diff --git a/include/exec/ramblock.h b/include/exec/ramblock.h
index 07d50864d8..664701b759 100644
--- a/include/exec/ramblock.h
+++ b/include/exec/ramblock.h
@@ -59,6 +59,16 @@ struct RAMBlock {
      */
     unsigned long *clear_bmap;
     uint8_t clear_bmap_shift;
+
+    /*
+     * RAM block length that corresponds to the used_length on the migration
+     * source (after RAM block sizes were synchronized). Especially, after
+     * starting to run the guest, used_length and postcopy_length can differ.
+     * Used to register/unregister uffd handlers and as the size of the received
+     * bitmap. Receiving any page beyond this length will bail out, as it
+     * could not have been valid on the source.
+     */
+    ram_addr_t postcopy_length;
 };
 #endif
 #endif
diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index a36402722b..c68caf4e42 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -17,6 +17,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/rcu.h"
 #include "exec/target_page.h"
 #include "migration.h"
 #include "qemu-file.h"
@@ -31,6 +32,7 @@
 #include "qemu/error-report.h"
 #include "trace.h"
 #include "hw/boards.h"
+#include "exec/ramblock.h"
 
 /* Arbitrary limit on size of each discard command,
  * keeps them around ~200 bytes
@@ -456,6 +458,13 @@ static int init_range(RAMBlock *rb, void *opaque)
     ram_addr_t length = qemu_ram_get_used_length(rb);
     trace_postcopy_init_range(block_name, host_addr, offset, length);
 
+    /*
+     * Save the used_length before running the guest. In case we have to
+     * resize RAM blocks when syncing RAM block sizes from the source during
+     * precopy, we'll update it manually via the ram block notifier.
+     */
+    rb->postcopy_length = length;
+
     /*
      * We need the whole of RAM to be truly empty for postcopy, so things
      * like ROMs and any data tables built during init must be zero'd
@@ -478,7 +487,7 @@ static int cleanup_range(RAMBlock *rb, void *opaque)
     const char *block_name = qemu_ram_get_idstr(rb);
     void *host_addr = qemu_ram_get_host_addr(rb);
     ram_addr_t offset = qemu_ram_get_offset(rb);
-    ram_addr_t length = qemu_ram_get_used_length(rb);
+    ram_addr_t length = rb->postcopy_length;
     MigrationIncomingState *mis = opaque;
     struct uffdio_range range_struct;
     trace_postcopy_cleanup_range(block_name, host_addr, offset, length);
@@ -600,7 +609,7 @@ static int nhp_range(RAMBlock *rb, void *opaque)
     const char *block_name = qemu_ram_get_idstr(rb);
     void *host_addr = qemu_ram_get_host_addr(rb);
     ram_addr_t offset = qemu_ram_get_offset(rb);
-    ram_addr_t length = qemu_ram_get_used_length(rb);
+    ram_addr_t length = rb->postcopy_length;
     trace_postcopy_nhp_range(block_name, host_addr, offset, length);
 
     /*
@@ -644,7 +653,7 @@ static int ram_block_enable_notify(RAMBlock *rb, void *opaque)
     struct uffdio_register reg_struct;
 
     reg_struct.range.start = (uintptr_t)qemu_ram_get_host_addr(rb);
-    reg_struct.range.len = qemu_ram_get_used_length(rb);
+    reg_struct.range.len = rb->postcopy_length;
     reg_struct.mode = UFFDIO_REGISTER_MODE_MISSING;
 
     /* Now tell our userfault_fd that it's responsible for this area */
diff --git a/migration/ram.c b/migration/ram.c
index 1a5ff07997..ee5c3d5784 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -244,7 +244,7 @@ int64_t ramblock_recv_bitmap_send(QEMUFile *file,
         return -1;
     }
 
-    nbits = block->used_length >> TARGET_PAGE_BITS;
+    nbits = block->postcopy_length >> TARGET_PAGE_BITS;
 
     /*
      * Make sure the tmp bitmap buffer is big enough, e.g., on 32bit
@@ -3160,7 +3160,13 @@ static int ram_load_postcopy(QEMUFile *f)
                 break;
             }
 
-            if (!offset_in_ramblock(block, addr)) {
+            /*
+             * Relying on used_length is racy and can result in false positives.
+             * We might place pages beyond used_length in case RAM was shrunk
+             * while in postcopy, which is fine - trying to place via
+             * UFFDIO_COPY/UFFDIO_ZEROPAGE will never segfault.
+             */
+            if (!block->host || addr >= block->postcopy_length) {
                 error_report("Illegal RAM offset " RAM_ADDR_FMT, addr);
                 ret = -EINVAL;
                 break;
@@ -3757,6 +3763,7 @@ static void ram_mig_ram_block_resized(RAMBlockNotifier *n, void *host,
                              rb->idstr);
             }
         }
+        rb->postcopy_length = new_size;
         break;
     case POSTCOPY_INCOMING_NONE:
     case POSTCOPY_INCOMING_RUNNING:
