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

David Hildenbrand
2022-11-16T10:34:43Z

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@redhat.com>
---
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;
}
--
2.38.1