linux-kernel - Re: [PATCH 2/4] kaslr: check if the random addr is available

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Date:	Tue, 9 Sep 2014 15:41:18 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Baoquan He <bhe@...hat.com>
Cc:	Kees Cook <keescook@...omium.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Deutschmann <whissi@...ssi.de>,
	Dave Young <dyoung@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	WANG Chao <chaowang@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 2/4] kaslr: check if the random addr is available

On Sat, Sep 06, 2014 at 06:16:57AM +0800, Baoquan He wrote:
> On 09/05/14 at 10:16am, Kees Cook wrote:
> > On Fri, Sep 5, 2014 at 7:08 AM, Baoquan He <bhe@...hat.com> wrote:
> > > diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> > > index 7780a5b..d2a0eaa 100644
> > > --- a/arch/x86/boot/compressed/misc.c
> > > +++ b/arch/x86/boot/compressed/misc.c
> > > @@ -250,6 +250,11 @@ static void handle_relocations(void *output, unsigned long output_len)
> > >         }
> > >  #endif
> > >
> > > +       if (max_addr > CONFIG_RANDOMIZE_BASE_MAX_OFFSET) {
> > > +                debug_putstr("Random addr is not allowed. No relocation needed... \n");
> > > +                return;
> > > +        }
> > > +
> > 
> > It's not clear to me what this is fixing. In aslr.c,
> > process_e820_entry() should already make it impossible to select
> > max_addr > CONFIG_RANDOMIZE_BASE_MAX_OFFSET. If you're trying to
> > detect a non-kaslr boot, I think this is better handled in 1/4 where I
> > suggest examining the "output" location before/after
> > choose_kernel_location.
> > 
> > -Kees
> 

[CC hpa ]

> Hi Kees,
> 
> Yes, process_e820_entry() can make sure the choice+output_len <
> CONFIG_RANDOMIZE_BASE_MAX_OFFSET, but that can't stop other bootloaders
> to put kernel in region above CONFIG_RANDOMIZE_BASE_MAX_OFFSET.
> 
> E.g in kdump, we can set crashkernel=256M@...4M in cmdline. Then the 1st
> kernel will reserve 256M memory just at 1024M place. So if load kdump
> kernel now, the output will be 1024M before choose_kernel_location().
> With this value, output won't be changed in choose_kernel_location(),
> then it will do decompress(), then call handle_relocations(). Then since
> 1024 is not equal to LOAD_PHYSICAL_ADDR, it will start relocatoins
> handling. And this cause _text stamping into MODULES vaddr range. System
> will be exceptional.

Bao,

If you apply your first patch where output_orig == output, then
handle_relocations() will not do anything for x86_64 case and bail
out. That should take care of this issue. Isn't it? And we should
not require this patch.

Thanks
Vivek
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