[PULL 02/13] esp: always check current_req is not NULL before use in DMA callbacks

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

After issuing a SCSI command the SCSI layer can call the SCSIBusInfo .cancel
callback which resets both current_req and current_dev to NULL. If any data
is left in the transfer buffer (async_len != 0) then the next TI (Transfer
Information) command will attempt to reference the NULL pointer causing a
segfault.

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>
Tested-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20210407195801.685-2-mark.cave-ayland@ilande.co.uk>
---
 hw/scsi/esp.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
index d87e1a63db..a79196f3f3 100644
--- a/hw/scsi/esp.c
+++ b/hw/scsi/esp.c
@@ -496,6 +496,10 @@ static void do_dma_pdma_cb(ESPState *s)
         return;
     }
 
+    if (!s->current_req) {
+        return;
+    }
+
     if (to_device) {
         /* Copy FIFO data to device */
         len = MIN(s->async_len, ESP_FIFO_SZ);
@@ -527,11 +531,9 @@ static void do_dma_pdma_cb(ESPState *s)
         return;
     } else {
         if (s->async_len == 0) {
-            if (s->current_req) {
-                /* Defer until the scsi layer has completed */
-                scsi_req_continue(s->current_req);
-                s->data_in_ready = false;
-            }
+            /* Defer until the scsi layer has completed */
+            scsi_req_continue(s->current_req);
+            s->data_in_ready = false;
             return;
         }
 
@@ -604,6 +606,9 @@ static void esp_do_dma(ESPState *s)
         }
         return;
     }
+    if (!s->current_req) {
+        return;
+    }
     if (s->async_len == 0) {
         /* Defer until data is available.  */
         return;
@@ -713,6 +718,10 @@ static void esp_do_nodma(ESPState *s)
         return;
     }
 
+    if (!s->current_req) {
+        return;
+    }
+
     if (s->async_len == 0) {
         /* Defer until data is available.  */
         return;
-- 
2.20.1