linux-kernel - Re: [PATCH] 9p/xen: Fix end of loop tests for list_for_each

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Date:   Mon, 26 Jul 2021 05:46:33 +0900
From:   asmadeus@...ewreck.org
To:     Harshvardhan Jha <harshvardhan.jha@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Cc:     ericvh@...il.com, lucho@...kov.net, davem@...emloft.net,
        kuba@...nel.org, v9fs-developer@...ts.sourceforge.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] 9p/xen: Fix end of loop tests for list_for_each_entry

Harshvardhan Jha wrote on Sun, Jul 25, 2021 at 11:21:03PM +0530:
> The list_for_each_entry() iterator, "priv" in this code, can never be
> NULL so the warning would never be printed.

hm? priv won't be NULL but priv->client won't be client, so it will
return -EINVAL alright in practice?

This does fix an invalid read after the list head, so there's a real
bug, but the commit message needs fixing.

> 
> Signed-off-by: Harshvardhan Jha <harshvardhan.jha@...cle.com>
> ---
> From static analysis.  Not tested.

+Stefano in To - I also can't test xen right now :/
This looks functional to me but if you have a bit of time to spare just
a mount test can't hurt.

> ---
>  net/9p/trans_xen.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
> index f4fea28e05da..3ec1a51a6944 100644
> --- a/net/9p/trans_xen.c
> +++ b/net/9p/trans_xen.c
> @@ -138,7 +138,7 @@ static bool p9_xen_write_todo(struct xen_9pfs_dataring *ring, RING_IDX size)
>  
>  static int p9_xen_request(struct p9_client *client, struct p9_req_t *p9_req)
>  {
> -	struct xen_9pfs_front_priv *priv = NULL;
> +	struct xen_9pfs_front_priv *priv;
>  	RING_IDX cons, prod, masked_cons, masked_prod;
>  	unsigned long flags;
>  	u32 size = p9_req->tc.size;
> @@ -151,7 +151,7 @@ static int p9_xen_request(struct p9_client *client, struct p9_req_t *p9_req)
>  			break;
>  	}
>  	read_unlock(&xen_9pfs_lock);
> -	if (!priv || priv->client != client)
> +	if (list_entry_is_head(priv, &xen_9pfs_devs, list))
>  		return -EINVAL;
>  
>  	num = p9_req->tc.tag % priv->num_rings;
-- 
Dominique