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

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

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