linux-kernel - Re: [PATCH net] ax25: use after free in ax25

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Date:   Tue, 11 Jan 2022 12:56:52 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Hangyu Hua <hbh25y@...il.com>, jreuter@...na.de,
        ralf@...ux-mips.org, davem@...emloft.net, kuba@...nel.org
Cc:     linux-hams@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] ax25: use after free in ax25_connect


On 1/10/22 20:20, Hangyu Hua wrote:
> sk_to_ax25(sk) needs to be called after lock_sock(sk) to avoid UAF
> caused by a race condition.

Can you describe what race condition you have found exactly ?

sk pointer can not change.


> Signed-off-by: Hangyu Hua <hbh25y@...il.com>
> ---
>   net/ax25/af_ax25.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
> index cfca99e295b8..c5d62420a2a8 100644
> --- a/net/ax25/af_ax25.c
> +++ b/net/ax25/af_ax25.c
> @@ -1127,7 +1127,7 @@ static int __must_check ax25_connect(struct socket *sock,
>   	struct sockaddr *uaddr, int addr_len, int flags)
>   {
>   	struct sock *sk = sock->sk;
> -	ax25_cb *ax25 = sk_to_ax25(sk), *ax25t;
> +	ax25_cb *ax25, *ax25t;
>   	struct full_sockaddr_ax25 *fsa = (struct full_sockaddr_ax25 *)uaddr;
>   	ax25_digi *digi = NULL;
>   	int ct = 0, err = 0;
> @@ -1155,6 +1155,8 @@ static int __must_check ax25_connect(struct socket *sock,
>   
>   	lock_sock(sk);
>   
> +	ax25 = sk_to_ax25(sk);
> +
>   	/* deal with restarts */
>   	if (sock->state == SS_CONNECTING) {
>   		switch (sk->sk_state) {