Subject: Re: [PATCH] net/tls: Fix slab-use-after-free in tls_encrypt_done

From: Simon Horman <[email-masked]>
Date: Mon, 16 Oct 2023 11:50:15 +0200

On Thu, Oct 12, 2023 at 07:02:51PM +0800, Juntong Deng wrote:
> In the current implementation, ctx->async_wait.completion is completed
> after spin_lock_bh, which causes tls_sw_release_resources_tx to
> continue executing and return to tls_sk_proto_cleanup, then return

Hi Juntong Deng,

I'm slightly confused by "causes tls_sw_release_resources_tx to  continue
executing".

What I see in tls_sw_release_resources_tx() is:

        /* Wait for any pending async encryptions to complete */   
        spin_lock_bh(&ctx->encrypt_compl_lock);
        ctx->async_notify = true;
        pending = atomic_read(&ctx->encrypt_pending);
        spin_unlock_bh(&ctx->encrypt_compl_lock);  

Am I wrong in thinking the above will block because
(the same) ctx->encrypt_compl_lock is held in tls_encrypt_done?

> to tls_sk_proto_close, and after that enter tls_sw_free_ctx_tx to kfree
> the entire struct tls_context (including ctx->encrypt_compl_lock).
> 
> Since ctx->encrypt_compl_lock has been freed, subsequent spin_unlock_bh
> will result in slab-use-after-free error. Due to SMP, even using
> spin_lock_bh does not prevent tls_sw_release_resources_tx from continuing
> on other CPUs. After tls_sw_release_resources_tx is woken up, there is no
> attempt to hold ctx->encrypt_compl_lock again, therefore everything
> described above is possible.
> 
> The fix is to put complete(&ctx->async_wait.completion) after
> spin_unlock_bh, making the release after the unlock. Since complete is
> only executed if pending is 0, which means this is the last record, there
> is no need to worry about race condition causing duplicate completes.
> 
> Reported-by: [email-masked]
> Closes: https://syzkaller.appspot.com/bug?extid=29c22ea2d6b2c5fd2eae
> Signed-off-by: Juntong Deng <[email-masked]>
> ---
>  net/tls/tls_sw.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
> index 270712b8d391..7abe5a6aa989 100644
> --- a/net/tls/tls_sw.c
> +++ b/net/tls/tls_sw.c
> @@ -441,6 +441,7 @@ static void tls_encrypt_done(void *data, int err)
>  	struct sk_msg *msg_en;
>  	bool ready = false;
>  	struct sock *sk;
> +	int async_notify;
>  	int pending;
>  
>  	msg_en = &rec->msg_encrypted;
> @@ -482,10 +483,11 @@ static void tls_encrypt_done(void *data, int err)
>  
>  	spin_lock_bh(&ctx->encrypt_compl_lock);
>  	pending = atomic_dec_return(&ctx->encrypt_pending);
> +	async_notify = ctx->async_notify;
> +	spin_unlock_bh(&ctx->encrypt_compl_lock);
>  
> -	if (!pending && ctx->async_notify)
> +	if (!pending && async_notify)
>  		complete(&ctx->async_wait.completion);
> -	spin_unlock_bh(&ctx->encrypt_compl_lock);
>  
>  	if (!ready)
>  		return;
> -- 
> 2.39.2
> 
>