Re: [PATCH mm-unstable v1 05/20] mm: add early FAULT_FLAG_WRITE consistency checks

[Vlastimil Babka]

On 11/16/22 11:26, David Hildenbrand wrote:
> Let's catch abuse of FAULT_FLAG_WRITE early, such that we don't have to
> care in all other handlers and might get "surprises" if we forget to do
> so.
> 
> Write faults without VM_MAYWRITE don't make any sense, and our
> maybe_mkwrite() logic could have hidden such abuse for now.
> 
> Write faults without VM_WRITE on something that is not a COW mapping is
> similarly broken, and e.g., do_wp_page() could end up placing an
> anonymous page into a shared mapping, which would be bad.
> 
> This is a preparation for reliable R/O long-term pinning of pages in
> private mappings, whereby we want to make sure that we will never break
> COW in a read-only private mapping.
> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/memory.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index e014435a87db..c4fa378ec2a0 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5170,6 +5170,14 @@ static vm_fault_t sanitize_fault_flags(struct vm_area_struct *vma,
>  		 */
>  		if (!is_cow_mapping(vma->vm_flags))
>  			*flags &= ~FAULT_FLAG_UNSHARE;
> +	} else if (*flags & FAULT_FLAG_WRITE) {
> +		/* Write faults on read-only mappings are impossible ... */
> +		if (WARN_ON_ONCE(!(vma->vm_flags & VM_MAYWRITE)))
> +			return VM_FAULT_SIGSEGV;
> +		/* ... and FOLL_FORCE only applies to COW mappings. */
> +		if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE) &&
> +				 !is_cow_mapping(vma->vm_flags)))
> +			return VM_FAULT_SIGSEGV;
>  	}
>  	return 0;
>  }