--- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
@@ -619,6 +619,14 @@ static int i915_gem_userptr_get_pages(st
 				      GFP_KERNEL |
 				      __GFP_NORETRY |
 				      __GFP_NOWARN);
+		/*
+		 * Using __get_user_pages_fast() with a read-only
+		 * access is questionable. A read-only page may be
+		 * COW-broken, and then this might end up giving
+		 * the wrong side of the COW..
+		 *
+		 * We may or may not care.
+		 */
 		if (pvec) /* defer to worker if malloc fails */
 			pinned = __get_user_pages_fast(obj->userptr.ptr,
 						       num_pages,
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -161,13 +161,22 @@ static int follow_pfn_pte(struct vm_area
 }
 
 /*
- * FOLL_FORCE can write to even unwritable pte's, but only
- * after we've gone through a COW cycle and they are dirty.
+ * FOLL_FORCE or a forced COW break can write even to unwritable pte's,
+ * but only after we've gone through a COW cycle and they are dirty.
  */
 static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
 {
-	return pte_write(pte) ||
-		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
+	return pte_write(pte) || ((flags & FOLL_COW) && pte_dirty(pte));
+}
+
+/*
+ * A (separate) COW fault might break the page the other way and
+ * get_user_pages() would return the page from what is now the wrong
+ * VM. So we need to force a COW break at GUP time even for reads.
+ */
+static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags)
+{
+	return is_cow_mapping(vma->vm_flags) && (flags & FOLL_GET);
 }
 
 static struct page *follow_page_pte(struct vm_area_struct *vma,
@@ -823,12 +832,18 @@ static long __get_user_pages(struct task
 				goto out;
 			}
 			if (is_vm_hugetlb_page(vma)) {
+				if (should_force_cow_break(vma, foll_flags))
+					foll_flags |= FOLL_WRITE;
 				i = follow_hugetlb_page(mm, vma, pages, vmas,
 						&start, &nr_pages, i,
-						gup_flags, nonblocking);
+						foll_flags, nonblocking);
 				continue;
 			}
 		}
+
+		if (should_force_cow_break(vma, foll_flags))
+			foll_flags |= FOLL_WRITE;
+
 retry:
 		/*
 		 * If we have a pending SIGKILL, don't keep faulting pages and
@@ -2316,6 +2331,10 @@ static bool gup_fast_permitted(unsigned
  *
  * If the architecture does not support this function, simply return with no
  * pages pinned.
+ *
+ * Careful, careful! COW breaking can go either way, so a non-write
+ * access can get ambiguous page results. If you call this function without
+ * 'write' set, you'd better be sure that you're ok with that ambiguity.
  */
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			  struct page **pages)
@@ -2343,6 +2362,12 @@ int __get_user_pages_fast(unsigned long
 	 *
 	 * We do not adopt an rcu_read_lock(.) here as we also want to
 	 * block IPIs that come from THPs splitting.
+	 *
+	 * NOTE! We allow read-only gup_fast() here, but you'd better be
+	 * careful about possible COW pages. You'll get _a_ COW page, but
+	 * not necessarily the one you intended to get depending on what
+	 * COW event happens after this. COW may break the page copy in a
+	 * random direction.
 	 */
 
 	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
@@ -2415,10 +2440,17 @@ int get_user_pages_fast(unsigned long st
 	if (unlikely(!access_ok((void __user *)start, len)))
 		return -EFAULT;
 
+	/*
+	 * The FAST_GUP case requires FOLL_WRITE even for pure reads,
+	 * because get_user_pages() may need to cause an early COW in
+	 * order to avoid confusing the normal COW routines. So only
+	 * targets that are already writable are safe to do by just
+	 * looking at the page tables.
+	 */
 	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
 	    gup_fast_permitted(start, end)) {
 		local_irq_disable();
-		gup_pgd_range(addr, end, gup_flags, pages, &nr);
+		gup_pgd_range(addr, end, gup_flags | FOLL_WRITE, pages, &nr);
 		local_irq_enable();
 		ret = nr;
 	}
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1454,13 +1454,12 @@ out_unlock:
 }
 
 /*
- * FOLL_FORCE can write to even unwritable pmd's, but only
- * after we've gone through a COW cycle and they are dirty.
+ * FOLL_FORCE or a forced COW break can write even to unwritable pmd's,
+ * but only after we've gone through a COW cycle and they are dirty.
  */
 static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
 {
-	return pmd_write(pmd) ||
-	       ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
+	return pmd_write(pmd) || ((flags & FOLL_COW) && pmd_dirty(pmd));
 }
 
 struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
