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According to POSIX the grantpt() function does the following: The grantpt() function shall change the mode and ownership of the slave pseudo-terminal device associated with its master pseudo-terminal counterpart. The fildes argument is a file descriptor that refers to a master pseudo-terminal device. The user ID of the slave shall be set to the real UID of the calling process and the group ID shall be set to an unspecified group ID. The permission mode of the slave pseudo-terminal shall be set to readable and writable by the owner, and writable by the group. Historically the GNU libc has been responsible to setup the permission mode to 0620 and the group to 'tty' usually number 5, using the pt_chown helper, badly known for its security issues. With the creation of the devpts filesytem in the Linux kernel, this responsibility has been moved to the Linux kernel. The system is responsible to mount the devpts filesystem in /dev/pts with the options gid=5 and mode=0620. In that case the GNU libc has nothing to do and pt_chown is not need anymore. So far so good. The problem is that by default the devpts filesystem is shared between all mounts, and that contrary to other filesystem, the mount options are honored at the second mount, including for the default mount options. Given it corresponds to mode=0600 without gid parameter (that is the filesystem GID of the creating process), it's common to see systems where the devpts filesystem is mounted using these options. It is enough to run a "mount -t devpts devpts /mychroot/dev/pts" to come into this situation, and it's unfortunately wrongly used in a lot of scripts dealing with chroots, or for creating virtual machines images. When this happens the GNU libc tries to fix the group and permission mode of the pty nodes, and given it fails to do so for non-root users, grantpt() almost always fail. It means users are not able to open new terminals. This patch changes grantpt() to not enforce this anymore, while still enforcing minimum security measures to the permission mode. Therefore the responsibility to follow POSIX is now shared at the system level, i.e. kernel + system scripts + GNU libc. It stops trying to change the group, and makes the pty node readable and writable by the owner, and writable by the group only when originally writable and when the group is the tty one. As a result, on a system wrongly mounted with gid=0 and mode=0600, the pty nodes won't be accessible by the tty group, but the grantpt() function will succeed and users will have a working system. The system is not fully POSIX compliant (which might be an admin choice to default to "mesg n" mode), but the GNU libc is not to blame here, as without the pt_chown helper it can't do anything. With this patch there should not be any reason left to build the GNU libc with the --enable-pt_chown configure option on a GNU/Linux system. |
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