[PATCH 1/1] drm/ttm: Fix COW check

[Christian König]

Am 10.07.21 um 02:28 schrieb Felix Kuehling:
> KFD Thunk maps invisible VRAM BOs with PROT_NONE, MAP_PRIVATE.
> is_cow_mapping returns true for these mappings. Add a check for
> vm_flags & VM_WRITE to avoid mmap failures on private read-only or
> PROT_NONE mappings.
>
> v2: protect against mprotect making a mapping writable after the fact
>
> Fixes: f91142c62161 ("drm/ttm: nuke VM_MIXEDMAP on BO mappings v3")
> Signed-off-by: Felix Kuehling <Felix.Kuehling at amd.com>
> Signed-off-by: Alex Deucher <alexander.deucher at amd.com>
> ---
>   drivers/gpu/drm/ttm/ttm_bo_vm.c | 13 ++++++++++++-
>   1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> index f56be5bc0861..9ad211ede611 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> @@ -542,17 +542,28 @@ int ttm_bo_vm_access(struct vm_area_struct *vma, unsigned long addr,
>   }
>   EXPORT_SYMBOL(ttm_bo_vm_access);
>   
> +static int ttm_bo_vm_mprotect(struct vm_area_struct *vma, unsigned long start,
> +			      unsigned long end, unsigned long newflags)
> +{
> +	/* Enforce no COW since would have really strange behavior with it. */
> +	if (is_cow_mapping(newflags) && (newflags & VM_WRITE))
> +		return -EINVAL;
> +
> +	return 0;
> +}
> +
>   static const struct vm_operations_struct ttm_bo_vm_ops = {
>   	.fault = ttm_bo_vm_fault,
>   	.open = ttm_bo_vm_open,
>   	.close = ttm_bo_vm_close,
>   	.access = ttm_bo_vm_access,
> +	.mprotect = ttm_bo_vm_mprotect,

The problem is most drivers implement their own copy of this structure.

You need to add this to those occasions as well, just search for 
references to ttm_bo_vm_open.

Regards,
Christian.

>   };
>   
>   int ttm_bo_mmap_obj(struct vm_area_struct *vma, struct ttm_buffer_object *bo)
>   {
>   	/* Enforce no COW since would have really strange behavior with it. */
> -	if (is_cow_mapping(vma->vm_flags))
> +	if (is_cow_mapping(vma->vm_flags) && (vma->vm_flags & VM_WRITE))
>   		return -EINVAL;
>   
>   	ttm_bo_get(bo);