[PATCH] hw/scsi/scsi-disk: Disallow block sizes smaller than 512 [CVE-2023-42467]

[Thomas Huth <thuth@redhat.com>]

We are doing things like

    nb_sectors /= (s->qdev.blocksize / BDRV_SECTOR_SIZE);

in the code here (e.g. in scsi_disk_emulate_mode_sense()), so if
the blocksize is smaller than BDRV_SECTOR_SIZE (=512), this crashes
with a division by 0 exception. Thus disallow block sizes of 256
bytes to avoid this situation.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1813
CVE: 2023-42467
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 hw/scsi/scsi-disk.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c
index e0d79c7966..477ee2bcd4 100644
--- a/hw/scsi/scsi-disk.c
+++ b/hw/scsi/scsi-disk.c
@@ -1628,9 +1628,10 @@ static void scsi_disk_emulate_mode_select(SCSIDiskReq *r, uint8_t *inbuf)
          * Since the existing code only checks/updates bits 8-15 of the block
          * size, restrict ourselves to the same requirement for now to ensure
          * that a block size set by a block descriptor and then read back by
-         * a subsequent SCSI command will be the same
+         * a subsequent SCSI command will be the same. Also disallow a block
+         * size of 256 since we cannot handle anything below BDRV_SECTOR_SIZE.
          */
-        if (bs && !(bs & ~0xff00) && bs != s->qdev.blocksize) {
+        if (bs && !(bs & ~0xfe00) && bs != s->qdev.blocksize) {
             s->qdev.blocksize = bs;
             trace_scsi_disk_mode_select_set_blocksize(s->qdev.blocksize);
         }
-- 
2.41.0