ovl: Prevent rw remount when it should be ro mount

Overlayfs should be mounted read-only when upper-fs is read-only or nonexistent.
But now it can be remounted read-write and this can cause kernel panic.
So we should prevent read-write remount when the above situation happens.

Signed-off-by: Seunghun Lee <waydi1@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

1 files changed, 12 insertions, 0 deletions

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 9e94f4acef1a..b90952f528b1 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -525,10 +525,22 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
         return 0;
 }
 
+static int ovl_remount(struct super_block *sb, int *flags, char *data)
+{
+        struct ovl_fs *ufs = sb->s_fs_info;
+
+        if (!(*flags & MS_RDONLY) &&
+            (!ufs->upper_mnt || (ufs->upper_mnt->mnt_sb->s_flags & MS_RDONLY)))
+                return -EROFS;
+
+        return 0;
+}
+
 static const struct super_operations ovl_super_operations = {
         .put_super      = ovl_put_super,
         .statfs         = ovl_statfs,
         .show_options   = ovl_show_options,
+        .remount_fs     = ovl_remount,
 };
 
 enum {