linux-kernel - Re: [PATCH] x86/dumpstack: Dump user space code correctly again

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Message-ID: <20200723081900.GA16138@lst.de>
Date:   Thu, 23 Jul 2020 10:19:00 +0200
From:   Christoph Hellwig <hch@....de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "H.J. Lu" <hjl.tools@...il.com>, x86@...nel.org,
        Christoph Hellwig <hch@....de>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH] x86/dumpstack: Dump user space code correctly again

On Wed, Jul 22, 2020 at 07:54:15PM +0200, Thomas Gleixner wrote:
> Subject: x86/dumpstack: Dump user space code correctly again
> From: Thomas Gleixner <tglx@...utronix.de>
> Date: Wed, 22 Jul 2020 10:39:54 +0200
> 
> H.J. reported that post 5.7 a segfault of a user space task does not longer
> dump the Code bytes when /proc/sys/debug/exception-trace is enabled. It
> prints 'Code: Bad RIP value.' instead.
> 
> This was broken by a recent change which made probe_kernel_read() reject
> non-kernel addresses.
> 
> Update show_opcodes() so it retrieves user space opcodes via
> copy_from_user_nmi().
> 
> Fixes: 98a23609b103 ("maccess: always use strict semantics for probe_kernel_read")
> Reported-by: H.J. Lu <hjl.tools@...il.com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Looks good, and also cleans up the code nicely:

Reviewed-by: Christoph Hellwig <hch@....de>

But one question below:

> +	/*
> +	 * Make sure userspace isn't trying to trick us into dumping kernel
> +	 * memory by pointing the userspace instruction pointer at it.
> +	 */
> +	if (__chk_range_not_ok(src, nbytes, TASK_SIZE_MAX))
> +		return -EINVAL;
> +
> +	return copy_from_user_nmi(buf, (void __user *)src, nbytes);

copy_from_user_nmi already contains a:

	if (__range_not_ok(from, n, TASK_SIZE))
		return n;

what is the reason it checks for TASK_SIZE vs TASK_SIZE_MAX, and why
do we need both checks?