pxelinux: workaround for EFI CSM bug - BEV stack overwrite on localboot

On at least some machines with an EFI-based BIOS, the Compatibility
Service Module (CSM) puts the BEV stack at an address where the boot
loader is likely to overwrite it.  Implement a workaround which forces
INT 18h instead if an EFI signature is detected in memory.

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

1 files changed, 75 insertions, 0 deletions

diff --git a/core/pxelinux.asm b/core/pxelinux.asm
index 190f4c66..b2745146 100644
--- a/core/pxelinux.asm
+++ b/core/pxelinux.asm
@@ -268,6 +268,81 @@ _start1:
 		call writestr_early
 
 ;
+; Look to see if we are on an EFI CSM system.  Some EFI
+; CSM systems put the BEV stack in low memory, which means
+; a return to the PXE stack will crash the system.  However,
+; INT 18h works reliably, so in that case hack the stack and
+; point the "return address" to an INT 18h instruction.
+;
+; Hack the stack instead of the much simpler "just invoke INT 18h
+; if we want to reset", so that chainloading other NBPs will work.
+;
+efi_csm_workaround:
+		les bp,[InitStack]	; GS:SP -> original stack
+		les bx,[es:bp+44]	; Return address
+		cmp word [es:bx],18CDh	; Already pointing to INT 18h?
+		je .skip
+
+		; Search memory from E0000 to FFFFF for a $EFI structure
+		mov bx,0E000h
+.scan_mem:
+		mov es,bx
+		cmp dword [es:0],'IFE$'	; $EFI is byte-reversed...
+		jne .not_here
+		;
+		; Verify the table.  We don't check the checksum because
+		; it seems some CSMs leave it at zero.
+		;
+		movzx cx,byte [es:5]	; Table length
+		cmp cx,83		; 83 bytes is the current length...
+		jae .found_it
+
+.not_here:
+		inc bx
+		jnz .scan_mem
+		jmp .skip		; No $EFI structure found
+
+		;
+		; Found a $EFI structure.  Move down the original stack
+		; and put an INT 18h instruction there instead.
+		;
+.found_it:
+%if USE_PXE_PROVIDED_STACK
+		mov cx,efi_csm_hack_size
+		mov si,sp
+		sub sp,cx
+		mov di,sp
+		mov ax,ss
+		mov es,ax
+		sub [InitStack],cx
+		sub [BaseStack],cx
+%else
+		les si,[InitStack]
+		lea di,[si-efi_csm_hack_size]
+		mov [InitStack],di
+%endif
+		lea cx,[bp+52]		; End of the stack we care about
+		sub cx,si
+		es rep movsb
+		mov [es:di-8],di	; Clobber the return address
+		mov [es:di-6],es
+		mov si,efi_csm_hack
+		mov cx,efi_csm_hack_size
+		rep movsb
+
+.skip:	
+
+		section .data
+		alignz 4
+efi_csm_hack:
+		int 18h
+		jmp 0F000h:0FFF0h
+		hlt
+efi_csm_hack_size equ $-efi_csm_hack
+
+		section .text
+
+;
 ; Assume API version 2.1, in case we find the !PXE structure without
 ; finding the PXENV+ structure.  This should really look at the Base
 ; Code ROM ID structure in have_pxe, but this is adequate for now --