netdev - Re: [PATCH v1 net] netlink: Add sock_i_ino_irqsaved() for __netlink_diag

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Message-ID: <CANn89iL0n5Prem6Cjc6jkdAq6jm5AOYXWgn=i80UPsnNZE6WQw@mail.gmail.com>
Date: Mon, 26 Jun 2023 09:21:30 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org, 
	syzbot+5da61cf6a9bc1902d422@...kaller.appspotmail.com
Subject: Re: [PATCH v1 net] netlink: Add sock_i_ino_irqsaved() for __netlink_diag_dump().

On Sun, Jun 25, 2023 at 6:14 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> syzbot reported a warning in __local_bh_enable_ip(). [0]
>
> Commit 8d61f926d420 ("netlink: fix potential deadlock in
> netlink_set_err()") converted read_lock(&nl_table_lock) to
> read_lock_irqsave() in __netlink_diag_dump() to prevent a deadlock.
>
> However, __netlink_diag_dump() calls sock_i_ino() that uses
> read_lock_bh() and read_unlock_bh().  read_unlock_bh() finally
> enables BH even though it should stay disabled until the following
> read_unlock_irqrestore().
>
> Using read_lock() in sock_i_ino() would trigger a lockdep splat
> in another place that was fixed in commit f064af1e500a ("net: fix
> a lockdep splat"), so let's add another function that would be safe
> to use under BH disabled.
>
> [0]:
>
> Fixes: 8d61f926d420 ("netlink: fix potential deadlock in netlink_set_err()")
> Reported-by: syzbot+5da61cf6a9bc1902d422@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=5da61cf6a9bc1902d422
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> ---
>

Hi Kuniyuki, thanks for the fix, I mistakenly released this syzbot
bug/report the other day ;)

I wonder if we could use __sock_i_ino() instead of sock_i_ino_bh_disabled(),
and perhaps something like the following to have less copy/pasted code ?

diff --git a/net/core/sock.c b/net/core/sock.c
index 6e5662ca00fe5638881db11c71c46169d59a2746..146a83c50c5d329fee2e833c4f2ba29e896d7766
100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2550,13 +2550,25 @@ kuid_t sock_i_uid(struct sock *sk)
 }
 EXPORT_SYMBOL(sock_i_uid);

-unsigned long sock_i_ino(struct sock *sk)
+/* Must be called while interrupts are disabled. */
+unsigned long __sock_i_ino(struct sock *sk)
 {
        unsigned long ino;

-       read_lock_bh(&sk->sk_callback_lock);
+       read_lock(&sk->sk_callback_lock);
        ino = sk->sk_socket ? SOCK_INODE(sk->sk_socket)->i_ino : 0;
-       read_unlock_bh(&sk->sk_callback_lock);
+       read_unlock(&sk->sk_callback_lock);
+       return ino;
+}
+EXPORT_SYMBOL(__sock_i_ino);
+
+unsigned long sock_i_ino(struct sock *sk)
+{
+       unsigned long ino;
+
+       local_bh_disable();
+       ino = __sock_i_ino(sk);
+       local_bh_enable();
        return ino;
 }
 EXPORT_SYMBOL(sock_i_ino);