[PATCH 1/3] Fix buffer overrun when suspending VMs with many vCPUs

[Owen Smith <owen.smith@xxxxxxxxx>]

Dynamically allocate the KDPC array when there are more active vCPUs that
would fit into the single PAGE_SIZE region that is pre-allocated.

__Section is defined as a PAGE_SIZE region, which can only contain a limited
number of KDPC objects in addition to the SYNC_CONTEXT header. When there is
insufficient space for a KDPC object per vCPU, dynamically allocate space for
the KDPC array. This prevents a buffer overrun when accessing indexes with
64 vCPUs. Dynamic allocation will allow this to expand to many vCPUs beyond
64.

Signed-off-by: Owen Smith <owen.smith@xxxxxxxxx>
---
 src/xenbus/sync.c | 50 ++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 47 insertions(+), 3 deletions(-)

diff --git a/src/xenbus/sync.c b/src/xenbus/sync.c
index fdfaf2d..4418c8e 100644
--- a/src/xenbus/sync.c
+++ b/src/xenbus/sync.c
@@ -96,12 +96,43 @@ typedef struct  _SYNC_CONTEXT {
     LONG            ProcessorCount;
     SYNC_REQUEST    Request;
     LONG            CompletionCount;
+    PKDPC           Dpcs;
     KDPC            Dpc[1];
 } SYNC_CONTEXT, *PSYNC_CONTEXT;
 
 static PSYNC_CONTEXT    SyncContext = (PVOID)__Section;
 static LONG             SyncOwner = -1;
 
+#define MAX_DPCS_PER_PAGE   ((PAGE_SIZE - sizeof(SYNC_CONTEXT)) / sizeof(KDPC))
+#define SYNC_POOL_TAG       'CNYS'
+
+static FORCEINLINE PVOID
+__SyncAllocate(
+    IN  ULONG   Size
+    )
+{
+    PVOID       Buffer;
+
+    if (Size == 0)
+        return NULL;
+
+    Buffer = ExAllocatePoolWithTag(NonPagedPool,
+                                   Size,
+                                   SYNC_POOL_TAG);
+    if (Buffer != NULL)
+        RtlZeroMemory(Buffer, Size);
+
+    return Buffer;
+}
+
+static FORCEINLINE VOID
+__SyncFree(
+    IN  PVOID   Buffer
+    )
+{
+    ExFreePoolWithTag(Buffer, SYNC_POOL_TAG);
+}
+
 static FORCEINLINE VOID
 __SyncAcquire(
     IN  LONG    Index
@@ -343,15 +374,23 @@ SyncCapture(
     Context->Late = Late;
 
     Context->CompletionCount = 0;
-    KeMemoryBarrier();
+
+    Context->Dpcs = &Context->Dpc[0];
 
     Context->ProcessorCount = 
KeQueryActiveProcessorCountEx(ALL_PROCESSOR_GROUPS);
+    if (Context->ProcessorCount >= MAX_DPCS_PER_PAGE) {
+        Context->Dpcs = __SyncAllocate(Context->ProcessorCount * sizeof(KDPC));
+        ASSERT(Context->Dpcs != NULL);
+    }
+
+    KeMemoryBarrier();
 
     for (Index = 0; Index < Context->ProcessorCount; Index++) {
-        PKDPC       Dpc = &Context->Dpc[Index];
+        PKDPC       Dpc = &Context->Dpcs[Index];
         NTSTATUS    status;
 
-        ASSERT3U((ULONG_PTR)(Dpc + 1), <, (ULONG_PTR)__Section + PAGE_SIZE);
+        // Imply Context->Dpcs == &Context->Dpc[0] ->
+        //  ASSERT3U((ULONG_PTR)(Dpc + 1), <, (ULONG_PTR)__Section + 
PAGE_SIZE);
 
         status = KeGetProcessorNumberFromIndex(Index, &ProcNumber);
         ASSERT(NT_SUCCESS(status));
@@ -489,6 +528,11 @@ SyncRelease(
 
     __SyncWait();
 
+    if (Context->ProcessorCount >= MAX_DPCS_PER_PAGE) {
+        ASSERT3P(Context->Dpcs, !=, &Context->Dpc[0]);
+        __SyncFree(Context->Dpcs);
+    }
+
     RtlZeroMemory(Context, PAGE_SIZE);
 
     __SyncRelease();
-- 
2.39.0.windows.1