linux-kernel - Re: [PATCH v9 3/5] x86/KASLR: Randomize virtual address separately

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Date:	Fri, 17 Jun 2016 10:35:35 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	Baoquan He <bhe@...hat.com>, Borislav Petkov <bp@...e.de>,
	Yinghai Lu <yinghai@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	"H.J. Lu" <hjl.tools@...il.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 3/5] x86/KASLR: Randomize virtual address separately


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Kees Cook <keescook@...omium.org> wrote:
> 
> > -unsigned char *choose_random_location(unsigned long input,
> > -				      unsigned long input_size,
> > -				      unsigned long output,
> > -				      unsigned long output_size)
> > +void choose_random_location(unsigned long input,
> > +			    unsigned long input_size,
> > +			    unsigned long *output,
> > +			    unsigned long output_size,
> > +			    unsigned long *virt_addr)
> >  {
> > -	unsigned long choice = output;
> >  	unsigned long random_addr;
> >  
> > +	/* By default, keep output position unchanged. */
> > +	*virt_addr = *output;
> 
> So I applied this, after fixing a conflict with a recent hibernation related 
> change, but it would be nice to further clean up the types in this file, in 
> particular could we please propagate 'const' for all input-only pointers?
> 
> For example in the above function it would be obvious at a glance if it said 
> something like:
> 
>  void choose_random_location(unsigned long input,
> 			    unsigned long input_size,
> 			    const unsigned long *output,
> 			    unsigned long output_size,
> 			    unsigned long *virt_addr)
> 
> when reading such a function prototype I can immediately tell: 'yeah, while it's 
> named "output", it's in fact a read-only input parameter - the _real_ output of 
> the function is 'virt_addr'.)

Doh, so I managed to confuse myself by looking at the unpatched function only. 
This patch in fact starts writing to 'output':

+               /* Update the new physical address location. */
+               if (*output != random_addr) {
+                       add_identity_map(random_addr, output_size);
+                       *output = random_addr;
+               }

At which point 'output' cannot be const, and in fact it might be beneficial that 
'virt_addr' is passed in by a pointer as well.

The comment of the function definitely needs to be updated:

  * it takes the input and output pointers as 'unsigned long'.

... which is not true anymore.

I also find the type flow and naming for the 'output' pointer very confusing. We 
have:

asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
                                  unsigned char *input_data,
                                  unsigned long input_len,
                                  unsigned char *output,
                                  unsigned long output_len)

...

        choose_random_location((unsigned long)input_data, input_len,
                                (unsigned long *)&output,
                                max(output_len, kernel_total_size),
                                &virt_addr);


void choose_random_location(unsigned long input,
                            unsigned long input_size,
                            unsigned long *output,
                            unsigned long output_size,
                            unsigned long *virt_addr)

...

                        *output = random_addr;


it is very easy to confuse 'unsigned long *output' with the 'char *output' pointer 
to the output stream! But in reality this is a double pointer and we want to use 
it to change the pointer.

So at minimum we should rename 'output' in choose_random_location() to something 
like 'output_ptr' - but even better would be to just preserve its natural type and 
use 'char * const *' and do a single type cast when setting it.

Same goes for 'virt_addr'.

Agreed?

Thanks,

	Ingo