pty_setowner and tty permissions

[Damien Miller]

On Wed, 27 Aug 2014, Corinna Vinschen wrote:

> Hi,
> 
> 
> while looking into Cygwin's tty code, I stumbled over this problem:
> 
> Every time you log in to Cygwin via sshd, the pty's permissions are
> set like this:
> 
>   $ ls -l `tty`
>   crw--w--w- 1 user   group           136, 2 Aug 27 13:06 /dev/pty2
> 
> Since Cygwin sets the permissions more tight to begin with, I was
> wondering why the permissions are this open.  Turns out, sshd sets
> them like this:
> 
>   /* Determine the group to make the owner of the tty. */
>   grp = getgrnam("tty");
>   if (grp) {
> 	  gid = grp->gr_gid;
> 	  mode = S_IRUSR | S_IWUSR | S_IWGRP;
>   } else {
> 	  gid = pw->pw_gid;
> 	  mode = S_IRUSR | S_IWUSR | S_IWGRP | S_IWOTH;
>   }
> 
> On Windows no group called "tty" exists, so sshd always sets the
> permissions to 0622 on Cygwin.
> 
> My question is, isn't that a security problem?  Shouldn't the
> permissions set to 0600 if a "tty" group doesn't exist, otherwise
> everyone can write to the user's tty?  What am I missing?

I think the intention was to allow tools like wall(1) and write(1)
to function on systems without a "tty" group, but IMO it's better
to let the admin decide that.

-d