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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>
This commit is contained in:
David Hildenbrand 2022-11-16 11:26:44 +01:00 committed by Andrew Morton
parent cdc5021cda
commit 79881fed60
1 changed files with 8 additions and 0 deletions
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8
mm/memory.c
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@ -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;
}