bootsect: zero memory above a chainloaded boot sector/NBP

Windows RIS has been reported to make inappropriate use of data found
in uninitialized memory.  To avoid that, clear memory between
a chainloaded boot sector or NBP and Free Base Memory.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

3 files changed, 16 insertions, 3 deletions

diff --git a/NEWS b/NEWS
index 5a18525b..2ab59465 100644
--- a/NEWS
+++ b/NEWS
@@ -2,6 +2,10 @@ Starting with 1.47, changes marked with SYSLINUX, PXELINUX, ISOLINUX
 or EXTLINUX apply to that specific program only; other changes apply
 to all derivatives.
 
+Changes in 3.83:
+	* PXELINUX: clear memory before handing over to a chainloaded
+	  NBP.  This may help avoid a bug in Windows RIS.
+
 Changes in 3.82:
 	* isohybrid: fix the -partok logic for loading from a partition.
 	* ISOLINUX: deal with systems which return from INT 13h with
diff --git a/core/bootsect.inc b/core/bootsect.inc
index b107eb5c..3ea36bf5 100644
--- a/core/bootsect.inc
+++ b/core/bootsect.inc
@@ -84,11 +84,20 @@ load_bootsec:
 
 ;
 ; replace_bootstrap for the special case where we have exactly one
-; descriptor.
+; descriptor, based in low memory.  We will generate a second descriptor
+; to clear remaining FBM.
 ;
 
 replace_bootstrap_one:
-		push word 1			; Length of descriptor list
+		mov eax,[trackbuf]		; Base address
+		add eax,[trackbuf+8]		; Length
+		movzx edx,word [BIOS_fbm]
+		shl edx,10			; Free Base Memory
+		sub edx,eax
+		mov [trackbuf+12],eax
+		or dword [trackbuf+16],-1	; Zero memory
+		mov [trackbuf+20],edx
+		push word 2			; Length of descriptor list
 		; Fall through
 
 ;
diff --git a/version b/version
index 2fc94187..f2649fb3 100644
--- a/version
+++ b/version
@@ -1 +1 @@
-3.82 2009
+3.83 2009