Re: Fix PVFB backend to validate frontend's frame buffer description

[Pat Campbell <plc@novell.com>]

Markus Armbruster wrote:
> A buggy or malicious frontend can describe its shared framebuffer to
> the backend in a way that makes the backend map an arbitrary amount of
>
>   
snipped out, see inline question below.
>
> diff -r 0a8fc1a62796 tools/ioemu/hw/xenfb.c
> --- a/tools/ioemu/hw/xenfb.c	Mon May 12 11:19:09 2008 +0100
> +++ b/tools/ioemu/hw/xenfb.c	Tue May 13 14:53:58 2008 +0200
> @@ -28,8 +28,6 @@
>  #ifndef BTN_LEFT
>  #define BTN_LEFT 0x110 /* from <linux/input.h> */
>  #endif
> -
> -// FIXME defend against malicious frontend?
>  
>  struct xenfb;
>  
> @@ -484,6 +482,68 @@ void xenfb_shutdown(struct xenfb *xenfb)
>  	free(xenfb);
>  }
>  
> +static int xenfb_configure_fb(struct xenfb *xenfb, size_t fb_len_lim,
> +			      int width, int height, int depth,
> +			      size_t fb_len, int offset, int row_stride)
> +{
> +	size_t mfn_sz = sizeof(*((struct xenfb_page *)0)->pd);
> +	size_t pd_len = sizeof(((struct xenfb_page *)0)->pd) / mfn_sz;
> +	size_t fb_pages = pd_len * XC_PAGE_SIZE / mfn_sz;
> +	size_t fb_len_max = fb_pages * XC_PAGE_SIZE;
> +	int max_width, max_height;
> +
> +	if (fb_len_lim > fb_len_max) {
> +		fprintf(stderr,
> +			"FB: fb size limit %zu exceeds %zu, corrected\n",
> +			fb_len_lim, fb_len_max);
> +		fb_len_lim = fb_len_max;
> +	}
> +	if (fb_len > fb_len_lim) {
> +		fprintf(stderr,
> +			"FB: frontend fb size %zu limited to %zu\n",
> +			fb_len, fb_len_lim);
>   
Do we need to set fb_len to fb_len_lim here?
fb_len = fb_len_lim;
> +	}
> +	if (depth != 8 && depth != 16 && depth != 24 && depth != 32) {
> +		fprintf(stderr,
> +			"FB: can't handle frontend fb depth %d\n",
> +			depth);
> +		return -1;
> +	}
> +	if (row_stride < 0 || row_stride > fb_len) {
> +		fprintf(stderr,
> +			"FB: invalid frontend stride %d\n", row_stride);
> +		return -1;
> +	}
> +	max_width = row_stride / (depth / 8);
> +	if (width < 0 || width > max_width) {
> +		fprintf(stderr,
> +			"FB: invalid frontend width %d limited to %d\n",
> +			width, max_width);
> +		width = max_width;
> +	}
> +	if (offset < 0 || offset >= fb_len) {
> +		fprintf(stderr,
> +			"FB: invalid frontend offset %d (max %zu)\n",
> +			offset, fb_len - 1);
> +		return -1;
> +	}
> +	max_height = (fb_len - offset) / row_stride;
> +	if (height < 0 || height > max_height) {
> +		fprintf(stderr,
> +			"FB: invalid frontend height %d limited to %d\n",
> +			height, max_height);
> +		height = max_height;
> +	}
> +	xenfb->fb_len = fb_len;
> +	xenfb->row_stride = row_stride;
> +	xenfb->depth = depth;
> +	xenfb->width = width;
> +	xenfb->height = height;
> +	xenfb->offset = offset;
> +	fprintf(stderr, "Framebuffer %dx%dx%d offset %d stride %d\n",
> +		width, height, depth, offset, row_stride);
> +	return 0;
> +}
>  
>  static void xenfb_on_fb_event(struct xenfb *xenfb)
>  {
> @@ -514,16 +574,18 @@ static void xenfb_on_fb_event(struct xen
>  			    || h != event->update.height) {
>  				fprintf(stderr, "%s bogus update clipped\n",
>  					xenfb->fb.nodename);
> -				break;
>  			}
>  			xenfb_guest_copy(xenfb, x, y, w, h);
>  			break;
>  		case XENFB_TYPE_RESIZE:
> -			xenfb->width  = event->resize.width;
> -			xenfb->height = event->resize.height;
> -			xenfb->depth = event->resize.depth;
> -			xenfb->row_stride = event->resize.stride;
> -			xenfb->offset = event->resize.offset;
> +			if (xenfb_configure_fb(xenfb, xenfb->fb_len,
> +					       event->resize.width,
> +					       event->resize.height,
> +					       event->resize.depth,
> +					       xenfb->fb_len,
> +					       event->resize.offset,
> +					       event->resize.stride) < 0)
> +				break;
>  			dpy_colourdepth(xenfb->ds, xenfb->depth);
>  			dpy_resize(xenfb->ds, xenfb->width, xenfb->height, xenfb->row_stride);
>  			if (xenfb->ds->shared_buf)
> @@ -745,29 +807,18 @@ static int xenfb_read_frontend_fb_config
>          xenfb_xs_printf(xenfb->xsh, xenfb->fb.nodename, "request-update", "1");
>          xenfb->refresh_period = -1;
>  
> -        /* TODO check for permitted ranges */
> -        fb_page = xenfb->fb.page;
> -        xenfb->depth = fb_page->depth;
> -        xenfb->width = fb_page->width;
> -        xenfb->height = fb_page->height;
> -        /* TODO check for consistency with the above */
> -        xenfb->fb_len = fb_page->mem_length;
> -        xenfb->row_stride = fb_page->line_length;
> -
> -        /* Protect against hostile frontend, limit fb_len to max allowed */
>          if (xenfb_xs_scanf1(xenfb->xsh, xenfb->fb.nodename, "videoram", "%d",
>                              &videoram) < 0)
>                  videoram = 0;
> -        videoram = videoram * 1024 * 1024;
> -        if (videoram && xenfb->fb_len > videoram) {
> -                fprintf(stderr, "Framebuffer requested length of %zd exceeded allowed %d\n",
> -                        xenfb->fb_len, videoram);
> -                xenfb->fb_len = videoram;
> -                if (xenfb->row_stride * xenfb->height > xenfb->fb_len)
> -                        xenfb->height = xenfb->fb_len / xenfb->row_stride;
> -        }
> -        fprintf(stderr, "Framebuffer depth %d width %d height %d line %d\n",
> -                fb_page->depth, fb_page->width, fb_page->height, fb_page->line_length);
> +	fb_page = xenfb->fb.page;
> +	if (xenfb_configure_fb(xenfb, videoram * 1024 * 1024U,
> +			       fb_page->width, fb_page->height, fb_page->depth,
> +			       fb_page->mem_length, 0, fb_page->line_length)
> +	    < 0) {
> +		errno = EINVAL;
> +		return -1;
> +	}
> +
>          if (xenfb_map_fb(xenfb, xenfb->fb.otherend_id) < 0)
>  		return -1;
>  
>
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