Changeset 101

Timestamp:

Apr 8, 2017, 12:26:55 AM (8 years ago)

Author:

Ben Rietbroek

Message:

Prevent LVM CRC-check indicating OK on zero sectors [v1.1.1-testing]

When 'LVM_InitCRCTable' is not called yet, an LVM CRC-check on a sector
with all zeros would indicate a valid LVM CRC. Eventhough this is not
the case _after_ 'LVM_InitCRCTable' has been called, we rule out this
situation to guard agains coding bugs.

CAUTION:
This is a testbuild !
AirBoot uses the BIOS to access disks and a small coding error can trash
partition tables or other vital disk structures. You are advised to make
backups of TRACK0 and EBRs before using this testbuild. More info at:
​https://rousseaux.github.io/netlabs.air-boot/pdf/AirBoot-v1.1.0-manual.pdf

Location:

trunk/bootcode

Files:

• regular/other.asm (modified) (1 diff)
• special/lvm.asm (modified) (1 diff)

trunk/bootcode/regular/other.asm

@@ -562 +562 @@
         ret
 MBR_TeletypeSyncPos     EndP
+
+; Check if a memory block is all zeros
+; IN  : BX pointer to memblock
+;       CX length to check, zero length is interpreted as block is zero
+; OUT : ZF=1 block if all zeros
+; NOTE: Segment used is DS, which should be the same as ES
+IsMemBlockZero  Proc    Near    Uses ax di
+        push    es          ; Save ES just to be sure
+        push    ds          ; Segment to use
+        pop     es          ; Pop in ES because ES is required for scasb
+        mov     di, bx      ; Pointer to memblock
+        xor     al, al      ; Compare to zero
+        repe    scasb       ; Scan the block, will leave ZF=1 if all zeros
+        mov     bx, di      ; Update pointer (points past last byte compared)
+        pop     es          ; Restore ES
+        ret
+IsMemBlockZero  EndP
+
+; Check if a loaded sector is all zeros
+; IN  : BX pointer to memblock
+; OUT : ZF=1 block if all zeros
+; NOTE: Segment used is DS, which should be the same as ES
+IsSectorZero    Proc    Near    Uses cx
+        mov     cx, sector_size ; Normal size of a sector (512 bytes)
+        call    IsMemBlockZero  ; Check the memory block
+        ret
+IsSectorZero    EndP

trunk/bootcode/special/lvm.asm

@@ -112 +112 @@
 ;       Out: Carry set, if LVM CRC valid
 ; Destroyed: None
-LVM_CheckSectorCRC              Proc Near   Uses ax dx
-        call    LVM_GetSectorCRC
+LVM_CheckSectorCRC              Proc Near   Uses ax bx dx
+        mov     bx, si
+        call    IsSectorZero                ; Zero sector implies bad CRC
+        jz      LVMCSCRC_BadCRC
+        call    LVM_GetSectorCRC            ; Only use after CRC table is valid
         cmp     ax, word ptr [si+LocLVM_CRC]
         jne     LVMCSCRC_BadCRC
         cmp     dx, word ptr [si+LocLVM_CRC+2]
         jne     LVMCSCRC_BadCRC
-        stc
+        stc                                 ; Indicate CRC is OK
         ret
     LVMCSCRC_BadCRC:
-        clc
+        clc                                 ; Indicate BAD CRC
         ret
 LVM_CheckSectorCRC              EndP