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[PATCH v4 for-6.0 07/12] esp: don't underflow cmdfifo in do_cmd()

From: Mark Cave-Ayland
Subject: [PATCH v4 for-6.0 07/12] esp: don't underflow cmdfifo in do_cmd()
Date: Wed, 7 Apr 2021 20:57:56 +0100

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>
 hw/scsi/esp.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
index 4decbbfc29..7f49522e1d 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;
     /* 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;

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