[PATCH] drm/ttm: nuke VM_MIXEDMAP on BO mappings

[Thomas Hellström (Intel)]

On 6/2/21 10:30 AM, Christian König wrote:
> We discussed if that is really the right approach for quite a while now, but
> digging deeper into a bug report on arm turned out that this is actually
> horrible broken right now.
>
> The reason for this is that vmf_insert_mixed_prot() always tries to grab
> a reference to the underlaying page on architectures without
> ARCH_HAS_PTE_SPECIAL and as far as I can see also enabled GUP.
>
> So nuke using VM_MIXEDMAP here and use VM_PFNMAP instead.
>
> Also set VM_SHARED, not 100% sure if that is needed with VM_PFNMAP, but better
> save than sorry.
>
> Signed-off-by: Christian König <christian.koenig at amd.com>
> Bugs: https://gitlab.freedesktop.org/drm/amd/-/issues/1606#note_936174
> ---
>   drivers/gpu/drm/ttm/ttm_bo_vm.c | 29 +++++++----------------------
>   1 file changed, 7 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> index 9bd15cb39145..bf86ae849340 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> @@ -359,12 +359,7 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
>   		 * at arbitrary times while the data is mmap'ed.
>   		 * See vmf_insert_mixed_prot() for a discussion.
>   		 */
> -		if (vma->vm_flags & VM_MIXEDMAP)
> -			ret = vmf_insert_mixed_prot(vma, address,
> -						    __pfn_to_pfn_t(pfn, PFN_DEV),
> -						    prot);
> -		else
> -			ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
> +		ret = vmf_insert_pfn_prot(vma, address, pfn, prot);

I think vmwgfx still uses MIXEDMAP. (Which is ofc same bug and should be 
changed).

>   
>   		/* Never error on prefaulted PTEs */
>   		if (unlikely((ret & VM_FAULT_ERROR))) {
> @@ -411,15 +406,9 @@ vm_fault_t ttm_bo_vm_dummy_page(struct vm_fault *vmf, pgprot_t prot)
>   	pfn = page_to_pfn(page);
>   
>   	/* Prefault the entire VMA range right away to avoid further faults */
> -	for (address = vma->vm_start; address < vma->vm_end; address += PAGE_SIZE) {
> -
> -		if (vma->vm_flags & VM_MIXEDMAP)
> -			ret = vmf_insert_mixed_prot(vma, address,
> -						    __pfn_to_pfn_t(pfn, PFN_DEV),
> -						    prot);
> -		else
> -			ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
> -	}
> +	for (address = vma->vm_start; address < vma->vm_end;
> +	     address += PAGE_SIZE)
> +		ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
>   
>   	return ret;
>   }
> @@ -576,14 +565,10 @@ static void ttm_bo_mmap_vma_setup(struct ttm_buffer_object *bo, struct vm_area_s
>   
>   	vma->vm_private_data = bo;
>   
> -	/*
> -	 * We'd like to use VM_PFNMAP on shared mappings, where
> -	 * (vma->vm_flags & VM_SHARED) != 0, for performance reasons,
> -	 * but for some reason VM_PFNMAP + x86 PAT + write-combine is very
> -	 * bad for performance. Until that has been sorted out, use
> -	 * VM_MIXEDMAP on all mappings. See freedesktop.org bug #75719
> +	/* Enforce VM_SHARED here since no driver backend actually supports COW
> +	 * on TTM buffer object mappings.

I think by default all TTM drivers support COW mappings in the sense 
that written data never makes it to the bo but stays in anonymous pages, 
although I can't find a single usecase. So comment should be changed to 
state that they are useless for us and that we can't support COW 
mappings with VM_PFNMAP.

>   	 */
> -	vma->vm_flags |= VM_MIXEDMAP;
> +	vma->vm_flags |= VM_PFNMAP | VM_SHARED;

Hmm, shouldn't we refuse COW mappings instead, like my old patch on this 
subject did? In theory someone could be setting up what she thinks is a 
private mapping to a shared buffer object, and write sensitive data to 
it, which will immediately leak. It's a simple check, could open-code if 
necessary.

>   	vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
>   }
>   

/Thomas