mm: userfaultfd: fix UFFDIO_CONTINUE on fallocated shmem pages

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>

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index c9bab48..3bbaf5f 100644 (file)
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -227,7 +227,10 @@ static int mcontinue_atomic_pte(struct mm_struct *dst_mm,
        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) {