[PATCH v2 2/4] hw/scsi/megasas: Assert cdb_len is valid in megasas_handle_scsi()

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

cdb_len can not be zero... (or less than 6) here, else we have a
out-of-bound read first in scsi_cdb_length():

 71 int scsi_cdb_length(uint8_t *buf)
 72 {
 73     int cdb_len;
 74
 75     switch (buf[0] >> 5) {
 76     case 0:
 77         cdb_len = 6;
 78         break;

Then another out-of-bound read when the size returned by
scsi_cdb_length() is used.

Figured out after reviewing:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg757937.html

And reproduced fuzzing:

  qemu-fuzz-i386: hw/scsi/megasas.c:1679: int megasas_handle_scsi(MegasasState *, MegasasCmd *, int):
  Assertion `len > 0 && cdb_len >= len' failed.
  ==1689590== ERROR: libFuzzer: deadly signal
      #8 0x7f7a5d918e75 in __assert_fail (/lib64/libc.so.6+0x34e75)
      #9 0x55a1b95cf6d4 in megasas_handle_scsi hw/scsi/megasas.c:1679:5
      #10 0x55a1b95cf6d4 in megasas_handle_frame hw/scsi/megasas.c:1975:24
      #11 0x55a1b95cf6d4 in megasas_mmio_write hw/scsi/megasas.c:2132:9
      #12 0x55a1b981972e in memory_region_write_accessor softmmu/memory.c:491:5
      #13 0x55a1b981972e in access_with_adjusted_size softmmu/memory.c:552:18
      #14 0x55a1b981972e in memory_region_dispatch_write softmmu/memory.c:1501:16
      #15 0x55a1b97f0ab0 in flatview_write_continue softmmu/physmem.c:2759:23
      #16 0x55a1b97ec3f2 in flatview_write softmmu/physmem.c:2799:14
      #17 0x55a1b97ec3f2 in address_space_write softmmu/physmem.c:2891:18
      #18 0x55a1b985c7cd in cpu_outw softmmu/ioport.c:70:5
      #19 0x55a1b99577ac in qtest_process_command softmmu/qtest.c:481:13

Inspired-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
Inspired-by: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 hw/scsi/megasas.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/hw/scsi/megasas.c b/hw/scsi/megasas.c
index 1a5fc5857db..f5ad4425b5b 100644
--- a/hw/scsi/megasas.c
+++ b/hw/scsi/megasas.c
@@ -1667,6 +1667,7 @@ static int megasas_handle_scsi(MegasasState *s, MegasasCmd *cmd,
 {
     uint8_t *cdb;
     int target_id, lun_id, cdb_len;
+    int len = -1;
     bool is_write;
     struct SCSIDevice *sdev = NULL;
     bool is_logical = (frame_cmd == MFI_CMD_LD_SCSI_IO);
@@ -1676,6 +1677,10 @@ static int megasas_handle_scsi(MegasasState *s, MegasasCmd *cmd,
     lun_id = cmd->frame->header.lun_id;
     cdb_len = cmd->frame->header.cdb_len;
 
+    if (cdb_len > 0) {
+        len = scsi_cdb_length(cdb);
+    }
+    assert(len > 0 && cdb_len >= len);
     if (is_logical) {
         if (target_id >= MFI_MAX_LD || lun_id != 0) {
             trace_megasas_scsi_target_not_present(
-- 
2.26.2