Re: [Bug 210655] ptrace.2: documentation is incorrect about access checking threads in same thread group
From: Ted Estes
Date: Tue Dec 15 2020 - 21:22:27 EST
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@xxxxxxxxx> 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