linux-kernel - Re: [PATCH] hvc/xen: lock console list traversal

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Message-ID: <alpine.DEB.2.22.394.2211291416540.4039@ubuntu-linux-20-04-desktop>
Date:   Tue, 29 Nov 2022 14:25:12 -0800 (PST)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Roger Pau Monne <roger.pau@...rix.com>
cc:     linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Jan Beulich <jbeulich@...e.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] hvc/xen: lock console list traversal

On Tue, 29 Nov 2022, Roger Pau Monne wrote:
> The currently lockless access to the xen console list in
> vtermno_to_xencons() is incorrect, as additions and removals from the
> list can happen anytime, and as such the traversal of the list to get
> the private console data for a given termno needs to happen with the
> lock held.  Note users that modify the list already do so with the
> lock taken.
> 
> While there switch from using list_for_each_entry_safe to
> list_for_each_entry: the current entry cursor won't be removed as
> part of the code in the loop body, so using the _safe variant is
> pointless.
> 
> Fixes: 02e19f9c7cac ('hvc_xen: implement multiconsole support')
> Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
> ---
>  drivers/tty/hvc/hvc_xen.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
> index d65741983837..117dc48f980e 100644
> --- a/drivers/tty/hvc/hvc_xen.c
> +++ b/drivers/tty/hvc/hvc_xen.c
> @@ -53,17 +53,22 @@ static DEFINE_SPINLOCK(xencons_lock);
>  
>  static struct xencons_info *vtermno_to_xencons(int vtermno)
>  {
> -	struct xencons_info *entry, *n, *ret = NULL;
> +	struct xencons_info *entry, *ret = NULL;
> +	unsigned long flags;
>  
> -	if (list_empty(&xenconsoles))
> -			return NULL;
> +	spin_lock_irqsave(&xencons_lock, flags);

If xencons_lock requires irqsave then we need to change all the
xencons_lock spinlocks to call irqsave, including the ones in
xen_hvm_console_init if they can happen at runtime.


> +	if (list_empty(&xenconsoles)) {
> +		spin_unlock_irqrestore(&xencons_lock, flags);
> +		return NULL;
> +	}
>  
> -	list_for_each_entry_safe(entry, n, &xenconsoles, list) {
> +	list_for_each_entry(entry, &xenconsoles, list) {
>  		if (entry->vtermno == vtermno) {
>  			ret  = entry;
>  			break;
>  		}
>  	}
> +	spin_unlock_irqrestore(&xencons_lock, flags);
>  
>  	return ret;
>  }
> -- 
> 2.37.3
>