Commit 79881fed authored by David Hildenbrand's avatar David Hildenbrand Committed by Andrew Morton
Browse files

mm: add early FAULT_FLAG_WRITE consistency checks 

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.

Link: https://lkml.kernel.org/r/20221116102659.70287-6-david@redhat.com


Signed-off-by: default avatarDavid Hildenbrand <david@redhat.com>
Reviewed-by: default avatarVlastimil Babka <vbabka@suse.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent cdc5021c

mm/memory.c

+8 −0
Original line number Diff line number Diff line
@@ -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;

}