Re: [PATCH] fixed rtnl deadlock from gtp

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Daniel Yang <danielyangkang@gmail.com>
Date: Mon, 30 Sep 2024 18:55:54 -0700
> Fixes deadlock described in this bug:
> https://syzkaller.appspot.com/bug?extid=e953a8f3071f5c0a28fd.
> Specific crash report here:
> https://syzkaller.appspot.com/text?tag=CrashReport&x=14670e07980000.
> 
> DESCRIPTION OF ISSUE
> Deadlock: sk_lock-AF_INET --> &smc->clcsock_release_lock --> rtnl_mutex
> 
> rtnl_mutex->sk_lock-AF_INET
> rtnetlink_rcv_msg() acquires rtnl_lock() and calls rtnl_newlink(), which
> eventually calls gtp_newlink() which calls lock_sock() to attempt to
> acquire sk_lock.

Is the deadlock real ?

From the lockdep splat, the gtp's sk_protocol is verified to be
IPPROTO_UDP before holding lock_sock(), so it seems just a labeling
issue.
https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/drivers/net/gtp.c?id=9410645520e9b820069761f3450ef6661418e279#n1674


> 
> sk_lock-AF_INET->&smc->clcsock_release_lock
> smc_sendmsg() calls lock_sock() to acquire sk_lock, then calls
> smc_switch_to_fallback() which attempts to acquire mutex_lock(&smc->...).
> 
> &smc->clcsock_release_lock->rtnl_mutex
> smc_setsockopt() calls mutex_lock(&smc->...). smc->...->setsockopt() is
> called, which calls nf_setsockopt() which attempts to acquire
> rtnl_lock() in some nested call in start_sync_thread() in ip_vs_sync.c.
> 
> FIX:
> In smc_switch_to_fallback(), separate the logic into inline function
> __smc_switch_to_fallback(). In smc_sendmsg(), lock ordering can be
> modified and the functionality of smc_switch_to_fallback() is
> encapsulated in the __smc_switch_to_fallback() function.