esp: don't underflow cmdfifo in do_cmd()

If the guest tries to execute a CDB when cmdfifo is not empty before the start
of the message out phase then clearing the message out phase data will cause
cmdfifo to underflow due to cmdfifo_cdb_offset being larger than the amount of
data within.

Since this can only occur by issuing deliberately incorrect instruction
sequences, ensure that the maximum length of esp_fifo_pop_buf() is limited to
the size of the data 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: <20210407195801.685-8-mark.cave-ayland@ilande.co.uk>

1 files changed, 4 insertions, 2 deletions

diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
index 904fa3179c..d3b105b703 100644
--- a/hw/scsi/esp.c
+++ b/hw/scsi/esp.c
@@ -319,13 +319,15 @@ static void do_busid_cmd(ESPState *s, uint8_t busid)
 
 static void do_cmd(ESPState *s)
 {
-    uint8_t busid = fifo8_pop(&s->cmdfifo);
+    uint8_t busid = esp_fifo_pop(&s->cmdfifo);
+    int len;
 
     s->cmdfifo_cdb_offset--;
 
     /* Ignore extended messages for now */
     if (s->cmdfifo_cdb_offset) {
-        esp_fifo_pop_buf(&s->cmdfifo, NULL, s->cmdfifo_cdb_offset);
+        len = MIN(s->cmdfifo_cdb_offset, fifo8_num_used(&s->cmdfifo));
+        esp_fifo_pop_buf(&s->cmdfifo, NULL, len);
         s->cmdfifo_cdb_offset = 0;
     }