When about to execute a SCSI command, ensure that cmdfifo is not empty and
current_dev is non-NULL. This can happen if the guest tries to execute a TI
(Transfer Information) command without issuing one of the select commands
first.

Buglink: https://bugs.launchpad.net/qemu/+bug/1910723
Buglink: https://bugs.launchpad.net/qemu/+bug/1909247
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
---
 hw/scsi/esp.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
index bbcbfa4a91..ae362c9dfb 100644
--- a/hw/scsi/esp.c
+++ b/hw/scsi/esp.c
@@ -286,6 +286,9 @@ static void do_busid_cmd(ESPState *s, uint8_t busid)
     trace_esp_do_busid_cmd(busid);
     lun = busid & 7;
     cmdlen = fifo8_num_used(&s->cmdfifo);
+    if (!cmdlen || !s->current_dev) {
+        return;
+    }
     buf = (uint8_t *)fifo8_pop_buf(&s->cmdfifo, cmdlen, &n);
 
     current_lun = scsi_device_find(&s->bus, 0, s->current_dev->id, lun);
-- 
2.20.1

On 3/17/21 12:30 AM, Mark Cave-Ayland wrote:
> When about to execute a SCSI command, ensure that cmdfifo is not empty and
> current_dev is non-NULL. This can happen if the guest tries to execute a TI
> (Transfer Information) command without issuing one of the select commands
> first.
> 
> Buglink: https://bugs.launchpad.net/qemu/+bug/1910723
> Buglink: https://bugs.launchpad.net/qemu/+bug/1909247
> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> ---
>  hw/scsi/esp.c | 3 +++
>  1 file changed, 3 insertions(+)

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

Hi Mark,

On 210316 2330, Mark Cave-Ayland wrote:
> When about to execute a SCSI command, ensure that cmdfifo is not empty and
> current_dev is non-NULL. This can happen if the guest tries to execute a TI
> (Transfer Information) command without issuing one of the select commands
> first.
> 
> Buglink: https://bugs.launchpad.net/qemu/+bug/1910723

^ Can't reproduce this one anymore

> Buglink: https://bugs.launchpad.net/qemu/+bug/1909247

However, this still seems to cause a UAF:
https://bugs.launchpad.net/qemu/+bug/1909247/comments/6

-Alex

> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> ---
>  hw/scsi/esp.c | 3 +++
>  1 file changed, 3 insertions(+)