Re: [PATCH v2 net] ipv6: fib6: fix NULL deref in fib6_walk_continue() on multi-batch dump

[Kuniyuki Iwashima <kuniyu@google.com>]

From: Pengfei Zhang <zhangfeionline@gmail.com>
Date: Thu, 25 Jun 2026 12:41:01 +0800
> From: Pengfei Zhang <zhangpengfei16@xiaomi.com>
> 
> inet6_dump_fib() saves its progress in cb->args[1] as a positional
> index within the current hash chain.  Between batches the RTNL lock
> is released,

nit: RTNL has been removed from IPv6 FIB, simply say like

  Between batches, a concurrent fib6_new_table() can insert ...

> so a concurrent fib6_new_table() can insert a new table
> at the chain head, shifting all existing entries.  The saved index
> then lands on a different table, causing fib6_dump_table() to set
> w->root to the wrong table while w->node still points into the
> previous one.  fib6_walk_continue() dereferences w->node->parent
> (NULL) and panics:
> 
>   BUG: kernel NULL pointer dereference, address: 0000000000000008
>   RIP: 0010:fib6_walk_continue+0x6e/0x170
>   Call Trace:
>    <TASK>
>    fib6_dump_table.isra.0+0xc5/0x240
>    inet6_dump_fib+0xf6/0x420
>    rtnl_dumpit+0x30/0xa0
>    netlink_dump+0x15b/0x460
>    netlink_recvmsg+0x1d6/0x2a0
>    ____sys_recvmsg+0x17a/0x190
> 
> Fix by storing tb->tb6_id in cb->args[1] instead of a positional
> index.  On resume, skip entries until the id matches; a concurrent
> head-insert can never match the saved id, so the walker always
> resumes on the correct table.
> 
> Fixes: 1b43af5480c3 ("[IPV6]: Increase number of possible routing tables to 2^32")
> Signed-off-by: Pengfei Zhang <zhangfeionline@gmail.com>

SOB does not match the Author of the patch (the first From: line).


> ---
>  net/ipv6/ip6_fib.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
> index fc95738de..bda492634 100644
> --- a/net/ipv6/ip6_fib.c
> +++ b/net/ipv6/ip6_fib.c
> @@ -636,11 +636,11 @@ static int inet6_dump_fib(struct sk_buff *skb, struct netlink_callback *cb)
>  	};
>  	const struct nlmsghdr *nlh = cb->nlh;
>  	struct net *net = sock_net(skb->sk);
> -	unsigned int e = 0, s_e;
>  	struct hlist_head *head;
>  	struct fib6_walker *w;
>  	struct fib6_table *tb;
>  	unsigned int h, s_h;
> +	u32 s_id;

nit: please keep the reverse xmas tree order.
https://docs.kernel.org/7.1/process/maintainer-netdev.html#local-variable-ordering-reverse-xmas-tree-rcs


>  	int err = 0;
>  
>  	rcu_read_lock();
> @@ -701,23 +701,22 @@ static int inet6_dump_fib(struct sk_buff *skb, struct netlink_callback *cb)
>  	}
>  
>  	s_h = cb->args[0];
> -	s_e = cb->args[1];
> +	s_id = cb->args[1];
>  
> -	for (h = s_h; h < FIB6_TABLE_HASHSZ; h++, s_e = 0) {
> -		e = 0;
> +	for (h = s_h; h < FIB6_TABLE_HASHSZ; h++, s_id = 0) {
>  		head = &net->ipv6.fib_table_hash[h];
>  		hlist_for_each_entry_rcu(tb, head, tb6_hlist) {
> -			if (e < s_e)
> -				goto next;
> +			if (s_id && tb->tb6_id != s_id)
> +				continue;
> +			s_id = 0;
> +
> +			cb->args[1] = tb->tb6_id;
>  			err = fib6_dump_table(tb, skb, cb);
>  			if (err != 0)
>  				goto out;
> -next:
> -			e++;
>  		}
>  	}
>  out:
> -	cb->args[1] = e;
>  	cb->args[0] = h;
>  
>  unlock:
> -- 
> 2.34.1