Re: [PATCH 1/2] qemu-kvm: extboot: Keep variables in RAM

Anthony Liguori
2010-02-18T20:35:41Z

On 02/18/2010 10:13 AM, Jan Kiszka wrote:
> Instead of saving the old INT 0x13 and 0x19 handlers in ROM which fails
> under QEMU as it enforces protection, keep them in spare vectors of the
> interrupt table, namely INT 0x80 and 0x81.
>
> Signed-off-by: Jan Kiszka<jan.kiszka@siemens.com>
>

commit a4492b03932ea3c9762372f3e15e8c6526ee56c6
Author: H. Peter Anvin <hpa@zytor.com>
Date: Fri Jul 18 11:22:59 2008 -0700

kvm: extboot: don't use interrupt vectors $0x2b and $0x2c

extboot's use of interrupt vectors $0x2b and $0x2c is unsafe, as these
interrupt vectors fall in the OS-use range (0x20-0x3f). Furthermore,
it's unnecessary: we can keep a local pointer instead of hooking
another interrupt as long as we can write to our own segment.

Make the extboot segment writable, and use local variables to hold the
old link pointers.

If this turns out to cause problems, we should probably switch to
using vectors in the 0xc0-0xef range, and/or other BIOS-reserved
memory.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

Sounds like 0x80/0x81 is probably not the best choice. hpa: any
suggestions?

Regards,

Anthony Liguori

> ---
>
> Don't forget to update extboot.bin after merging both patches.
>
> pc-bios/optionrom/extboot.S | 41 ++++++++++++++++++++++++++++++-----------
> 1 files changed, 30 insertions(+), 11 deletions(-)
>
> diff --git a/pc-bios/optionrom/extboot.S b/pc-bios/optionrom/extboot.S
> index 1e60f68..1eeb172 100644
> --- a/pc-bios/optionrom/extboot.S
> +++ b/pc-bios/optionrom/extboot.S
> @@ -19,6 +19,9 @@
> * Authors: Anthony Liguori<aliguori@us.ibm.com>
> */
>
> +#define OLD_INT19 (0x80 * 4) /* re-use INT 0x80 BASIC vector */
> +#define OLD_INT13 (0x81 * 4) /* re-use INT 0x81 BASIC vector */
> +
> .code16
> .text
> .global _start
> @@ -37,7 +40,7 @@ _start:
>
> /* save old int 19 */
> mov (0x19*4), %eax
> - mov %eax, %cs:old_int19
> + mov %eax, (OLD_INT19)
>
> /* install out int 19 handler */
> movw $int19_handler, (0x19*4)
> @@ -48,6 +51,7 @@ _start:
> lret
>
> int19_handler:
> + push %eax /* reserve space for lret */
> push %eax
> push %bx
> push %cx
> @@ -69,7 +73,7 @@ int19_handler:
> 1: /* hook int13: intb(0x404) == 1 */
> /* save old int 13 to int 2c */
> mov (0x13*4), %eax
> - mov %eax, %cs:old_int13
> + mov %eax, (OLD_INT13)
>
> /* install our int 13 handler */
> movw $int13_handler, (0x13*4)
> @@ -90,15 +94,21 @@ int19_handler:
>
> 3: /* fall through: inb(0x404) == 0 */
> /* restore previous int $0x19 handler */
> - mov %cs:old_int19,%eax
> + mov (OLD_INT19),%eax
> mov %eax,(0x19*4)
> -
> +
> + /* write old handler as return address onto stack */
> + push %bp
> + mov %sp, %bp
> + mov %eax, 14(%bp)
> + pop %bp
> +
> pop %ds
> pop %dx
> pop %cx
> pop %bx
> pop %eax
> - ljmpw *%cs:old_int19
> + lret
>
> #define FLAGS_CF 0x01
>
> @@ -626,7 +636,21 @@ terminate_disk_emulation:
> int13_handler:
> cmp $0x80, %dl
> je 1f
> - ljmpw *%cs:old_int13
> +
> + /* write old handler as return address onto stack */
> + push %eax
> + push %eax
> + push %ds
> + push %bp
> + mov %sp, %bp
> + xor %ax, %ax
> + mov %ax, %ds
> + mov (OLD_INT13), %eax
> + mov %eax, 8(%bp)
> + pop %bp
> + pop %ds
> + pop %eax
> + lret
> 1:
> cmp $0x0, %ah
> jne 1f
> @@ -686,10 +710,5 @@ int13_handler:
> int $0x18 /* boot failed */
> iret
>
> -/* Variables */
> -.align 4, 0
> -old_int13: .long 0
> -old_int19: .long 0
> -
> .align 512, 0
> _end:
> --
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