[PATCH] mm/userfaultfd: Replace kmap/kmap_atomic() with kmap_local_page()

[ira . weiny]

From: Ira Weiny <ira.weiny@xxxxxxxxx>

kmap() and kmap_atomic() are being deprecated in favor of
kmap_local_page() which is appropriate for any thread local context.[1]

A recent locking bug report with userfaultfd showed that the conversion
of these kmap_atomic()'s required care with regard to the prevention of
deadlock.[2]

Complete kmap conversion in userfaultfd by replacing the kmap() and
kmap_atomic() calls with kmap_local_page().  When replacing the
kmap_atomic() call ensure page faults continue to be disabled to support
the correct fall back behavior and add a comment to inform future souls
of the requirement.

[1] https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@xxxxxxxxx/
[2] https://lore.kernel.org/all/Y1Mh2S7fUGQ%2FiKFR@iweiny-desk3/

Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
---
 mm/userfaultfd.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index e24e8a47ce8a..c5db06f4d28d 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -157,11 +157,18 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
 		if (!page)
 			goto out;
 
-		page_kaddr = kmap_atomic(page);
+		page_kaddr = kmap_local_page(page);
+		/*
+		 * The mmap_lock is held here.  Disable page faults to
+		 * prevent deadlock should copy_from_user() fault.  The
+		 * copy will be retried outside the mmap_lock.
+		 */
+		pagefault_disable();
 		ret = copy_from_user(page_kaddr,
 				     (const void __user *) src_addr,
 				     PAGE_SIZE);
-		kunmap_atomic(page_kaddr);
+		pagefault_enable();
+		kunmap_local(page_kaddr);
 
 		/* fallback to copy_from_user outside mmap_lock */
 		if (unlikely(ret)) {
@@ -646,11 +653,11 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
 			mmap_read_unlock(dst_mm);
 			BUG_ON(!page);
 
-			page_kaddr = kmap(page);
+			page_kaddr = kmap_local_page(page);
 			err = copy_from_user(page_kaddr,
 					     (const void __user *) src_addr,
 					     PAGE_SIZE);
-			kunmap(page);
+			kunmap_local(page_kaddr);
 			if (unlikely(err)) {
 				err = -EFAULT;
 				goto out;
-- 
2.37.2