diff mbox series

[PULL,13/14] virtio-ccw: fix virtio_set_ind_atomic

Message ID 20200703100650.621212-14-cohuck@redhat.com
State New
Headers show
Series s390x update | expand

Commit Message

Cornelia Huck July 3, 2020, 10:06 a.m. UTC 
From: Halil Pasic <pasic@linux.ibm.com>

The atomic_cmpxchg() loop is broken because we occasionally end up with
old and _old having different values (a legit compiler can generate code
that accessed *ind_addr again to pick up a value for _old instead of
using the value of old that was already fetched according to the
rules of the abstract machine). This means the underlying CS instruction
may use a different old (_old) than the one we intended to use if
atomic_cmpxchg() performed the xchg part.

Let us use volatile to force the rules of the abstract machine for
accesses to *ind_addr. Let us also rewrite the loop so, we that the
new old is used to compute the new desired value if the xchg part
is not performed.

Fixes: 7e7494627f ("s390x/virtio-ccw: Adapter interrupt support.")
Reported-by: Andre Wild <Andre.Wild1@ibm.com>
Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20200616045035.51641-2-pasic@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
---
 hw/s390x/virtio-ccw.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

Comments

Halil Pasic Sept. 8, 2020, 9:34 a.m. UTC | #1 
On Fri,  3 Jul 2020 12:06:49 +0200
Cornelia Huck <cohuck@redhat.com> wrote:

> From: Halil Pasic <pasic@linux.ibm.com>
> 
> The atomic_cmpxchg() loop is broken because we occasionally end up with
> old and _old having different values (a legit compiler can generate code
> that accessed *ind_addr again to pick up a value for _old instead of
> using the value of old that was already fetched according to the
> rules of the abstract machine). This means the underlying CS instruction
> may use a different old (_old) than the one we intended to use if
> atomic_cmpxchg() performed the xchg part.
> 

[..]

I believe this fix should be considered for stable. Unfortunately we
didn't think about it back then.

Regards,
Halil
diff mbox series

Patch

diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
index c1f4bb1d331d..3c988a000b5b 100644
--- a/hw/s390x/virtio-ccw.c
+++ b/hw/s390x/virtio-ccw.c
@@ -786,9 +786,10 @@  static inline VirtioCcwDevice *to_virtio_ccw_dev_fast(DeviceState *d)
 static uint8_t virtio_set_ind_atomic(SubchDev *sch, uint64_t ind_loc,
                                      uint8_t to_be_set)
 {
-    uint8_t ind_old, ind_new;
+    uint8_t expected, actual;
     hwaddr len = 1;
-    uint8_t *ind_addr;
+    /* avoid  multiple fetches */
+    uint8_t volatile *ind_addr;
 
     ind_addr = cpu_physical_memory_map(ind_loc, &len, true);
     if (!ind_addr) {
@@ -796,14 +797,15 @@  static uint8_t virtio_set_ind_atomic(SubchDev *sch, uint64_t ind_loc,
                      __func__, sch->cssid, sch->ssid, sch->schid);
         return -1;
     }
+    actual = *ind_addr;
     do {
-        ind_old = *ind_addr;
-        ind_new = ind_old | to_be_set;
-    } while (atomic_cmpxchg(ind_addr, ind_old, ind_new) != ind_old);
-    trace_virtio_ccw_set_ind(ind_loc, ind_old, ind_new);
-    cpu_physical_memory_unmap(ind_addr, len, 1, len);
+        expected = actual;
+        actual = atomic_cmpxchg(ind_addr, expected, expected | to_be_set);
+    } while (actual != expected);
+    trace_virtio_ccw_set_ind(ind_loc, actual, actual | to_be_set);
+    cpu_physical_memory_unmap((void *)ind_addr, len, 1, len);
 
-    return ind_old;
+    return actual;
 }
 
 static void virtio_ccw_notify(DeviceState *d, uint16_t vector)