[PATCH v1 2/4] mm/gup: Make follow_page() succeed again on PROT_NONE PTEs/PMDs

[David Hildenbrand <david@redhat.com>]

We accidentally enforced PROT_NONE PTE/PMD permission checks for
follow_page() like we do for get_user_pages() and friends. That was
undesired, because follow_page() is usually only used to lookup a currently
mapped page, not to actually access it. Further, follow_page() does not
actually trigger fault handling, but instead simply fails.

Let's restore that behavior by conditionally setting FOLL_FORCE if
FOLL_WRITE is not set. This way, for example KSM and migration code will
no longer fail on PROT_NONE mapped PTEs/PMDS.

Handling this internally doesn't require us to add any new FOLL_FORCE
usage outside of GUP code.

While at it, refuse to accept FOLL_FORCE: we don't even perform VMA
permission checks like in check_vma_flags(), so especially
FOLL_FORCE|FOLL_WRITE would be dodgy.

This issue was identified by code inspection. We'll add some
documentation regarding FOLL_FORCE next.

Reported-by: Peter Xu <peterx@redhat.com>
Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()")
Cc: <stable@vger.kernel.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/gup.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/gup.c b/mm/gup.c
index 2493ffa10f4b..da9a5cc096ac 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -841,9 +841,17 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 	if (vma_is_secretmem(vma))
 		return NULL;
 
-	if (WARN_ON_ONCE(foll_flags & FOLL_PIN))
+	if (WARN_ON_ONCE(foll_flags & (FOLL_PIN | FOLL_FORCE)))
 		return NULL;
 
+	/*
+	 * Traditionally, follow_page() succeeded on PROT_NONE-mapped pages
+	 * but failed follow_page(FOLL_WRITE) on R/O-mapped pages. Let's
+	 * keep these semantics by setting FOLL_FORCE if FOLL_WRITE is not set.
+	 */
+	if (!(foll_flags & FOLL_WRITE))
+		foll_flags |= FOLL_FORCE;
+
 	page = follow_page_mask(vma, address, foll_flags, &ctx);
 	if (ctx.pgmap)
 		put_dev_pagemap(ctx.pgmap);
-- 
2.41.0