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SubjectRe: [RFC PATCH 3/3a] ptrace: add _ptrace_may_access()
On 05/06, Roland McGrath wrote:
>
> > I was going to cleanup this later. Because I think that
> > __ptrace_may_access() should die. It has only one caller, mm_for_maps().
>
> CC'ing Al Viro, who wrote mm_for_maps() (and no one has touched it since,
> see commit 831830b).
>
> > I will re-check, but it looks a bit strange. More precisely, I just
> > can't understand it. Why we can't just do
> >
> > struct mm_struct *mm_for_maps(struct task_struct *task)
> > {
> > struct mm_struct *mm = get_task_mm(task);
> >
> > if (mm && mm != current->mm && !ptrace_may_access()) {
> > mmput(mm);
> > mm = NULL;
> > }
> >
> > return mm;
> > }
>
> That seems fine to me. I suspect the old code just predated the PF_KTHREAD
> check in get_task_mm() and excluding the borrowed-mm window races was the
> only reason for using task_lock() that way.
>
> > ? We do not care if this task exits and clears ->mm right before
> > or after ptrace_may_access(), and this is possible eith the current
> > code too once it drops tasklist.
>
> I agree.

Great. Will try to make the patches soon.

And I forgot to mention, there is another reason to kill __ptrace_may_access.
Because we can "narrow" the critical section protected by task_lock(). Not
for performance of course, just for clarity:

/* the callers of ptrace_may_access should be fixed */

int ptrace_may_access(struct task_struct *task, unsigned int mode)
{
const struct cred *cred = current_cred(), *tcred;
int ret = 0;

/* 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 (task == current)
return ret;

rcu_read_lock();
tcred = __task_cred(task);
if ((cred->uid != tcred->euid ||
cred->uid != tcred->suid ||
cred->uid != tcred->uid ||
cred->gid != tcred->egid ||
cred->gid != tcred->sgid ||
cred->gid != tcred->gid) &&
!capable(CAP_SYS_PTRACE))
ret = -EPERM;
rcu_read_unlock();
if (ret)
return ret;
/* kill rmb ? */

task_lock(task);
if (!task->mm || !get_dumpable(task->mm)) {
if (!capable(CAP_SYS_PTRACE))
ret = -EPERM;
task_unclock(task);
if (ret)
return ret;

return security_ptrace_may_access(task, mode);
}

Btw, "[PATCH 3/3]" notes that security_ptrace_may_access() is called without
task_lock(), this note "leaked" from this change in future ;)

But firsty I'll try to grep/recheck this all.

Oleg.



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