Re: [PATCH v2] hvc/xen: prevent concurrent accesses to the shared ring

From: Stefano Stabellini
Date: Wed Nov 30 2022 - 20:08:18 EST


On Wed, 30 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.

For domU_read_console: I don't mean to block this patch but we need to
be sure about the semantics of hv_ops.get_chars. Either it is expected
to be already locked, then we definitely shouldn't add another lock to
domU_read_console. Or it is not expected to be already locked, then we
should add the lock.

My impression is that it is expected to be already locked, but I think
we need Greg or Jiri to confirm one way or the other.

Aside from that the rest looks fine.



> Fixes: b536b4b96230 ('xen: use the hvc console infrastructure for Xen console')
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> Changes since v1:
> - Properly initialize the introduced lock in all paths.
> ---
> drivers/tty/hvc/hvc_xen.c | 19 +++++++++++++++++--
> 1 file changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
> index 7c23112dc923..e63c1761a361 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);
> @@ -234,6 +245,7 @@ static int xen_hvm_console_init(void)
> info = kzalloc(sizeof(struct xencons_info), GFP_KERNEL);
> if (!info)
> return -ENOMEM;
> + spin_lock_init(&info->ring_lock);
> } else if (info->intf != NULL) {
> /* already configured */
> return 0;
> @@ -270,6 +282,7 @@ static int xen_hvm_console_init(void)
>
> static int xencons_info_pv_init(struct xencons_info *info, int vtermno)
> {
> + spin_lock_init(&info->ring_lock);
> info->evtchn = xen_start_info->console.domU.evtchn;
> /* GFN == MFN for PV guest */
> info->intf = gfn_to_virt(xen_start_info->console.domU.mfn);
> @@ -318,6 +331,7 @@ static int xen_initial_domain_console_init(void)
> info = kzalloc(sizeof(struct xencons_info), GFP_KERNEL);
> if (!info)
> return -ENOMEM;
> + spin_lock_init(&info->ring_lock);
> }
>
> info->irq = bind_virq_to_irq(VIRQ_CONSOLE, 0, false);
> @@ -472,6 +486,7 @@ static int xencons_probe(struct xenbus_device *dev,
> info = kzalloc(sizeof(struct xencons_info), GFP_KERNEL);
> if (!info)
> return -ENOMEM;
> + spin_lock_init(&info->ring_lock);
> dev_set_drvdata(&dev->dev, info);
> info->xbdev = dev;
> info->vtermno = xenbus_devid_to_vtermno(devid);
> --
> 2.37.3
>