[PATCH 06/12] libxenguest: guard against overflow from too large p2m when checkpointing

[Jan Beulich <jbeulich@xxxxxxxx>]

struct xc_sr_record's length field has just 32 bits. Fill it early and
check that the calculated value hasn't overflowed. Additionally check
for counter overflow early - there's no point even trying to allocate
any memory in such an event.

While there also limit an induction variable's type to unsigned long:
There's no gain from it being uint64_t.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Of course looping over test_bit() is pretty inefficient, but given that
I have no idea how to test this code I wanted to restrict changes to
what can sensibly be seen as no worse than before from just looking at
the changes.

--- a/tools/libs/guest/xg_sr_restore.c
+++ b/tools/libs/guest/xg_sr_restore.c
@@ -450,7 +450,8 @@ static int send_checkpoint_dirty_pfn_lis
     xc_interface *xch = ctx->xch;
     int rc = -1;
     unsigned int count, written;
-    uint64_t i, *pfns = NULL;
+    unsigned long i;
+    uint64_t *pfns = NULL;
     struct iovec *iov = NULL;
     struct xc_sr_record rec = {
         .type = REC_TYPE_CHECKPOINT_DIRTY_PFN_LIST,
@@ -469,16 +470,28 @@ static int send_checkpoint_dirty_pfn_lis
 
     for ( i = 0, count = 0; i < ctx->restore.p2m_size; i++ )
     {
-        if ( test_bit(i, dirty_bitmap) )
-            count++;
+        if ( test_bit(i, dirty_bitmap) && !++count )
+            break;
     }
 
+    if ( i < ctx->restore.p2m_size )
+    {
+        ERROR("Too many dirty pfns");
+        goto err;
+    }
+
+    rec.length = count * sizeof(*pfns);
+    if ( rec.length / sizeof(*pfns) != count )
+    {
+        ERROR("Too many (%u) dirty pfns", count);
+        goto err;
+    }
 
-    pfns = malloc(count * sizeof(*pfns));
+    pfns = malloc(rec.length);
     if ( !pfns )
     {
-        ERROR("Unable to allocate %zu bytes of memory for dirty pfn list",
-              count * sizeof(*pfns));
+        ERROR("Unable to allocate %u bytes of memory for dirty pfn list",
+              rec.length);
         goto err;
     }
 
@@ -504,8 +517,6 @@ static int send_checkpoint_dirty_pfn_lis
         goto err;
     }
 
-    rec.length = count * sizeof(*pfns);
-
     iov[0].iov_base = &rec.type;
     iov[0].iov_len = sizeof(rec.type);
 
@@ -513,7 +524,7 @@ static int send_checkpoint_dirty_pfn_lis
     iov[1].iov_len = sizeof(rec.length);
 
     iov[2].iov_base = pfns;
-    iov[2].iov_len = count * sizeof(*pfns);
+    iov[2].iov_len = rec.length;
 
     if ( writev_exact(ctx->restore.send_back_fd, iov, 3) )
     {