fix(flash): split writes that cross a flash page

changelog.md

@@ -18,6 +18,7 @@ it and is upstream Adafruit history, kept for provenance.
 - A DFU session started after a SoftDevice update no longer ends before it begins.
 - Build change, no behavior change: the bootloader is now compiled with `-flto`, which shrinks every image by about 3.3 KB of flash. Section addresses and SoftDevice call sites are unchanged.
 - The lazy-erase packet buffer holds 16 packets instead of 8. Costs about 2 KB of `.bss`.
+- Hardening, no behavior change: `flash_nrf5x_write` splits a write that crosses a 4 KB page instead of running past its page buffer. The UF2 path accepts only 256 B blocks at 256 B-aligned addresses, so no current caller crosses a page.
 - A UF2 whose application blocks start below the installed SoftDevice's end and carry no SoftDevice (no block at 0x1000) is now rejected as WRONG FILE: every block of the file is rejected and nothing is written. An app-only image built for a different SoftDevice would have overwritten the last page of the installed one. SoftDevice+app images, app-only images at the application start, and `CURRENT.UF2` restores are unaffected; the check assumes the host writes the file front to back.
 
 ## OTAFIX 2.5

src/flash_nrf5x.c

@@ -60,14 +60,25 @@ void flash_nrf5x_flush (bool need_erase)
 
 void flash_nrf5x_write (uint32_t dst, void const *src, int len, bool need_erase)
 {
-  uint32_t newAddr = dst & ~(FLASH_PAGE_SIZE - 1);
-
-  if ( newAddr != _fl_addr )
+  while ( len > 0 )
   {
-    flash_nrf5x_flush(need_erase);
-    _fl_addr = newAddr;
-    memcpy(_fl_buf, (void *) newAddr, FLASH_PAGE_SIZE);
+    uint32_t newAddr = dst & ~(FLASH_PAGE_SIZE - 1);
+
+    if ( newAddr != _fl_addr )
+    {
+      flash_nrf5x_flush(need_erase);
+      _fl_addr = newAddr;
+      memcpy(_fl_buf, (void *) newAddr, FLASH_PAGE_SIZE);
+    }
+
+    uint32_t offset = dst & (FLASH_PAGE_SIZE - 1);
+    uint32_t n = FLASH_PAGE_SIZE - offset;
+    if ( n > (uint32_t) len ) n = (uint32_t) len;
+
+    memcpy(_fl_buf + offset, src, n);
+
+    dst += n;
+    src = (uint8_t const *) src + n;
+    len -= (int) n;
   }
-  memcpy(_fl_buf + (dst & (FLASH_PAGE_SIZE - 1)), src, len);
 }
-