linux-kernel - Re: [PATCH 6/6] x86: kaslr: report kernel offset on panic

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Message-ID: <20130426221301.GA26059@pd.tnic>
Date:	Sat, 27 Apr 2013 00:13:01 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Kees Cook <keescook@...omium.org>
Cc:	linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Jarkko Sakkinen <jarkko.sakkinen@...el.com>,
	Matthew Garrett <mjg@...hat.com>,
	Matt Fleming <matt.fleming@...el.com>,
	Eric Northup <digitaleric@...gle.com>,
	Dan Rosenberg <drosenberg@...curity.com>,
	Julien Tinnes <jln@...gle.com>, Will Drewry <wad@...omium.org>
Subject: Re: [PATCH 6/6] x86: kaslr: report kernel offset on panic

On Fri, Apr 26, 2013 at 12:03:25PM -0700, Kees Cook wrote:
> When the system panics, include the kernel offset in the report to assist
> in debugging.
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  arch/x86/kernel/setup.c |   24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index fae9134..95a33b1 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -830,6 +830,18 @@ static void __init trim_low_memory_range(void)
>  }
>  	
>  /*
> + * Dump out kernel offset information on panic.
> + */
> +static int
> +dump_kernel_offset(struct notifier_block *self, unsigned long v, void *p)
> +{
> +	pr_emerg("Kernel Offset: 0x%lx\n",
> +		 (unsigned long)&_text - __START_KERNEL);

So what's wrong with subtracting the offset from the function addresses
on the stack so that traces can show the addresses as they are in
vmlinux, completely agnostic of any randomization?

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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