Re: [Qemu-devel] [PATCH v1] ps2: check PS2Queue pointers in post_load routine

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/11/2017 08:51, P J P wrote:
> From: Prasad J Pandit <pjp@fedoraproject.org>
> 
> During Qemu guest migration, a destination process invokes ps2
> post_load function. In that, if 'rptr' and 'count' values were
> invalid, it could lead to OOB access or infinite loop issue.
> Add check to avoid it.
> 
> Reported-by: Cyrille Chatras <cyrille.chatras@orange.com>
> Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
> ---
>  hw/input/ps2.c | 21 +++++++++------------
>  1 file changed, 9 insertions(+), 12 deletions(-)
> 
> Update: avoid changing field type, add range check on values
>   -> https://lists.gnu.org/archive/html/qemu-devel/2017-11/msg02815.html
> 
> diff --git a/hw/input/ps2.c b/hw/input/ps2.c
> index f388a23c8e..de171a28dd 100644
> --- a/hw/input/ps2.c
> +++ b/hw/input/ps2.c
> @@ -1225,24 +1225,21 @@ static void ps2_common_reset(PS2State *s)
>  static void ps2_common_post_load(PS2State *s)
>  {
>      PS2Queue *q = &s->queue;
> -    int size;
> -    int i;
> -    int tmp_data[PS2_QUEUE_SIZE];
> +    uint8_t i, size;
> +    uint8_t tmp_data[PS2_QUEUE_SIZE];

Hi Prasad,

you don't need to change the invalid values to sane ones.  Instead, make
ps2_common_post_load return an int (just like the .post_load member of
VMStateDescription).  You can then detect out of range count/rptr/wptr
and return -1 for bad indices.

In the callers,

	ps2_common_post_load(ps2);

	return 0;

can then become simply

	return ps2_common_post_load(ps2);

Thanks,

Paolo

>      /* set the useful data buffer queue size, < PS2_QUEUE_SIZE */
> -    size = q->count > PS2_QUEUE_SIZE ? 0 : q->count;
> +    size = (q->count < 0 || q->count > PS2_QUEUE_SIZE) ? 0 : q->count;
>  
>      /* move the queue elements to the start of data array */
> -    if (size > 0) {
> -        for (i = 0; i < size; i++) {
> -            /* move the queue elements to the temporary buffer */
> -            tmp_data[i] = q->data[q->rptr];
> -            if (++q->rptr == 256) {
> -                q->rptr = 0;
> -            }
> +    for (i = 0; i < size; i++) {
> +        if (q->rptr < 0 || q->rptr >= sizeof(q->data)) {
> +            q->rptr = 0;
>          }
> -        memcpy(q->data, tmp_data, size);
> +        tmp_data[i] = q->data[q->rptr++];
>      }
> +    memcpy(q->data, tmp_data, size);
> +
>      /* reset rptr/wptr/count */
>      q->rptr = 0;
>      q->wptr = size;
>