[PATCH] fdc: check for illegal dma length calculation

[Jon Maloy <jmaloy@redhat.com>]

The function fdctrl_start_transfer() calculates the dma data length
wrongly when certain boundary conditions are fulfilled. We have
noticed that the if ((fdctrl->fifo[5] - fdctrl->fifo[6]) > 1) we get
a dma length that will be interpreted as negative by the next function
in the chain, fdctrl_transfer_handler(). This leads to a crash.

Rather than trying to fix this obscure calculation, we just check if
the harmful condition is fulfilled, and return without action if that
is the case. Since this is a condition that can only be created by a
malicious user we deem this solution safe.

This fix is intended to address CVE-2021-3507.

Signed-off-by: Jon Maloy <jmaloy@redhat.com>
---
 hw/block/fdc.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/hw/block/fdc.c b/hw/block/fdc.c
index 21d18ac2e3..80a1f1750a 100644
--- a/hw/block/fdc.c
+++ b/hw/block/fdc.c
@@ -1532,6 +1532,11 @@ static void fdctrl_start_transfer(FDCtrl *fdctrl, int direction)
         if (fdctrl->fifo[0] & 0x80)
             tmp += fdctrl->fifo[6];
         fdctrl->data_len *= tmp;
+        if (tmp < 0) {
+            FLOPPY_DPRINTF("calculated illegal data_len=%u, tmp=%i\n",
+                           fdctrl->data_len, tmp);
+            return;
+        }
     }
     fdctrl->eot = fdctrl->fifo[6];
     if (fdctrl->dor & FD_DOR_DMAEN) {
-- 
2.31.1