[PATCH v9 3/5] proc: Disable cancellation of subset=pid option

[Alexey Gladkov <legion@kernel.org>]

When procfs is mounted with subset=pid option, where is no way to
remount it with this option removed. This is done in order not to make
visible what ever was hidden since some checks occur during mount.

This patch makes the limitation explicit and prints an error message.

Signed-off-by: Alexey Gladkov <legion@kernel.org>
---
 fs/proc/root.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/fs/proc/root.c b/fs/proc/root.c
index 6d18f9ee0375..05558654df31 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -223,7 +223,7 @@ static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
 	return 0;
 }
 
-static void proc_apply_options(struct proc_fs_info *fs_info,
+static int proc_apply_options(struct proc_fs_info *fs_info,
 			       struct fs_context *fc,
 			       struct user_namespace *user_ns)
 {
@@ -233,13 +233,17 @@ static void proc_apply_options(struct proc_fs_info *fs_info,
 		fs_info->pid_gid = make_kgid(user_ns, ctx->gid);
 	if (ctx->mask & (1 << Opt_hidepid))
 		fs_info->hide_pid = ctx->hidepid;
-	if (ctx->mask & (1 << Opt_subset))
+	if (ctx->mask & (1 << Opt_subset)) {
+		if (ctx->pidonly != PROC_PIDONLY_ON && fs_info->pidonly == PROC_PIDONLY_ON)
+			return invalf(fc, "proc: subset=pid cannot be unset\n");
 		fs_info->pidonly = ctx->pidonly;
+	}
 	if (ctx->mask & (1 << Opt_pidns) &&
 	    !WARN_ON_ONCE(fc->purpose == FS_CONTEXT_FOR_RECONFIGURE)) {
 		put_pid_ns(fs_info->pid_ns);
 		fs_info->pid_ns = get_pid_ns(ctx->pid_ns);
 	}
+	return 0;
 }
 
 static int proc_fill_super(struct super_block *s, struct fs_context *fc)
@@ -255,7 +259,9 @@ static int proc_fill_super(struct super_block *s, struct fs_context *fc)
 
 	fs_info->pid_ns = get_pid_ns(ctx->pid_ns);
 	fs_info->mounter_cred = get_cred(fc->cred);
-	proc_apply_options(fs_info, fc, current_user_ns());
+	ret = proc_apply_options(fs_info, fc, current_user_ns());
+	if (ret)
+		return ret;
 
 	/* User space would break if executables or devices appear on proc */
 	s->s_iflags |= SB_I_USERNS_VISIBLE | SB_I_NOEXEC | SB_I_NODEV;
@@ -304,8 +310,7 @@ static int proc_reconfigure(struct fs_context *fc)
 
 	sync_filesystem(sb);
 
-	proc_apply_options(fs_info, fc, current_user_ns());
-	return 0;
+	return proc_apply_options(fs_info, fc, current_user_ns());
 }
 
 static int proc_get_tree(struct fs_context *fc)
-- 
2.53.0