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

[Alejandro Colomar (man-pages)]

On 12/16/20 12:23 AM, Alejandro Colomar (man-pages) wrote:
> 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,

Sorry, I should have said "that code never returns 0".

> 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.
>>