diff mbox series

[RFC,05/19] mm: add early FAULT_FLAG_WRITE consistency checks

Message ID 20221107161740.144456-6-david@redhat.com
State Superseded
Headers show
Series mm/gup: remove FOLL_FORCE usage from drivers (reliable R/O long-term pinning) | expand

Commit Message

David Hildenbrand Nov. 7, 2022, 4:17 p.m. UTC 
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 mbox series

Patch

diff --git a/mm/memory.c b/mm/memory.c
index fe131273217a..826353da7b23 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5159,6 +5159,14 @@  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;
 }