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

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Date:   Tue, 6 Jun 2017 18:00:04 -0600
From:   David Ahern <dsahern@...il.com>
To:     Ben Greear <greearb@...delatech.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: Repeatable inet6_dump_fib crash in stock 4.12.0-rc4+

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) {