Re: [PATCH] drm/bufs: Fix Spectre v1 vulnerability

[Daniel Vetter]

On Tue, Oct 16, 2018 at 11:55:49AM +0200, Gustavo A. R. Silva wrote:
> idx can be indirectly controlled by user-space, hence leading to a
> potential exploitation of the Spectre variant 1 vulnerability.
> 
> This issue was detected with the help of Smatch:
> 
> drivers/gpu/drm/drm_bufs.c:1420 drm_legacy_freebufs() warn: potential
> spectre issue 'dma->buflist' [r] (local cap)
> 
> Fix this by sanitizing idx before using it to index dma->buflist
> 
> Notice that given that speculation windows are large, the policy is
> to kill the speculation on the first load and not worry if it can be
> completed with a dependent load/store [1].
> 
> [1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>

Applied since it's correct, but I dropped the cc: stable. This code is
very dead and full of security issues, and spectre is the least of your
worries. If you want to a stab at fixing the real spectre issues in drm,
then look anywhere that isn't full of drm_legacy_* functions.

The most important file is probably drm_ioctl.c.
-Daniel

> ---
>  drivers/gpu/drm/drm_bufs.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_bufs.c b/drivers/gpu/drm/drm_bufs.c
> index 7412aca..d7d10ca 100644
> --- a/drivers/gpu/drm/drm_bufs.c
> +++ b/drivers/gpu/drm/drm_bufs.c
> @@ -36,6 +36,8 @@
>  #include <drm/drmP.h>
>  #include "drm_legacy.h"
>  
> +#include <linux/nospec.h>
> +
>  static struct drm_map_list *drm_find_matching_map(struct drm_device *dev,
>  						  struct drm_local_map *map)
>  {
> @@ -1417,6 +1419,7 @@ int drm_legacy_freebufs(struct drm_device *dev, void *data,
>  				  idx, dma->buf_count - 1);
>  			return -EINVAL;
>  		}
> +		idx = array_index_nospec(idx, dma->buf_count);
>  		buf = dma->buflist[idx];
>  		if (buf->file_priv != file_priv) {
>  			DRM_ERROR("Process %d freeing buffer not owned\n",
> -- 
> 2.7.4
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch