linux-kernel - Re: [PATCH v3 1/3] ptrace: Provide ___ptrace_may_access() that can be applied on arbitrary tasks

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Date:   Wed, 5 Sep 2018 20:29:07 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Andrea Arcangeli <aarcange@...hat.com>
cc:     Andi Kleen <ak@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "Schaufler, Casey" <casey.schaufler@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        Oleg Nesterov <oleg@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v3 1/3] ptrace: Provide ___ptrace_may_access() that can
 be applied on arbitrary tasks

On Wed, 5 Sep 2018, Andrea Arcangeli wrote:

> ptrace_has_cap(tcred->user_ns, mode) is supposed to eventually lockup
> hard if called from scheduler as it does some locking, and we fixed
> that already half a year ago.
> 
> Not sure how it's still unfixed in Jiri's codebase after so long, or
> if it's an issue specific to 3.10 and upstream gets away without this.

We haven't got any lockup reports in our kernels (and we do carry a 
variant of this patch), so it might be somehow specific to 3.10.

> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index eb7862f185ff..4a8d0dd73c93 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -285,7 +285,8 @@ int ___ptrace_may_access(struct task_struct *tracer,
>  	    gid_eq(caller_gid, tcred->sgid) &&
>  	    gid_eq(caller_gid, tcred->gid))
>  		goto ok;
> -	if (ptrace_has_cap(tcred->user_ns, mode))
> +	if (!(mode & PTRACE_MODE_NOACCESS_CHK) &&
> +	    ptrace_has_cap(tcred->user_ns, mode))
>  		goto ok;
>  	rcu_read_unlock();
>  	return -EPERM;
> @@ -296,7 +297,8 @@ ok:
>  		dumpable = get_dumpable(task->mm);
>  	rcu_read_lock();
>  	if (dumpable != SUID_DUMP_USER &&
> -	    !ptrace_has_cap(__task_cred(task)->user_ns, mode)) {
> +	    ((mode & PTRACE_MODE_NOACCESS_CHK) ||
> +	     !ptrace_has_cap(__task_cred(task)->user_ns, mode))) {
>  		rcu_read_unlock();
>  		return -EPERM;

I will look into this whether it's still applicable or not, thanks a lot 
for the pointer.

(and no, my testing of the patch I sent on current tree didn't produce any 
hangs -- was there a reliable way to trigger it on 3.10?).

Thanks,

-- 
Jiri Kosina
SUSE Labs