[PATCH v3 1/1] drm/ttm: Fix COW check

[Daniel Vetter]

On Mon, Jul 12, 2021 at 06:06:36PM -0400, Felix Kuehling wrote:
> 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
> v3: update driver-specific vm_operations_structs
> 
> 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>

So looking at vmf_insert_pfn_prot() and the comment there we can't have
VM_PFNMAP and is_cow_mapping ever be true, or things break. On platforms
without pte_special at least.

So I'm not sure this is a great idea, and definitely not for all drivers
...

Can we clear VM_MAYWRITE instead to force this to be a non-cow mapping
instead?
-Daniel

> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c  |  3 ++-
>  drivers/gpu/drm/nouveau/nouveau_gem.c    |  3 ++-
>  drivers/gpu/drm/radeon/radeon_gem.c      |  3 ++-
>  drivers/gpu/drm/ttm/ttm_bo_vm.c          | 14 +++++++++++++-
>  drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c |  1 +
>  include/drm/ttm/ttm_bo_api.h             |  4 ++++
>  6 files changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> index b3404c43a911..1aa750a6a5d2 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> @@ -79,7 +79,8 @@ static const struct vm_operations_struct amdgpu_gem_vm_ops = {
>  	.fault = amdgpu_gem_fault,
>  	.open = ttm_bo_vm_open,
>  	.close = ttm_bo_vm_close,
> -	.access = ttm_bo_vm_access
> +	.access = ttm_bo_vm_access,
> +	.mprotect = ttm_bo_vm_mprotect
>  };
>  
>  static void amdgpu_gem_object_free(struct drm_gem_object *gobj)
> diff --git a/drivers/gpu/drm/nouveau/nouveau_gem.c b/drivers/gpu/drm/nouveau/nouveau_gem.c
> index 5b27845075a1..164ea564bb7a 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_gem.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_gem.c
> @@ -70,7 +70,8 @@ static const struct vm_operations_struct nouveau_ttm_vm_ops = {
>  	.fault = nouveau_ttm_fault,
>  	.open = ttm_bo_vm_open,
>  	.close = ttm_bo_vm_close,
> -	.access = ttm_bo_vm_access
> +	.access = ttm_bo_vm_access,
> +	.mprotect = ttm_bo_vm_mprotect
>  };
>  
>  void
> diff --git a/drivers/gpu/drm/radeon/radeon_gem.c b/drivers/gpu/drm/radeon/radeon_gem.c
> index 458f92a70887..c19ad07eb7b5 100644
> --- a/drivers/gpu/drm/radeon/radeon_gem.c
> +++ b/drivers/gpu/drm/radeon/radeon_gem.c
> @@ -77,7 +77,8 @@ static const struct vm_operations_struct radeon_gem_vm_ops = {
>  	.fault = radeon_gem_fault,
>  	.open = ttm_bo_vm_open,
>  	.close = ttm_bo_vm_close,
> -	.access = ttm_bo_vm_access
> +	.access = ttm_bo_vm_access,
> +	.mprotect = ttm_bo_vm_mprotect
>  };
>  
>  static void radeon_gem_object_free(struct drm_gem_object *gobj)
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> index f56be5bc0861..fb325bad5db6 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> @@ -542,17 +542,29 @@ int ttm_bo_vm_access(struct vm_area_struct *vma, unsigned long addr,
>  }
>  EXPORT_SYMBOL(ttm_bo_vm_access);
>  
> +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;
> +}
> +EXPORT_SYMBOL(ttm_bo_vm_mprotect);
> +
>  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,
>  };
>  
>  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);
> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c
> index e6b1f98ec99f..e4bf7dc99320 100644
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c
> @@ -61,6 +61,7 @@ int vmw_mmap(struct file *filp, struct vm_area_struct *vma)
>  		.fault = vmw_bo_vm_fault,
>  		.open = ttm_bo_vm_open,
>  		.close = ttm_bo_vm_close,
> +		.mprotect = ttm_bo_vm_mprotect,
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  		.huge_fault = vmw_bo_vm_huge_fault,
>  #endif
> diff --git a/include/drm/ttm/ttm_bo_api.h b/include/drm/ttm/ttm_bo_api.h
> index f681bbdbc698..40eb95875355 100644
> --- a/include/drm/ttm/ttm_bo_api.h
> +++ b/include/drm/ttm/ttm_bo_api.h
> @@ -605,6 +605,10 @@ void ttm_bo_vm_close(struct vm_area_struct *vma);
>  
>  int ttm_bo_vm_access(struct vm_area_struct *vma, unsigned long addr,
>  		     void *buf, int len, int write);
> +
> +int ttm_bo_vm_mprotect(struct vm_area_struct *vma, unsigned long start,
> +		       unsigned long end, unsigned long newflags);
> +
>  bool ttm_bo_delayed_delete(struct ttm_device *bdev, bool remove_all);
>  
>  vm_fault_t ttm_bo_vm_dummy_page(struct vm_fault *vmf, pgprot_t prot);
> -- 
> 2.32.0
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch