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@zytor.com>

diff --git a/core/diskstart.inc b/core/diskstart.inc
index c0ba52a..c0301d4 100644 (file)
--- 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