[PATCH 2/2] tools/migration: Fix potential overflow in send_checkpoint_dirty_pfn_list()

[Andrew Cooper <andrew.cooper3@citrix.com>]

'count * sizeof(*pfns)' can in principle overflow, but is implausible in
practice as the time between checkpoints is typically sub-second.
Nevertheless, simplify the code and remove the risk.

There is no need to loop over the bitmap to calculate count.  The number of
set bits is returned in xc_shadow_op_stats_t which is already collected (and
ignored).

Bounds check the count against what will fit in REC_LENGTH_MAX.  At the time
of writing, this allows up to 0xffffff pfns.  Rearrange the pfns loop to check
for errors both ways, not simply that there were more pfns than expected.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Ian Jackson <iwj@xenproject.org>
CC: Wei Liu <wl@xen.org>
CC: Juergen Gross <jgross@suse.com>
CC: Jan Beulich <JBeulich@suse.com>
CC: Olaf Hering <olaf@aepfle.de>
---
 tools/libs/guest/xg_sr_restore.c | 33 ++++++++++++++++-----------------
 1 file changed, 16 insertions(+), 17 deletions(-)

diff --git a/tools/libs/guest/xg_sr_restore.c b/tools/libs/guest/xg_sr_restore.c
index 07c9e291610b..bda04ee42e3f 100644
--- a/tools/libs/guest/xg_sr_restore.c
+++ b/tools/libs/guest/xg_sr_restore.c
@@ -425,7 +425,7 @@ static int send_checkpoint_dirty_pfn_list(struct xc_sr_context *ctx)
     int rc = -1;
     unsigned int count, written;
     uint64_t i, *pfns = NULL;
-    xc_shadow_op_stats_t stats = { 0, ctx->restore.p2m_size };
+    xc_shadow_op_stats_t stats;
     struct xc_sr_record rec = {
         .type = REC_TYPE_CHECKPOINT_DIRTY_PFN_LIST,
     };
@@ -444,14 +444,17 @@ static int send_checkpoint_dirty_pfn_list(struct xc_sr_context *ctx)
         goto err;
     }
 
-    for ( i = 0, count = 0; i < ctx->restore.p2m_size; i++ )
+    count = stats.dirty_count;
+
+    if ( ((REC_LENGTH_MAX - sizeof(rec)) / sizeof(*pfns)) < count )
     {
-        if ( test_bit(i, dirty_bitmap) )
-            count++;
+        ERROR("Too many PFNs (%u) to fit in record (limit %zu)", count,
+              ((REC_LENGTH_MAX - sizeof(rec)) / sizeof(*pfns)));
+        goto err;
     }
 
-
-    pfns = malloc(count * sizeof(*pfns));
+    iov[1].iov_len  = rec.length = count * sizeof(*pfns);
+    iov[1].iov_base = pfns       = malloc(rec.length);
     if ( !pfns )
     {
         ERROR("Unable to allocate %zu bytes of memory for dirty pfn list",
@@ -459,24 +462,20 @@ static int send_checkpoint_dirty_pfn_list(struct xc_sr_context *ctx)
         goto err;
     }
 
-    for ( i = 0, written = 0; i < ctx->restore.p2m_size; ++i )
+    for ( i = 0, written = 0; count && i < ctx->restore.p2m_size; ++i, --count )
     {
         if ( !test_bit(i, dirty_bitmap) )
             continue;
 
-        if ( written > count )
-        {
-            ERROR("Dirty pfn list exceed");
-            goto err;
-        }
-
         pfns[written++] = i;
     }
 
-    rec.length = count * sizeof(*pfns);
-
-    iov[1].iov_base = pfns;
-    iov[1].iov_len = rec.length;
+    if ( written != stats.dirty_count )
+    {
+        ERROR("Mismatch between dirty bitmap bits (%u), and dirty_count (%u)",
+              written, stats.dirty_count);
+        goto err;
+    }
 
     if ( writev_exact(ctx->restore.send_back_fd, iov, ARRAY_SIZE(iov)) )
     {
-- 
2.11.0