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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Axel Rasmussen <axelrasmussen@google.com>,
        Peter Xu <peterx@redhat.com>, Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: [PATCH 5.18 016/231] mm: userfaultfd: fix UFFDIO_CONTINUE on fallocated shmem pages
Date:   Tue, 19 Jul 2022 13:51:41 +0200
Message-Id: <20220719114715.404069935@linuxfoundation.org>
In-Reply-To: <20220719114714.247441733@linuxfoundation.org>
References: <20220719114714.247441733@linuxfoundation.org>
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From: Axel Rasmussen <axelrasmussen@google.com>

commit 73f37dbcfe1763ee2294c7717a1f571e27d17fd8 upstream.

When fallocate() is used on a shmem file, the pages we allocate can end up
with !PageUptodate.

Since UFFDIO_CONTINUE tries to find the existing page the user wants to
map with SGP_READ, we would fail to find such a page, since
shmem_getpage_gfp returns with a "NULL" pagep for SGP_READ if it discovers
!PageUptodate.  As a result, UFFDIO_CONTINUE returns -EFAULT, as it would
do if the page wasn't found in the page cache at all.

This isn't the intended behavior.  UFFDIO_CONTINUE is just trying to find
if a page exists, and doesn't care whether it still needs to be cleared or
not.  So, instead of SGP_READ, pass in SGP_NOALLOC.  This is the same,
except for one critical difference: in the !PageUptodate case, SGP_NOALLOC
will clear the page and then return it.  With this change, UFFDIO_CONTINUE
works properly (succeeds) on a shmem file which has been fallocated, but
otherwise not modified.

Link: https://lkml.kernel.org/r/20220610173812.1768919-1-axelrasmussen@google.com
Fixes: 153132571f02 ("userfaultfd/shmem: support UFFDIO_CONTINUE for shmem")
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 mm/userfaultfd.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -231,7 +231,10 @@ static int mcontinue_atomic_pte(struct m
 	struct page *page;
 	int ret;
 
-	ret = shmem_getpage(inode, pgoff, &page, SGP_READ);
+	ret = shmem_getpage(inode, pgoff, &page, SGP_NOALLOC);
+	/* Our caller expects us to return -EFAULT if we failed to find page. */
+	if (ret == -ENOENT)
+		ret = -EFAULT;
 	if (ret)
 		goto out;
 	if (!page) {