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SubjectRe: [Bug 210655] ptrace.2: documentation is incorrect about access checking threads in same thread group
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On 12/15/2020 6:01 PM, Jann Horn wrote:
> On Wed, Dec 16, 2020 at 12:25 AM Alejandro Colomar (man-pages)
> <alx.manpages@gmail.com> wrote:
>> On 12/16/20 12:23 AM, Alejandro Colomar (man-pages) wrote:
>>> 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):
>>>>> 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().
> Yes.
>
>>> 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);
>>> [...]
>>> }
> I said exactly that in my last mail:
>
>>>> 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.
> As I said, attaching indeed doesn't work. But that's not what "Ptrace
> access mode checking" means. As the first sentence of that section
> says:
>
> | Various parts of the kernel-user-space API (not just ptrace()
> | operations), require so-called "ptrace access mode" checks,
> | whose outcome determines whether an operation is
> | permitted (or, in a few cases, causes a "read" operation
> | to return sanitized data).
>
> You can find these places by grepping for \bptrace_may_access\b -
> operations like e.g. the get_robust_list() syscall will always succeed
> when inspecting other tasks in the caller's thread group thanks to
> this rule.

Ah, yes.  I missed that back reference while trying to digest that
rather meaty man page.  A grep on the man page source tree does show a
number of references to "ptrace access mode".

That said, the ptrace(2) man page also directly references the ptrace
access mode check under both PTRACE_ATTACH and PTACE_SEIZE:

| Permission to perform a PTRACE_ATTACH is governed by a ptrace | access
mode PTRACE_MODE_ATTACH_REALCREDS check; see below. As confirmed, the
"same thread group" rule does not apply to either of those operations. A
re-wording of rule 1 similar to this might help avoid confusion: 1. If
the calling thread and the target thread are in the same thread group:
a. For ptrace() called with PTRACE_ATTACH or PTRACE_SEIZE, access is
NEVER allowed. b. For all other so-called "ptrace access mode checks",
access is ALWAYS allowed. --Ted

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