linux-kernel - Re: [PATCH v4 2/9] x86/kthread,dumpstack: Set task_pt

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Date:   Wed, 17 Mar 2021 19:36:25 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH v4 2/9] x86/kthread,dumpstack: Set task_pt_regs->cs.RPL=3
 for kernel threads

On Wed, Mar 17, 2021 at 11:12:41AM -0700, Andy Lutomirski wrote:
> @@ -163,6 +163,19 @@ int copy_thread(unsigned long clone_flags, unsigned long sp, unsigned long arg,
>  	/* Kernel thread ? */
>  	if (unlikely(p->flags & PF_KTHREAD)) {
>  		memset(childregs, 0, sizeof(struct pt_regs));
> +
> +		/*
> +		 * Even though there is no real user state here, these
> +		 * are were user regs belong, and kernel_execve() will

s/were/where/

> +		 * overwrite them with user regs.  Put an obviously
> +		 * invalid value that nonetheless causes user_mode(regs)
> +		 * to return true in CS.
> +		 *
> +		 * This also prevents the unwinder from thinking that there
> +		 * is invalid kernel state at the top of the stack.
> +		 */
> +		childregs->cs = 3;
> +
>  		kthread_frame_init(frame, sp, arg);
>  		return 0;
>  	}

Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>

-- 
Josh