0x17f/0x410
>> do_group_exit+0xd4/0x2a0
>> __x64_sys_exit_group+0x3e/0x50
>> do_syscall_64+0x38/0xb0
>> entry_SYSCALL_64_after_hwframe+0x63/0xcd
>>
>> The cause of the issue is that sock_put() from __tun_detach() drops
>> last reference count for struct net, and then notifier_call_chain()
>> from netdev_state_change() accesses that struct net.
>>
>> This patch fixes the issue by calling sock_put() from tun_detach()
>> after all necessary accesses for the struct net has done.
>>
>> Link: https://syzkaller.appspot.com/bug?id=96eb7f1ce75ef933697f24eeab928c4a716edefe [1]
>> Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
>
> Please add a Fixes: tag, once you identified which commit added this bug.

I got it too. I'll send v2 patch.

Thanks,
Shigeru

>> ---
>> drivers/net/tun.c | 6 +++++-
>> 1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index 7a3ab3427369..ce9fcf4c8ef4 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -686,7 +686,6 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
>> if (tun)
>> xdp_rxq_info_unreg(&tfile->xdp_rxq);
>> ptr_ring_cleanup(&tfile->tx_ring, tun_ptr_free);
>> - sock_put(&tfile->sk);
>> }
>> }
>>
>> @@ -702,6 +701,11 @@ static void tun_detach(struct tun_file *tfile, bool clean)
>> if (dev)
>> netdev_state_change(dev);
>> rtnl_unlock();
>> +
>> + if (clean) {
>> + synchronize_rcu();
>> + sock_put(&tfile->sk);
>> + }
>> }
>>
>> static void tun_detach_all(struct net_device *dev)
>> --
>> 2.38.1
>>
>

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