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

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On 07.11.22 20:03, Nadav Amit wrote:
On Nov 7, 2022, at 8:17 AM, David Hildenbrand <david@xxxxxxxxxx> wrote:

!! External Email

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>
---
mm/memory.c | 8 ++++++++
1 file changed, 8 insertions(+)

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;

Not sure about the WARN_*(). Seems as if it might trigger in benign even if
rare scenarios, e.g., mprotect() racing with page-fault.


We most certainly would want to catch any such broken/racy cases. There are no benign cases I could possibly think of.

Page faults need the mmap lock in read. mprotect() / VMA changes need the mmap lock in write. Whoever calls handle_mm_fault() is supposed to properly check VMA permissions.


--
Thanks,

David / dhildenb




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