linux-kernel - Re: [PATCH v3.1 09/10] x86/unwind: add ORC unwinder

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Date:   Thu, 20 Jul 2017 16:16:25 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jiri Slaby <jslaby@...e.cz>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH v3.1 09/10] x86/unwind: add ORC unwinder

On Thu, Jul 20, 2017 at 09:12:16AM +0200, Jiri Slaby wrote:
> On 07/14/2017, 07:22 PM, Josh Poimboeuf wrote:
> > +void __unwind_start(struct unwind_state *state, struct task_struct *task,
> > +		    struct pt_regs *regs, unsigned long *first_frame)
> > +{
> > +	memset(state, 0, sizeof(*state));
> > +	state->task = task;
> > +
> > +	/*
> > +	 * Refuse to unwind the stack of a task while it's executing on another
> > +	 * CPU.  This check is racy, but that's ok: the unwinder has other
> > +	 * checks to prevent it from going off the rails.
> > +	 */
> > +	if (task_on_another_cpu(task))
> > +		goto done;
> > +
> > +	if (regs) {
> > +		if (user_mode(regs))
> > +			goto done;
> > +
> > +		state->ip = regs->ip;
> > +		state->sp = kernel_stack_pointer(regs);
> > +		state->bp = regs->bp;
> > +		state->regs = regs;
> > +		state->full_regs = true;
> > +		state->signal = true;
> > +
> > +	} else if (task == current) {
> > +		asm volatile("lea (%%rip), %0\n\t"
> > +			     "mov %%rsp, %1\n\t"
> > +			     "mov %%rbp, %2\n\t"
> > +			     : "=r" (state->ip), "=r" (state->sp),
> > +			       "=r" (state->bp));
> > +
> > +	} else {
> > +		struct inactive_task_frame *frame = (void *)task->thread.sp;
> > +
> > +		state->ip = frame->ret_addr;
> > +		state->sp = task->thread.sp;
> > +		state->bp = frame->bp;
> 
> I wonder, if the reads from 'frame' should have READ_ONCE_NOCHECK for
> the same reason as in:
> commit 84936118bdf37bda513d4a361c38181a216427e0
> Author: Josh Poimboeuf <jpoimboe@...hat.com>
> Date:   Mon Jan 9 12:00:23 2017 -0600
> 
>     x86/unwind: Disable KASAN checks for non-current tasks
> ?

Yeah, maybe so.  Since the task_on_another_cpu() check above is racy,
here it's remotely possible that the task has since starting executing
and has poisoned the stack memory we're about to read.

I don't know how realistic that scenario is, but it wouldn't hurt to add
a couple of READ_ONCE_NOCHECKs here for the 'frame' dereferences.

-- 
Josh