From ac96edb501b6313955813ec4b4ae475867cf3751 Mon Sep 17 00:00:00 2001
From: Lokesh Gidra <lokeshgidra@google.com>
Date: Wed, 17 Jan 2024 14:37:29 -0800
Subject: [PATCH] BACKPORT: userfaultfd: fix mmap_changing checking in
 mfill_atomic_hugetlb

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>

(cherry picked from commit 67695f18d55924b2013534ef3bdc363bc9e14605)
Conflicts:
	mm/userfaultfd.c

1. Update mfill_atomic_hugetlb() parameters to pass 'wp_copy' and 'mode'
   instead of 'flags'.

Bug: 320478828
Change-Id: I11ef09b2b8e477c32cc731205fd48b25bcbd020f
Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
---
 mm/userfaultfd.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 9149a95282e0..af165216fbbd 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -328,7 +328,8 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
 					      unsigned long src_start,
 					      unsigned long len,
 					      enum mcopy_atomic_mode mode,
-					      bool wp_copy)
+					      bool wp_copy,
+					      atomic_t *mmap_changing)
 {
 	int vm_shared = dst_vma->vm_flags & VM_SHARED;
 	ssize_t err;
@@ -445,6 +446,15 @@ retry:
 				goto out;
 			}
 			mmap_read_lock(dst_mm);
+			/*
+			 * 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 && atomic_read(mmap_changing)) {
+				err = -EAGAIN;
+				break;
+			}
 
 			dst_vma = NULL;
 			goto retry;
@@ -481,7 +491,8 @@ extern ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
 				      unsigned long src_start,
 				      unsigned long len,
 				      enum mcopy_atomic_mode mode,
-				      bool wp_copy);
+				      bool wp_copy,
+				      atomic_t *mmap_changing);
 #endif /* CONFIG_HUGETLB_PAGE */
 
 static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
@@ -602,7 +613,7 @@ retry:
 	if (is_vm_hugetlb_page(dst_vma))
 		return  __mcopy_atomic_hugetlb(dst_mm, dst_vma, dst_start,
 					       src_start, len, mcopy_mode,
-					       wp_copy);
+					       wp_copy, mmap_changing);
 
 	if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
 		goto out_unlock;