[PULL 2/2] xen-mapcache: Avoid entry->lock overflow

[Anthony PERARD via <qemu-devel@nongnu.org>]

From: Ross Lagerwall <ross.lagerwall@citrix.com>

In some cases, a particular mapcache entry may be mapped 256 times
causing the lock field to wrap to 0. For example, this may happen when
using emulated NVME and the guest submits a large scatter-gather write.
At this point, the entry map be remapped causing QEMU to write the wrong
data or crash (since remap is not atomic).

Avoid this overflow by increasing the lock field to a uint32_t and also
detect it and abort rather than continuing regardless.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Message-Id: <20220124104450.152481-1-ross.lagerwall@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
---
 hw/i386/xen/xen-mapcache.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
index bd47c3d672..f2ef977963 100644
--- a/hw/i386/xen/xen-mapcache.c
+++ b/hw/i386/xen/xen-mapcache.c
@@ -52,7 +52,7 @@ typedef struct MapCacheEntry {
     hwaddr paddr_index;
     uint8_t *vaddr_base;
     unsigned long *valid_mapping;
-    uint8_t lock;
+    uint32_t lock;
 #define XEN_MAPCACHE_ENTRY_DUMMY (1 << 0)
     uint8_t flags;
     hwaddr size;
@@ -355,6 +355,12 @@ static uint8_t *xen_map_cache_unlocked(hwaddr phys_addr, hwaddr size,
     if (lock) {
         MapCacheRev *reventry = g_malloc0(sizeof(MapCacheRev));
         entry->lock++;
+        if (entry->lock == 0) {
+            fprintf(stderr,
+                    "mapcache entry lock overflow: "TARGET_FMT_plx" -> %p\n",
+                    entry->paddr_index, entry->vaddr_base);
+            abort();
+        }
         reventry->dma = dma;
         reventry->vaddr_req = mapcache->last_entry->vaddr_base + address_offset;
         reventry->paddr_index = mapcache->last_entry->paddr_index;
-- 
Anthony PERARD