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Subject[PATCH] mm/userfaultfd: Replace kmap/kmap_atomic() with kmap_local_page()
Date
From: Ira Weiny <ira.weiny@intel.com>

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@intel.com/
[2] https://lore.kernel.org/all/Y1Mh2S7fUGQ%2FiKFR@iweiny-desk3/

Cc: Matthew Wilcox <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
---
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
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