Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for shadow stack

On Fri, 2018-07-13 at 11:26 -0700, Dave Hansen wrote:

quoted

On 07/11/2018 10:05 AM, Yu-cheng Yu wrote:

quoted

My understanding is that we don't want to follow write pte if the page
is shared as read-only.  For a SHSTK page, that is (R/O + DIRTY_SW),
which means the SHSTK page has not been COW'ed.  Is that right?

Let's look at the code again:

quoted

-static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
+static inline bool can_follow_write_pte(pte_t pte, unsigned int flags,
+					bool shstk)
 {
+	bool pte_cowed = shstk ? is_shstk_pte(pte):pte_dirty(pte);
+
 	return pte_write(pte) ||
-		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
+		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_cowed);
 }

This is another case where the naming of pte_*() is biting us vs. the
perversion of the PTE bits.  The lack of comments and explanation inthe
patch is compounding the confusion.

We need to find a way to differentiate "someone can write to this PTE"
from "the write bit is set in this PTE".

In this particular hunk, we need to make it clear that pte_write() is
*never* true for shadowstack PTEs.  In other words, shadow stack VMAs
will (should?) never even *see* a pte_write() PTE.

I think this is a case where you just need to bite the bullet and
bifurcate can_follow_write_pte().  Just separate the shadowstack and
non-shadowstack parts.

In case I don't understand the exact issue.
What about the following.

diff --git a/mm/gup.c b/mm/gup.c
index fc5f98069f4e..45a0837b27f9 100644
--- a/mm/gup.c
+++ b/mm/gup.c

@@ -70,6 +70,12 @@ static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)

 		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
 }
 
+static inline bool can_follow_write_shstk_pte(pte_t pte, unsigned int flags)
+{
+	return ((flags & FOLL_FORCE) && (flags & FOLL_COW) &&
+		is_shstk_pte(pte));
+}
+
 static struct page *follow_page_pte(struct vm_area_struct *vma,
 		unsigned long address, pmd_t *pmd, unsigned int flags)
 {

@@ -105,9 +111,16 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,

 	}
 	if ((flags & FOLL_NUMA) && pte_protnone(pte))
 		goto no_page;
-	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
-		pte_unmap_unlock(ptep, ptl);
-		return NULL;
+	if (flags & FOLL_WRITE) {
+		if (is_shstk_mapping(vma->vm_flags)) {
+			if (!can_follow_write_shstk_pte(pte, flags)) {
+				pte_unmap_unlock(ptep, ptl);
+				return NULL;
+			}
+		} else if (!can_follow_write_pte(pte, flags) {
+			pte_unmap_unlock(ptep, ptl);
+			return NULL;
+		}