linux-kernel - Re: [PATCH v2 2/2] x86: Rewrite ret_from

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Message-ID: <20230719205050.GG3529734@hirez.programming.kicks-ass.net>
Date:   Wed, 19 Jul 2023 22:50:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Brian Gerst <brgerst@...il.com>, jpoimboe@...nel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        alyssa.milburn@...ux.intel.com, keescook@...omium.org,
        joao@...rdrivepizza.com, tim.c.chen@...ux.intel.com,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v2 2/2] x86: Rewrite ret_from_fork() in C

On Wed, Jul 19, 2023 at 10:15:38PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 19, 2023 at 10:02:22PM +0200, Peter Zijlstra wrote:
> > On Wed, Jul 19, 2023 at 05:21:11PM +0200, Petr Mladek wrote:
> > 
> > > This patch broke livepatching. Kthreads never have a reliable stack.
> > > It works when I revert it.
> > 
> > > > +SYM_CODE_START(ret_from_fork_asm)
> > > > +	UNWIND_HINT_REGS
> > 
> > It works again when I change the above hint to UNWIND_HINT_END_OF_STACK,
> > so yeah. Doing this makes objtool unhappy with something else though,
> > so I'll go prod at things with something sharp...
> 
> 
> The below cures things; Josh, did I miss anything?
> 
> ---
>  arch/x86/entry/entry_64.S | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 91f6818884fa..cfe7882ea9ae 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -285,7 +285,14 @@ SYM_FUNC_END(__switch_to_asm)
>   */
>  .pushsection .text, "ax"
>  SYM_CODE_START(ret_from_fork_asm)
> -	UNWIND_HINT_REGS
> +	/*
> +	 * This is the start of the kernel stack; even through there's a regs
> +	 * set at the top, there is no real exception frame and one cannot
> +	 * unwind further. This is the end.
> +	 *
> +	 * This ensures stack unwinds of kernel threads hit a known good state.
> +	 */
> +	UNWIND_HINT_END_OF_STACK

So unwind_orc.c:unwind_next_frame() will terminate on this hint *or* on
user_mode(state->regs).

AFAICT way things are set up in copy_thread(), user_mode() will not be
true -- after all there is no usermode, the kthread would first have to
exec() something to create a usermode.

Yet I'm wondering if perhaps we should spoof the regs to make
user_mode() true and auto-terminate without this explicit hint.

Josh, do you remember the rationale for all this?