[PATCH 2/7] mm/hugetlb: Fix hugetlb_follow_page_mask() on permission checks

[Peter Xu <peterx@redhat.com>]

It seems hugetlb_follow_page_mask() was missing permission checks.  For
example, one follow_page() can get the hugetlb page with FOLL_WRITE even if
the page is read-only.

And it wasn't there even in the old follow_page_mask(), where we can
reference from before commit 57a196a58421 ("hugetlb: simplify hugetlb
handling in follow_page_mask").

Let's add them, namely, either the need to CoW due to missing write bit, or
proper CoR on !AnonExclusive pages over R/O pins to reject the follow page.
That brings this function closer to follow_hugetlb_page().

I just doubt how many of us care for that, for FOLL_PIN follow_page doesn't
really happen at all.  But we'll care, and care more if we switch over
slow-gup to use hugetlb_follow_page_mask().  We'll also care when to return
-EMLINK then, as that's the gup internal api to mean "we should do CoR".

When at it, switching the try_grab_page() to use WARN_ON_ONCE(), to be
clear that it just should never fail.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/hugetlb.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 82dfdd96db4c..9c261921b2cf 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6481,8 +6481,21 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
 	ptl = huge_pte_lock(h, mm, pte);
 	entry = huge_ptep_get(pte);
 	if (pte_present(entry)) {
-		page = pte_page(entry) +
-				((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
+		page = pte_page(entry);
+
+		if (gup_must_unshare(vma, flags, page)) {
+			/* Tell the caller to do Copy-On-Read */
+			page = ERR_PTR(-EMLINK);
+			goto out;
+		}
+
+		if ((flags & FOLL_WRITE) && !pte_write(entry)) {
+			page = NULL;
+			goto out;
+		}
+
+		page += ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
+
 		/*
 		 * Note that page may be a sub-page, and with vmemmap
 		 * optimizations the page struct may be read only.
@@ -6492,10 +6505,7 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
 		 * try_grab_page() should always be able to get the page here,
 		 * because we hold the ptl lock and have verified pte_present().
 		 */
-		if (try_grab_page(page, flags)) {
-			page = NULL;
-			goto out;
-		}
+		WARN_ON_ONCE(try_grab_page(page, flags));
 	}
 out:
 	spin_unlock(ptl);
-- 
2.40.1