netdev - Re: Repeatable inet6_dump_fib crash in stock 4.12.0-rc4+

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Message-ID: <1496795269.736.21.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Tue, 06 Jun 2017 17:27:49 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Ahern <dsahern@...il.com>
Cc:     Ben Greear <greearb@...delatech.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: Repeatable inet6_dump_fib crash in stock 4.12.0-rc4+

On Tue, 2017-06-06 at 18:00 -0600, David Ahern wrote:
> On 6/6/17 3:06 PM, Ben Greear wrote:
> > This bug has been around forever, and we recently got an intern and
> > stuck him with
> > trying to reproduce it on the latest kernel.  It is still here.  I'm not
> > super excited
> > about trying to fix this, but we can easily test patches if someone has a
> > patch to try.
> 
> Can you try this (whitespace damaged on paste, but it is moving the lock
> ahead of the fn_sernum check):
> 
> diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
> index deea901746c8..7a44c49055c0 100644
> --- a/net/ipv6/ip6_fib.c
> +++ b/net/ipv6/ip6_fib.c
> @@ -378,6 +378,7 @@ static int fib6_dump_table(struct fib6_table *table,
> struct sk_buff *skb,
>                         cb->args[5] = w->root->fn_sernum;
>                 }
>         } else {
> +               read_lock_bh(&table->tb6_lock);
>                 if (cb->args[5] != w->root->fn_sernum) {
>                         /* Begin at the root if the tree changed */
>                         cb->args[5] = w->root->fn_sernum;
> @@ -387,7 +388,6 @@ static int fib6_dump_table(struct fib6_table *table,
> struct sk_buff *skb,
>                 } else
>                         w->skip = 0;
> 
> -               read_lock_bh(&table->tb6_lock);
>                 res = fib6_walk_continue(w);
>                 read_unlock_bh(&table->tb6_lock);
>                 if (res <= 0) {


Good catch, but it looks like similar fix is needed a few lines before.

diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index deea901746c8570c5e801e40592c91e3b62812e0..b214443dc8346cef3690df7f27cc48a864028865 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -372,12 +372,13 @@ static int fib6_dump_table(struct fib6_table *table, struct sk_buff *skb,
 
 		read_lock_bh(&table->tb6_lock);
 		res = fib6_walk(net, w);
-		read_unlock_bh(&table->tb6_lock);
 		if (res > 0) {
 			cb->args[4] = 1;
 			cb->args[5] = w->root->fn_sernum;
 		}
+		read_unlock_bh(&table->tb6_lock);
 	} else {
+		read_lock_bh(&table->tb6_lock);
 		if (cb->args[5] != w->root->fn_sernum) {
 			/* Begin at the root if the tree changed */
 			cb->args[5] = w->root->fn_sernum;
@@ -387,7 +388,6 @@ static int fib6_dump_table(struct fib6_table *table, struct sk_buff *skb,
 		} else
 			w->skip = 0;
 
-		read_lock_bh(&table->tb6_lock);
 		res = fib6_walk_continue(w);
 		read_unlock_bh(&table->tb6_lock);
 		if (res <= 0) {