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Date:   Thu, 8 Jun 2023 18:26:04 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Andrew Morton <akpm@linux-foundation.org>
cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>,
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        Zack Rusin <zackr@vmware.com>, Jason Gunthorpe <jgg@ziepe.ca>,
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        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: [PATCH v2 15/32] mm/userfaultfd: allow pte_offset_map_lock() to
 fail
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Precedence: bulk

mfill_atomic_install_pte() and mfill_atomic_pte_zeropage() treat
failed pte_offset_map_lock() as -EAGAIN, which mfill_atomic() already
returns to user for a similar race.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/userfaultfd.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index e97a0b4889fc..5fd787158c70 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -76,7 +76,10 @@ int mfill_atomic_install_pte(pmd_t *dst_pmd,
 	if (flags & MFILL_ATOMIC_WP)
 		_dst_pte = pte_mkuffd_wp(_dst_pte);
 
+	ret = -EAGAIN;
 	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
+	if (!dst_pte)
+		goto out;
 
 	if (vma_is_shmem(dst_vma)) {
 		/* serialize against truncate with the page table lock */
@@ -121,6 +124,7 @@ int mfill_atomic_install_pte(pmd_t *dst_pmd,
 	ret = 0;
 out_unlock:
 	pte_unmap_unlock(dst_pte, ptl);
+out:
 	return ret;
 }
 
@@ -212,7 +216,10 @@ static int mfill_atomic_pte_zeropage(pmd_t *dst_pmd,
 
 	_dst_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr),
 					 dst_vma->vm_page_prot));
+	ret = -EAGAIN;
 	dst_pte = pte_offset_map_lock(dst_vma->vm_mm, dst_pmd, dst_addr, &ptl);
+	if (!dst_pte)
+		goto out;
 	if (dst_vma->vm_file) {
 		/* the shmem MAP_PRIVATE case requires checking the i_size */
 		inode = dst_vma->vm_file->f_inode;
@@ -231,6 +238,7 @@ static int mfill_atomic_pte_zeropage(pmd_t *dst_pmd,
 	ret = 0;
 out_unlock:
 	pte_unmap_unlock(dst_pte, ptl);
+out:
 	return ret;
 }
 
-- 
2.35.3