sdhci.c: Limit the maximum block size

It is possible for the guest to set an invalid block
size which is larger then the fifo_buffer[] array. This
could cause a buffer overflow.

To avoid this limit the maximum size of the blksize variable.
Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reported-by: Intel Security ATR <secure@intel.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: abe4c51f513290bbb85d1ee271cb1a3d463d7561.1444067470.git.alistair.francis@xilinx.com
Suggested-by: Igor Mitsyanko <i.mitsyanko@gmail.com>
Reported-by: Intel Security ATR <secure@intel.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

hw/sd/sdhci.c

@@ -1009,6 +1009,16 @@ sdhci_write(void *opaque, hwaddr offset, uint64_t val, unsigned size)
             MASKED_WRITE(s->blksize, mask, value);
             MASKED_WRITE(s->blkcnt, mask >> 16, value >> 16);
         }
+
+        /* Limit block size to the maximum buffer size */
+        if (extract32(s->blksize, 0, 12) > s->buf_maxsz) {
+            qemu_log_mask(LOG_GUEST_ERROR, "%s: Size 0x%x is larger than " \
+                          "the maximum buffer 0x%x", __func__, s->blksize,
+                          s->buf_maxsz);
+
+            s->blksize = deposit32(s->blksize, 0, 12, s->buf_maxsz);
+        }
+
         break;
     case SDHC_ARGUMENT:
         MASKED_WRITE(s->argument, mask, value);