[PULL 09/13] esp: don't overflow cmdfifo in get_cmd()

[Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>]

If the guest tries to read a CDB using DMA and cmdfifo is not empty then it is
possible to overflow cmdfifo.

Since this can only occur by issuing deliberately incorrect instruction
sequences, ensure that the maximum length of the CDB transferred to cmdfifo is
limited to the available free space within cmdfifo.

Buglink: https://bugs.launchpad.net/qemu/+bug/1909247
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20210407195801.685-9-mark.cave-ayland@ilande.co.uk>
---
 hw/scsi/esp.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
index d3b105b703..9d3fdb4398 100644
--- a/hw/scsi/esp.c
+++ b/hw/scsi/esp.c
@@ -243,6 +243,7 @@ static uint32_t get_cmd(ESPState *s, uint32_t maxlen)
         }
         if (s->dma_memory_read) {
             s->dma_memory_read(s->dma_opaque, buf, dmalen);
+            dmalen = MIN(fifo8_num_free(&s->cmdfifo), dmalen);
             fifo8_push_all(&s->cmdfifo, buf, dmalen);
         } else {
             if (esp_select(s) < 0) {
@@ -262,6 +263,7 @@ static uint32_t get_cmd(ESPState *s, uint32_t maxlen)
         if (n >= 3) {
             buf[0] = buf[2] >> 5;
         }
+        n = MIN(fifo8_num_free(&s->cmdfifo), n);
         fifo8_push_all(&s->cmdfifo, buf, n);
     }
     trace_esp_get_cmd(dmalen, target);
-- 
2.20.1