Re: [Linux-graphics-maintainer] [PATCH] drm/vmwgfx: Fix potential Spectre v1

[Thomas Hellstrom]

On 08/20/2018 10:53 PM, Deepak Singh Rawat wrote:
> Looks good to me based on my limited understanding. Thomas/Sinclair can
> could you please review and then we can include this in drm-fixes.
>
> Thanks,
> Deepak
>
> > arg.version is 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/vmwgfx/vmwgfx_execbuf.c:4526 vmw_execbuf_ioctl()
> > warn:
> > potential spectre issue 'copy_offset' [w]
> >
> > Fix this by sanitizing arg.version before using it to index copy_offset
> >
> > 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]
> >
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
> > ---
> >   drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c | 7 +++++--
> >   1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c
> > b/drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c
> > index 1f13457..ad91c6e 100644
> > --- a/drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c
> > +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c
> > @@ -25,6 +25,7 @@
> >    *
> >
> > **********************************************************
> > ****************/
> >   #include <linux/sync_file.h>
> > +#include <linux/nospec.h>
> >
> >   #include "vmwgfx_drv.h"
> >   #include "vmwgfx_reg.h"
> > @@ -4520,8 +4521,10 @@ int vmw_execbuf_ioctl(struct drm_device *dev,
> > unsigned long data,
> >   		return -EINVAL;
> >   	}
> >
> > -	if (arg.version > 1 &&
> > -	    copy_from_user(&arg.context_handle,
> > +	if (arg.version >= ARRAY_SIZE(copy_offset))
> > +		return -EFAULT;

I must admit my understanding of spectre workings in this case is 
limited, but why do you need to compare
arg.version against ARRAY_SIZE here, when it is already checked against 
DRM_VMW_EXECBUF_VERSION earlier?



> > +	arg.version = array_index_nospec(arg.version,
> > ARRAY_SIZE(copy_offset));
> > +	if (copy_from_user(&arg.context_handle,
> >   			   (void __user *) (data + copy_offset[0]),
> >   			   copy_offset[arg.version - 1] -
> >   			   copy_offset[0]) != 0)

Similarly, we want to perform this copy iff arg.version > 1. Why did you 
remove that check?

Thanks,
Thomas

> > --
> > 2.7.4
> >
> > _______________________________________________
> > Sent to linux-graphics-maintainer@xxxxxxxxxx