On 07.11.22 20:03, Nadav Amit wrote:
On Nov 7, 2022, at 8:17 AM, David Hildenbrand david@redhat.com wrote:
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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 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.