[PATCH v2] mm: userfaultfd: fix UFFDIO_CONTINUE on fallocated shmem pages

[Axel Rasmussen]

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.

Fixes: 153132571f02 ("userfaultfd/shmem: support UFFDIO_CONTINUE for shmem")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
---
 mm/userfaultfd.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 4f4892a5f767..07d3befc80e4 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -246,7 +246,10 @@ 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) {
-- 
2.36.1.476.g0c4daa206d-goog