Re: [Bug 210655] ptrace.2: documentation is incorrect about access checking threads in same thread group

[Jann Horn <jann@thejh.net>]

Am Tue, Dec 15, 2020 at 06:01:25PM +0100 schrieb Alejandro Colomar (man-pages):
> Hi,
> 
> There's a bug report: https://bugzilla.kernel.org/show_bug.cgi?id=210655
> 
> [[
> Under "Ptrace access mode checking", the documentation states:
>   "1. If the calling thread and the target thread are in the same thread
> group, access is always allowed."
> 
> This is incorrect. A thread may never attach to another in the same group.

No, that is correct. ptrace-mode access checks do always short-circuit for
tasks in the same thread group:

/* Returns 0 on success, -errno on denial. */
static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
{
[...]
        /* May we inspect the given task?
         * This check is used both for attaching with ptrace
         * and for allowing access to sensitive information in /proc.
         *
         * ptrace_attach denies several cases that /proc allows
         * because setting up the necessary parent/child relationship
         * or halting the specified task is impossible.
         */

        /* Don't let security modules deny introspection */
        if (same_thread_group(task, current))
                return 0;
[...]
}

As the comment explains, you can't actually *attach*
to another task in the same thread group; but that's
not because of the ptrace-style access check rules,
but because specifically *attaching* to another task
in the same thread group doesn't work.