linux-kernel - Re: [PATCH 3/3] x86: kernel base offset ASLR

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Message-ID: <20130406101037.GA3612@pd.tnic>
Date:	Sat, 6 Apr 2013 12:10:37 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Kees Cook <keescook@...omium.org>
Cc:	Julien Tinnes <jln@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"x86@...nel.org" <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>,
	Will Drewry <wad@...omium.org>
Subject: Re: [PATCH 3/3] x86: kernel base offset ASLR

On Fri, Apr 05, 2013 at 04:18:49PM -0700, Kees Cook wrote:
> There is already a hook in the patch that prints the offset:
> 
> +dump_kernel_offset(struct notifier_block *self, unsigned long v, void *p)
> +{
> +       pr_emerg("Kernel Offset: 0x%lx\n",
> +                (unsigned long)&_text - __START_KERNEL);
> ...
> +       atomic_notifier_chain_register(&panic_notifier_list,
> +                                       &kernel_offset_notifier);
> 
> But of course, this can get improved.

Yeah, this should probably be added to dump_trace(), i.e. something
which walks stack frames and dumps addresses. Because, in the panic
notifier, you're missing all those WARN* callsites, for example.

Also, I wonder whether it wouldn't be too hard to go even a step further
and compute the original, linker vmlinux addresses from the offsets
and dump a stack trace which looks exactly the same as if KASLR is
off. It'll probably need something to say KASLR was on when the trace
happened, though:

[  790.253365] Call Trace (KASLR):
[  790.254121]  [<ffffffff8110bc90>] ? __smpboot_create_thread+0x180/0x180
[  790.255428]  [<ffffffff810ff1df>] kthread+0xef/0x100
...

so that people who stare at this, know.

Because, in that case, you don't need both the panic notifier or the
userspace script massaging stack trace output anymore.

-- 
Regards/Gruss,
    Boris.

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