The current logic assumes that at least 1 byte is present in the FIFO when
executing a non-DMA SELATNS command, but this may not be the case if the
guest executes an invalid ESP command sequence.

Reported-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20240324191707.623175-11-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
---
 hw/scsi/esp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
index 1aac8f5564..f3aa5364cf 100644
--- a/hw/scsi/esp.c
+++ b/hw/scsi/esp.c
@@ -762,7 +762,8 @@ static void esp_do_nodma(ESPState *s)
 
         case CMD_SELATNS:
             /* Copy one byte from FIFO into cmdfifo */
-            len = esp_fifo_pop_buf(s, buf, 1);
+            len = esp_fifo_pop_buf(s, buf,
+                                   MIN(fifo8_num_used(&s->fifo), 1));
             len = MIN(fifo8_num_free(&s->cmdfifo), len);
             fifo8_push_all(&s->cmdfifo, buf, len);
 
-- 
2.39.2