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

[Philippe Mathieu-Daudé]

Hi Prasad,

On 8/13/19 12:05 PM, P J P wrote:
> +-- On Fri, 9 Aug 2019, 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 | 11 ++++++++++-
> |  1 file changed, 10 insertions(+), 1 deletion(-)
> | 
> | Update v3: raise an illegal instruction interrupt and exit
> |   -> https://lists.gnu.org/archive/html/qemu-devel/2019-08/msg01427.html
> | 
> | diff --git a/hw/scsi/lsi53c895a.c b/hw/scsi/lsi53c895a.c
> | index 10468c1ec1..e703ef4c9d 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 +1135,13 @@ static void lsi_execute_script(LSIState *s)
> |  
> |      s->istat1 |= LSI_ISTAT1_SRUN;
> |  again:
> | -    insn_processed++;
> | +    if (++insn_processed > LSI_MAX_INSN) {
> | +        trace_lsi_execute_script_tc_illegal();
> | +        lsi_script_dma_interrupt(s, LSI_DSTAT_IID);
> | +        lsi_disconnect(s);
> | +        trace_lsi_execute_script_stop();
> | +        return;

My understanding of Marcelo's explanation
(https://lists.gnu.org/archive/html/qemu-devel/2019-08/msg01427.html) is
we can kill insn_processed.

> | +    }
> |      insn = read_dword(s, s->dsp);
> |      if (!insn) {
> |          /* If we receive an empty opcode increment the DSP by 4 bytes
> | 
> 
> Ping...?!
> --
> Prasad J Pandit / Red Hat Product Security Team
> 47AF CE69 3A90 54AA 9045 1053 DD13 3D32 FE5B 041F
>