Hi Jann,
On 12/16/20 12:07 AM, Jann Horn wrote:
> 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;
> [...]
> }
AFAICS, that code always returns non-zero,
at least when called from ptrace_attach().
As you can see below,
__ptrace_may_access() is called some lines after
the code pointed to by the bug report.
static int ptrace_attach(struct task_struct *task, long request,
unsigned long addr,
unsigned long flags)
{
[...]
if (same_thread_group(task, current))
goto out;
/*
* Protect exec's credential calculations against our interference;
* SUID, SGID and LSM creds get determined differently
* under ptrace.
*/
retval = -ERESTARTNOINTR;
if (mutex_lock_interruptible(&task->signal->cred_guard_mutex))
goto out;
task_lock(task);
retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS);
[...]
}
Thanks,
Alex
>
> 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.
>
