Re: [PATCH v2 17/39] mm: Fixup places that call pte_mkwrite() directly

On Fri, 2022-10-14 at 17:52 +0200, Peter Zijlstra wrote:

On Thu, Sep 29, 2022 at 03:29:14PM -0700, Rick Edgecombe wrote:

quoted

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 7327b2573f7c..b49372c7de41 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c

@@ -63,6 +63,7 @@ int mfill_atomic_install_pte(struct mm_struct

*dst_mm, pmd_t *dst_pmd,
       int ret;
       pte_t _dst_pte, *dst_pte;
       bool writable = dst_vma->vm_flags & VM_WRITE;
+     bool shstk = dst_vma->vm_flags & VM_SHADOW_STACK;
       bool vm_shared = dst_vma->vm_flags & VM_SHARED;
       bool page_in_cache = page->mapping;
       spinlock_t *ptl;

@@ -83,9 +84,12 @@ int mfill_atomic_install_pte(struct mm_struct

*dst_mm, pmd_t *dst_pmd,
               writable = false;
       }
  
-     if (writable)
-             _dst_pte = pte_mkwrite(_dst_pte);
-     else
+     if (writable) {
+             if (shstk)
+                     _dst_pte = pte_mkwrite_shstk(_dst_pte);
+             else
+                     _dst_pte = pte_mkwrite(_dst_pte);
+     } else
               /*
                * We need this to make sure write bit removed; as
mk_pte()
                * could return a pte with write bit set.

Urgh.. that's unfortunate. But yeah, I don't see a way to make that
pretty either.

Nadav pointed out that:
entry = maybe_mkwrite(pte_mkdirty(entry), vma);

and:
if (vma->vm_flags & VM_WRITE)
	entry = pte_mkwrite(pte_mkdirty(entry));

Are not actually the same, because in the former the non-writable PTE
gets marked dirty. So I was actually going to add two more cases like
the ugly case.