libfs: Modify mount_pseudo_xattr to be clear it is not a userspace mount

[ Upstream commit 75422726b0f717d67db3283c2eb5bc14fa2619c5 ]

Add MS_KERNMOUNT to the flags that are passed.
Use sget_userns and force &init_user_ns instead of calling sget so that
even if called from a weird context the internal filesystem will be
considered to be in the intial user namespace.

Luis Ressel reported that the the failure to pass MS_KERNMOUNT into
mount_pseudo broke his in development graphics driver that uses the
generic drm infrastructure.  I am not certain the deriver was bug
free in it's usage of that infrastructure but since
mount_pseudo_xattr can never be triggered by userspace it is clearer
and less error prone, and less problematic for the code to be explicit.

Reported-by: Luis Ressel <aranea@aixah.de>
Tested-by: Luis Ressel <aranea@aixah.de>
Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/libfs.c b/fs/libfs.c
index 48826d4..9588780 100644 (file)
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -245,7 +245,8 @@ struct dentry *mount_pseudo_xattr(struct file_system_type *fs_type, char *name,
        struct inode *root;
        struct qstr d_name = QSTR_INIT(name, strlen(name));
 
-       s = sget(fs_type, NULL, set_anon_super, MS_NOUSER, NULL);
+       s = sget_userns(fs_type, NULL, set_anon_super, MS_KERNMOUNT|MS_NOUSER,
+                       &init_user_ns, NULL);
        if (IS_ERR(s))
                return ERR_CAST(s);