[PATCH] Security: Handle hidepid option correctly

[d17103513@gmail.com]

From: Cheng Yang <chengyang@xiaomi.com>

The proc_parse_options() call from proc_mount() runs only once at boot
time.  So on any later mount attempt, any mount options are ignored
because ->s_root is already initialized.
As a consequence, "mount -o <options>" will ignore the options.  The
only way to change mount options is "mount -o remount,<options>".
To fix this, parse the mount options unconditionally.

Signed-off-by: Cheng Yang <chengyang@xiaomi.com>
---
 fs/proc/inode.c | 4 ----
 fs/proc/root.c  | 3 +++
 2 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 5792f9e..ec509d1 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -493,13 +493,9 @@ struct inode *proc_get_inode(struct super_block *sb, struct proc_dir_entry *de)
 
 int proc_fill_super(struct super_block *s, void *data, int silent)
 {
-	struct pid_namespace *ns = get_pid_ns(s->s_fs_info);
 	struct inode *root_inode;
 	int ret;
 
-	if (!proc_parse_options(data, ns))
-		return -EINVAL;
-
 	/* User space would break if executables or devices appear on proc */
 	s->s_iflags |= SB_I_USERNS_VISIBLE | SB_I_NOEXEC | SB_I_NODEV;
 	s->s_flags |= SB_NODIRATIME | SB_NOSUID | SB_NOEXEC;
diff --git a/fs/proc/root.c b/fs/proc/root.c
index f4b1a9d..f5f3bf3 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -98,6 +98,9 @@ static struct dentry *proc_mount(struct file_system_type *fs_type,
 		ns = task_active_pid_ns(current);
 	}
 
+	if (!proc_parse_options(data, ns))
+		return ERR_PTR(-EINVAL);
+
 	return mount_ns(fs_type, flags, data, ns, ns->user_ns, proc_fill_super);
 }
 
-- 
1.9.1