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

[Paolo Bonzini]

On 08/08/19 08:33, P J P wrote:
> From: Prasad J Pandit <address@hidden>
> 
> When executing script in lsi_execute_script(), the LSI scsi
> adapter emulator advances 's->dsp' index to read next opcode.
> This can lead to an infinite loop if the next opcode is empty.
> Exit such loop after reading 10k empty opcodes.
> 
> Reported-by: Bugs SysSec <address@hidden>
> Signed-off-by: Prasad J Pandit <address@hidden>
> ---
>  hw/scsi/lsi53c895a.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/scsi/lsi53c895a.c b/hw/scsi/lsi53c895a.c
> index 10468c1ec1..c23a40525e 100644
> --- a/hw/scsi/lsi53c895a.c
> +++ b/hw/scsi/lsi53c895a.c
> @@ -1132,7 +1132,10 @@ static void lsi_execute_script(LSIState *s)
>  
>      s->istat1 |= LSI_ISTAT1_SRUN;
>  again:
> -    insn_processed++;
> +    if (++insn_processed > 10000) {
> +        s->waiting = LSI_NOWAIT;
> +        goto exitloop;
> +    }
>      insn = read_dword(s, s->dsp);
>      if (!insn) {
>          /* If we receive an empty opcode increment the DSP by 4 bytes
> @@ -1569,6 +1572,7 @@ again:
>              }
>          }
>      }
> +exitloop:
>      if (insn_processed > 10000 && s->waiting == LSI_NOWAIT) {
>          /* Some windows drivers make the device spin waiting for a memory
>             location to change.  If we have been executed a lot of code then
> 

I am not sure this is worth a CVE.  The kernel can cause QEMU to break,
but is there a practical case in which an unprivileged user can do that?

Paolo