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: David Hildenbrand <david@redhat.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memory.c b/mm/memory.c
index 07380ef..5e4df6b 100644 (file)
--- 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;
 }