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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@xmission.com>
This commit is contained in:
Eric W. Biederman 2015-12-08 00:36:51 -06:00
parent 985e5d856c
commit 93f0a88bd4
1 changed files with 4 additions and 9 deletions
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13
fs/devpts/inode.c
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@ -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);