[PATCH 4.19 08/52] userfaultfd: fix mmap_changing checking in mfill_atomic_hugetlb

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Lokesh Gidra <lokeshgidra@google.com>

commit 67695f18d55924b2013534ef3bdc363bc9e14605 upstream.

In mfill_atomic_hugetlb(), mmap_changing isn't being checked
again if we drop mmap_lock and reacquire it. When the lock is not held,
mmap_changing could have been incremented. This is also inconsistent
with the behavior in mfill_atomic().

Link: https://lkml.kernel.org/r/20240117223729.1444522-1-lokeshgidra@google.com
Fixes: df2cc96e77011 ("userfaultfd: prevent non-cooperative events vs mcopy_atomic races")
Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nicolas Geoffray <ngeoffray@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 mm/userfaultfd.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -179,6 +179,7 @@ static __always_inline ssize_t __mcopy_a
 					      unsigned long dst_start,
 					      unsigned long src_start,
 					      unsigned long len,
+					      bool *mmap_changing,
 					      bool zeropage)
 {
 	int vm_alloc_shared = dst_vma->vm_flags & VM_SHARED;
@@ -310,6 +311,15 @@ retry:
 				goto out;
 			}
 			down_read(&dst_mm->mmap_sem);
+			/*
+			 * If memory mappings are changing because of non-cooperative
+			 * operation (e.g. mremap) running in parallel, bail out and
+			 * request the user to retry later
+			 */
+			if (mmap_changing && READ_ONCE(*mmap_changing)) {
+				err = -EAGAIN;
+				break;
+			}
 
 			dst_vma = NULL;
 			goto retry;
@@ -391,6 +401,7 @@ extern ssize_t __mcopy_atomic_hugetlb(st
 				      unsigned long dst_start,
 				      unsigned long src_start,
 				      unsigned long len,
+				      bool *mmap_changing,
 				      bool zeropage);
 #endif /* CONFIG_HUGETLB_PAGE */
 
@@ -508,7 +519,8 @@ retry:
 	 */
 	if (is_vm_hugetlb_page(dst_vma))
 		return  __mcopy_atomic_hugetlb(dst_mm, dst_vma, dst_start,
-						src_start, len, zeropage);
+					       src_start, len, mmap_changing,
+					       zeropage);
 
 	if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
 		goto out_unlock;