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
