[syslinux:master] core: look for invalid MBR handover information address

[syslinux-bot for H. Peter Anvin]

Commit-ID:  5b1f5189081f677e4a578e2a6d52c60a7c082e1d
Gitweb:     http://syslinux.zytor.com/commit/5b1f5189081f677e4a578e2a6d52c60a7c082e1d
Author:     H. Peter Anvin <hpa at zytor.com>
AuthorDate: Wed, 11 Aug 2010 10:42:51 -0700
Committer:  H. Peter Anvin <hpa at zytor.com>
CommitDate: Wed, 11 Aug 2010 10:42:51 -0700

core: look for invalid MBR handover information address

The Win7 MBR doesn't provide proper handover information.  Instead, it
leaves DS:SI = 0:0x7e00, so reject that information as invalid on
grounds that it is pointed into the bootloader area.

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


---
 core/diskstart.inc |   15 ++++++++++++---
 1 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/core/diskstart.inc b/core/diskstart.inc
index c0ba52a..c0301d4 100644
--- a/core/diskstart.inc
+++ b/core/diskstart.inc
@@ -31,9 +31,10 @@ PartInfo	equ StackBuf
 .gpt		equ PartInfo+20
 FloppyTable	equ PartInfo+76
 ; Total size of PartInfo + FloppyTable == 76+16 = 92 bytes
-Hidden		equ StackBuf-20		; Partition offset
-OrigFDCTabPtr	equ StackBuf-12		; The 2nd high dword on the stack
-OrigESDI	equ StackBuf-8		; The high dword on the stack
+Hidden		equ StackBuf-24		; Partition offset (qword)
+OrigFDCTabPtr	equ StackBuf-16		; Original FDC table
+OrigDSSI	equ StackBuf-12		; DS:SI -> partinfo
+OrigESDI	equ StackBuf-8		; ES:DI -> $PnP structure
 DriveNumber	equ StackBuf-4		; Drive number
 StackHome	equ Hidden		; The start of the canonical stack
 
@@ -116,6 +117,8 @@ start:
 		push dx			; Save drive number (in DL)
 		push es			; Save initial ES:DI -> $PnP pointer
 		push di
+		push ds			; Save original DS:SI -> partinfo
+		push si
 		mov es,cx
 
 ;
@@ -174,6 +177,12 @@ floppy:
 ; Note: di points to beyond the end of PartInfo
 ;
 harddisk:
+		mov dx,[di-76-10]	; Original DS
+		mov si,[di-76-12]	; Original SI
+		shr si,4
+		add dx,si
+		cmp dx,PartInfo >> 4
+		jae .no_partition
 		test byte [di-76],7Fh	; Sanity check: "active flag" should
 		jnz .no_partition	; be 00 or 80
 		cmp [di-76+4],cl	; Sanity check: partition type != 0