Re: [PATCH] hvc/xen: prevent concurrent accesses to the shared ring

[Stefano Stabellini]

On Tue, 29 Nov 2022, Roger Pau Monne wrote:
> The hvc machinery registers both a console and a tty device based on
> the hv ops provided by the specific implementation.  Those two
> interfaces however have different locks, and there's no single locks
> that's shared between the tty and the console implementations, hence
> the driver needs to protect itself against concurrent accesses.
> Otherwise concurrent calls using the split interfaces are likely to
> corrupt the ring indexes, leaving the console unusable.
>
> Introduce a lock to xencons_info to serialize accesses to the shared
> ring.  This is only required when using the shared memory console,
> concurrent accesses to the hypercall based console implementation are
> not an issue.
> 
> Note the conditional logic in domU_read_console() is slightly modified
> so the notify_daemon() call can be done outside of the locked region:
> it's an hypercall and there's no need for it to be done with the lock
> held.
> 
> Fixes: b536b4b96230 ('xen: use the hvc console infrastructure for Xen console')
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> While the write handler (domU_write_console()) is used by both the
> console and the tty ops, that's not the case for the read side
> (domU_read_console()).  It's not obvious to me whether we could get
> concurrent poll calls from the poll_get_char tty hook, hence stay on
> the safe side also serialize read accesses in domU_read_console().

I think domU_read_console doesn't need it. struct hv_ops and struct
console are both already locked although independently locked.

I think we shouldn't add an unrequired lock there.


> ---
>  drivers/tty/hvc/hvc_xen.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
> index 7c23112dc923..d65741983837 100644
> --- a/drivers/tty/hvc/hvc_xen.c
> +++ b/drivers/tty/hvc/hvc_xen.c
> @@ -43,6 +43,7 @@ struct xencons_info {
>  	int irq;
>  	int vtermno;
>  	grant_ref_t gntref;
> +	spinlock_t ring_lock;
>  };
>  
>  static LIST_HEAD(xenconsoles);
> @@ -84,12 +85,15 @@ static int __write_console(struct xencons_info *xencons,
>  	XENCONS_RING_IDX cons, prod;
>  	struct xencons_interface *intf = xencons->intf;
>  	int sent = 0;
> +	unsigned long flags;
>  
> +	spin_lock_irqsave(&xencons->ring_lock, flags);
>  	cons = intf->out_cons;
>  	prod = intf->out_prod;
>  	mb();			/* update queue values before going on */
>  
>  	if ((prod - cons) > sizeof(intf->out)) {
> +		spin_unlock_irqrestore(&xencons->ring_lock, flags);
>  		pr_err_once("xencons: Illegal ring page indices");
>  		return -EINVAL;
>  	}
> @@ -99,6 +103,7 @@ static int __write_console(struct xencons_info *xencons,
>  
>  	wmb();			/* write ring before updating pointer */
>  	intf->out_prod = prod;
> +	spin_unlock_irqrestore(&xencons->ring_lock, flags);
>  
>  	if (sent)
>  		notify_daemon(xencons);
> @@ -141,16 +146,19 @@ static int domU_read_console(uint32_t vtermno, char *buf, int len)
>  	int recv = 0;
>  	struct xencons_info *xencons = vtermno_to_xencons(vtermno);
>  	unsigned int eoiflag = 0;
> +	unsigned long flags;
>  
>  	if (xencons == NULL)
>  		return -EINVAL;
>  	intf = xencons->intf;
>  
> +	spin_lock_irqsave(&xencons->ring_lock, flags);
>  	cons = intf->in_cons;
>  	prod = intf->in_prod;
>  	mb();			/* get pointers before reading ring */
>  
>  	if ((prod - cons) > sizeof(intf->in)) {
> +		spin_unlock_irqrestore(&xencons->ring_lock, flags);
>  		pr_err_once("xencons: Illegal ring page indices");
>  		return -EINVAL;
>  	}
> @@ -174,10 +182,13 @@ static int domU_read_console(uint32_t vtermno, char *buf, int len)
>  		xencons->out_cons = intf->out_cons;
>  		xencons->out_cons_same = 0;
>  	}
> +	if (!recv && xencons->out_cons_same++ > 1) {
> +		eoiflag = XEN_EOI_FLAG_SPURIOUS;
> +	}
> +	spin_unlock_irqrestore(&xencons->ring_lock, flags);
> +
>  	if (recv) {
>  		notify_daemon(xencons);
> -	} else if (xencons->out_cons_same++ > 1) {
> -		eoiflag = XEN_EOI_FLAG_SPURIOUS;
>  	}
>  
>  	xen_irq_lateeoi(xencons->irq, eoiflag);
> @@ -576,6 +587,7 @@ static int __init xen_hvc_init(void)
>  
>  		info = vtermno_to_xencons(HVC_COOKIE);
>  		info->irq = bind_evtchn_to_irq_lateeoi(info->evtchn);
> +		spin_lock_init(&info->ring_lock);

Don't we also need a call to spin_lock_init in xencons_connect_backend
and xen_cons_init and xenboot_console_setup ?


>  	}
>  	if (info->irq < 0)
>  		info->irq = 0; /* NO_IRQ */
> -- 
> 2.37.3
>