Re: [Qemu-devel] [PATCH v3 1/2] scsi: lsi: exit infinite loop while executing script (CVE-2019-12068)

[Paolo Bonzini]

On 14/08/19 12:25, P J P wrote:
> +-- On Tue, 13 Aug 2019, Paolo Bonzini wrote --+
> | After the first instruction is processed, "again" is only reached if 
> | s->waiting == LSI_NOWAIT.  Therefore, we could move the Windows hack to the 
> | beginning and remove the s->waiting condition.  The only change would be 
> | that it would also be triggered by all zero instructions, like this:
> | 
> | diff --git a/hw/scsi/lsi53c895a.c b/hw/scsi/lsi53c895a.c
> | index 10468c1..9d714af 100644
> | --- a/hw/scsi/lsi53c895a.c
> | +++ b/hw/scsi/lsi53c895a.c
> | @@ -185,6 +185,9 @@ static const char *names[] = {
> |  /* Flag set if this is a tagged command.  */
> |  #define LSI_TAG_VALID     (1 << 16)
> |  
> | +/* Maximum instructions to process. */
> | +#define LSI_MAX_INSN    10000
> | +
> |  typedef struct lsi_request {
> |      SCSIRequest *req;
> |      uint32_t tag;
> | @@ -1132,7 +1135,19 @@ static void lsi_execute_script(LSIState *s)
> |  
> |      s->istat1 |= LSI_ISTAT1_SRUN;
> |  again:
> | -    insn_processed++;
> | +    if (++insn_processed > LSI_MAX_INSN) {
> | +        /* Some windows drivers make the device spin waiting for a memory
> | +           location to change.  If we have been executed a lot of code then
> | +           assume this is the case and force an unexpected device 
> disconnect.
> | +           This is apparently sufficient to beat the drivers into 
> submission.
> | +         */
> | +        if (!(s->sien0 & LSI_SIST0_UDC)) {
> | +            qemu_log_mask(LOG_GUEST_ERROR,
> | +                          "lsi_scsi: inf. loop with UDC masked");
> | +        }
> | +        lsi_script_scsi_interrupt(s, LSI_SIST0_UDC, 0);
> | +        lsi_disconnect(s);
> ...
> |
> | Does it make sense? 
> 
> Yes, this'd also work, but need to return after lsi_disconnect(s), otherwise 
> loop would continue.
> 
> Should I send a revised patch? (with above change)

Yes, please.

Paolo