linux-kernel - [PATCH tty-next 6/6] devpts: Change the owner of /dev/pts/ptmx to the mounter of /dev/pts

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Message-Id: <20160914185338.15126-6-ebiederm@xmission.com>
Date:   Wed, 14 Sep 2016 13:53:38 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>
Cc:     "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Linux Containers <containers@...ts.linux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH tty-next 6/6] devpts: Change the owner of /dev/pts/ptmx to the mounter of /dev/pts

In 99.99% of the cases only root in a user namespace can mount /dev/pts
and in those cases the owner of /dev/pts/ptmx will remain root.root

In the oddball case where someone else has CAP_SYS_ADMIN this code
modifies the /dev/pts mount code to use current_fsuid and current_fsgid
as the values to use when creating the /dev/ptmx inode.  As is done
when any other file is created.

This is a code simplification, and it allows running without a root
user entirely.

Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 fs/devpts/inode.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index 17593d3fcdbe..442d1a7e671b 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -272,13 +272,8 @@ static int mknod_ptmx(struct super_block *sb)
 	struct dentry *root = sb->s_root;
 	struct pts_fs_info *fsi = DEVPTS_SB(sb);
 	struct pts_mount_opts *opts = &fsi->mount_opts;
-	kuid_t root_uid;
-	kgid_t root_gid;
-
-	root_uid = make_kuid(current_user_ns(), 0);
-	root_gid = make_kgid(current_user_ns(), 0);
-	if (!uid_valid(root_uid) || !gid_valid(root_gid))
-		return -EINVAL;
+	kuid_t ptmx_uid = current_fsuid();
+	kgid_t ptmx_gid = current_fsgid();
 
 	inode_lock(d_inode(root));
 
@@ -309,8 +304,8 @@ static int mknod_ptmx(struct super_block *sb)
 
 	mode = S_IFCHR|opts->ptmxmode;
 	init_special_inode(inode, mode, MKDEV(TTYAUX_MAJOR, 2));
-	inode->i_uid = root_uid;
-	inode->i_gid = root_gid;
+	inode->i_uid = ptmx_uid;
+	inode->i_gid = ptmx_gid;
 
 	d_add(dentry, inode);
 
-- 
2.8.3