[PATCH v3 12/17] esp.c: prevent cmdfifo overflow in esp_cdb_ready()

[Mark Cave-Ayland]

During normal use the cmdfifo will never wrap internally and cmdfifo_cdb_offset
will always indicate the start of the SCSI CDB. However it is possible that a
malicious guest could issue an invalid ESP command sequence such that cmdfifo
wraps internally and cmdfifo_cdb_offset could point beyond the end of the FIFO
data buffer.

Add an extra check to fifo8_peek_buf() to ensure that if the cmdfifo has wrapped
internally then esp_cdb_ready() will exit rather than allow scsi_cdb_length() to
access data outside the cmdfifo data buffer.

Reported-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
---
 hw/scsi/esp.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
index f47abc36d6..d8db33b921 100644
--- a/hw/scsi/esp.c
+++ b/hw/scsi/esp.c
@@ -429,13 +429,23 @@ static bool esp_cdb_ready(ESPState *s)
 {
     int len = fifo8_num_used(&s->cmdfifo) - s->cmdfifo_cdb_offset;
     const uint8_t *pbuf;
+    uint32_t n;
     int cdblen;
 
     if (len <= 0) {
         return false;
     }
 
-    pbuf = fifo8_peek_buf(&s->cmdfifo, len, NULL);
+    pbuf = fifo8_peek_buf(&s->cmdfifo, len, &n);
+    if (n < len) {
+        /*
+         * In normal use the cmdfifo should never wrap, but include this check
+         * to prevent a malicious guest from reading past the end of the
+         * cmdfifo data buffer below
+         */
+        return false;
+    }
+
     cdblen = scsi_cdb_length((uint8_t *)&pbuf[s->cmdfifo_cdb_offset]);
 
     return cdblen < 0 ? false : (len >= cdblen);
-- 
2.39.2