eCryptfs: fix possible fault in ecryptfs_sync_page

This will avoid a possible fault in ecryptfs_sync_page().

In the function, eCryptfs calls sync_page() method of a lower filesystem
without checking its existence.  However, there are many filesystems that
don't have this method including network filesystems such as NFS, AFS, and
so forth.  They may fail when an eCryptfs page is waiting for lock.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Acked-by: Michael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/ecryptfs/mmap.c b/fs/ecryptfs/mmap.c
index e4ab7bc14efe..fd3f94d4a668 100644
--- a/fs/ecryptfs/mmap.c
+++ b/fs/ecryptfs/mmap.c
@@ -834,7 +834,8 @@ static void ecryptfs_sync_page(struct page *page)
                 ecryptfs_printk(KERN_DEBUG, "find_lock_page failed\n");
                 return;
         }
-        lower_page->mapping->a_ops->sync_page(lower_page);
+        if (lower_page->mapping->a_ops->sync_page)
+                lower_page->mapping->a_ops->sync_page(lower_page);
         ecryptfs_printk(KERN_DEBUG, "Unlocking page with index = [0x%.16x]\n",
                         lower_page->index);
         unlock_page(lower_page);