Re: [PATCH net] ipv4: fib: free fib_alias with kfree_rcu() on insert error path

[Ido Schimmel]

On Sat, Jul 04, 2026 at 10:14:21AM -0700, Weiming Shi wrote:
> fib_table_insert() publishes new_fa into the leaf's fa_list with
> fib_insert_alias() before calling the fib entry notifiers. When a
> notifier fails, the error path removes new_fa with fib_remove_alias()
> (hlist_del_rcu) and frees it right away with kmem_cache_free().
> 
> fib_table_lookup() walks that list under rcu_read_lock() only, so a
> concurrent lookup that already reached new_fa keeps reading it after the
> free:
> 
>  BUG: KASAN: slab-use-after-free in fib_table_lookup (net/ipv4/fib_trie.c:1601)
>  Read of size 1 at addr ffff88810676d4eb by task exploit/297
>  Call Trace:
>   fib_table_lookup (net/ipv4/fib_trie.c:1601)
>   ip_route_output_key_hash_rcu (net/ipv4/route.c:2814)
>   ip_route_output_key_hash (net/ipv4/route.c:2705)
>   __ip4_datagram_connect (net/ipv4/datagram.c:49)
>   udp_connect (net/ipv4/udp.c:2144)
>   __sys_connect (net/socket.c:2167)
>   __x64_sys_connect (net/socket.c:2173)
>   do_syscall_64
>   entry_SYSCALL_64_after_hwframe
>  which belongs to the cache ip_fib_alias of size 56
> 
> Triggering the error path needs CAP_NET_ADMIN and a registered fib
> notifier that can reject a route; a netdevsim device whose IPv4 FIB
> resource is exhausted is enough.
> 
> Free new_fa with alias_free_mem_rcu(), as fib_table_delete() already
> does for a fib_alias removed from the trie.
> 
> Fixes: a6c76c17df02 ("ipv4: Notify route after insertion to the routing table")
> Reported-by: Xiang Mei <xmei5@xxxxxxx>
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Weiming Shi <bestswngs@xxxxxxxxx>

Reviewed-by: Ido Schimmel <idosch@xxxxxxxxxx>