Re: [PATCH net] netlink: fix potential deadlock in netlink_set_err()

[Jiri Pirko <jiri@resnulli.us>]

Wed, Jun 21, 2023 at 05:43:37PM CEST, edumazet@google.com wrote:
>syzbot reported a possible deadlock in netlink_set_err() [1]
>
>A similar issue was fixed in commit 1d482e666b8e ("netlink: disable IRQs
>for netlink_lock_table()") in netlink_lock_table()
>
>This patch adds IRQ safety to netlink_set_err() and __netlink_diag_dump()
>which were not covered by cited commit.
>
>[1]
>
>WARNING: possible irq lock inversion dependency detected
>6.4.0-rc6-syzkaller-00240-g4e9f0ec38852 #0 Not tainted
>
>syz-executor.2/23011 just changed the state of lock:
>ffffffff8e1a7a58 (nl_table_lock){.+.?}-{2:2}, at: netlink_set_err+0x2e/0x3a0 net/netlink/af_netlink.c:1612
>but this lock was taken by another, SOFTIRQ-safe lock in the past:
> (&local->queue_stop_reason_lock){..-.}-{2:2}
>
>and interrupts could create inverse lock ordering between them.
>
>other info that might help us debug this:
> Possible interrupt unsafe locking scenario:
>
>       CPU0                    CPU1
>       ----                    ----
>  lock(nl_table_lock);
>                               local_irq_disable();
>                               lock(&local->queue_stop_reason_lock);
>                               lock(nl_table_lock);
>  <Interrupt>
>    lock(&local->queue_stop_reason_lock);
>
> *** DEADLOCK ***
>
>Fixes: 1d482e666b8e ("netlink: disable IRQs for netlink_lock_table()")

I don't think that this "fixes" tag is correct. The referenced commit
is a fix to the same issue on a different codepath, not the one who
actually introduced the issue.

The code itself looks fine to me.