[PATCH 1/2] drm/etnaviv: Use FOLL_FORCE for userptr

[Lucas Stach]

Am Montag, dem 01.03.2021 um 10:52 +0100 schrieb Daniel Vetter:
> Nothing checks userptr.ro except this call to pup_fast, which means
> there's nothing actually preventing userspace from writing to this.
> Which means you can just read-only mmap any file you want, userptr it
> and then write to it with the gpu. Not good.

I agree about the "not good" part.

> The right way to handle this is FOLL_WRITE | FOLL_FORCE, which will
> break any COW mappings and update tracking for MAY_WRITE mappings so
> there's no exploit and the vm isn't confused about what's going on.
> For any legit use case there's no difference from what userspace can
> observe and do.

This however seems pretty heavy handed. Does this mean we do a full COW
cycle of the userpages on BO creation? This most likely kills a lot of
the performance benefits that one might seek by using userptr. If
that's the case I might still take this patch for stable, but then we
should rather just disallow writable GPU mappings to this BO.

Regards,
Lucas

> 
> Cc: stable at vger.kernel.org
> Cc: John Hubbard <jhubbard at nvidia.com>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> Cc: Lucas Stach <l.stach at pengutronix.de>
> Cc: Russell King <linux+etnaviv at armlinux.org.uk>
> Cc: Christian Gmeiner <christian.gmeiner at gmail.com>
> Cc: etnaviv at lists.freedesktop.org
> ---
>  drivers/gpu/drm/etnaviv/etnaviv_gem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.c b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> index 6d38c5c17f23..a9e696d05b33 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> @@ -689,7 +689,7 @@ static int etnaviv_gem_userptr_get_pages(struct etnaviv_gem_object *etnaviv_obj)
>  		struct page **pages = pvec + pinned;
>  
> 
> 
> 
> 
> 
> 
> 
>  		ret = pin_user_pages_fast(ptr, num_pages,
> -					  !userptr->ro ? FOLL_WRITE : 0, pages);
> +					  FOLL_WRITE | FOLL_FORCE, pages);
>  		if (ret < 0) {
>  			unpin_user_pages(pvec, pinned);
>  			kvfree(pvec);