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Date: Tue, 5 Dec 2017 23:57:40 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: "Tobin C. Harding" <me@...in.cc>
Cc: kernel-hardening@...ts.openwall.com, 
	Linus Torvalds <torvalds@...ux-foundation.org>, "Jason A. Donenfeld" <Jason@...c4.com>, 
	"Theodore Ts'o" <tytso@....edu>, Kees Cook <keescook@...omium.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Tycho Andersen <tycho@...ho.ws>, 
	"Roberts, William C" <william.c.roberts@...el.com>, Tejun Heo <tj@...nel.org>, 
	Jordan Glover <Golden_Miller83@...tonmail.ch>, Greg KH <gregkh@...uxfoundation.org>, 
	Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>, Ian Campbell <ijc@...lion.org.uk>, 
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <wilal.deacon@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Chris Fries <cfries@...gle.com>, Dave Weinstein <olorin@...gle.com>, 
	Daniel Micay <danielmicay@...il.com>, Djalal Harouni <tixxdz@...il.com>, 
	Radim Krčmář <rkrcmar@...hat.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, 
	David Miller <davem@...emloft.net>, Stephen Rothwell <sfr@...b.auug.org.au>, 
	Andrey Ryabinin <aryabinin@...tuozzo.com>, Alexander Potapenko <glider@...gle.com>, 
	Dmitry Vyukov <dvyukov@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
Subject: Re: [PATCH V11 3/5] printk: hash addresses printed with %p

Hi Tobin,

On Tue, Dec 5, 2017 at 9:44 PM, Tobin C. Harding <me@...in.cc> wrote:
> On Tue, Dec 05, 2017 at 09:20:57PM +0100, Geert Uytterhoeven wrote:
>> On Wed, Nov 29, 2017 at 3:05 AM, Tobin C. Harding <me@...in.cc> wrote:
>> > Currently there exist approximately 14 000 places in the kernel where
>> > addresses are being printed using an unadorned %p. This potentially
>> > leaks sensitive information regarding the Kernel layout in memory. Many
>> > of these calls are stale, instead of fixing every call lets hash the
>> > address by default before printing. This will of course break some
>> > users, forcing code printing needed addresses to be updated.
>> >
>> > Code that _really_ needs the address will soon be able to use the new
>> > printk specifier %px to print the address.
>>
>> > --- a/lib/vsprintf.c
>> > +++ b/lib/vsprintf.c
>>
>> > +/* Maps a pointer to a 32 bit unique identifier. */
>> > +static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec)
>> > +{
>> > +       unsigned long hashval;
>> > +       const int default_width = 2 * sizeof(ptr);
>> > +
>> > +       if (unlikely(!have_filled_random_ptr_key)) {
>> > +               spec.field_width = default_width;
>> > +               /* string length must be less than default_width */
>> > +               return string(buf, end, "(ptrval)", spec);
>> > +       }
>> > +
>> > +#ifdef CONFIG_64BIT
>> > +       hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);
>> > +       /*
>> > +        * Mask off the first 32 bits, this makes explicit that we have
>> > +        * modified the address (and 32 bits is plenty for a unique ID).
>> > +        */
>> > +       hashval = hashval & 0xffffffff;
>> > +#else
>> > +       hashval = (unsigned long)siphash_1u32((u32)ptr, &ptr_key);
>> > +#endif
>>
>> Would it make sense to keep the 3 lowest bits of the address?
>>
>> Currently printed pointers no longer have any correlation with the actual
>> alignment in memory of the object, which is a typical cause of a class of bugs.
>
> We'd have to keep the lowest 4 since we are printing in hex, right? This
> is easy enough to add. I wasn't the architect behind the hashing but I
> can do up a patch and see if anyone who knows crypto objects.

Lowest 3 is good enough for all natural types, up to long long.
We may still receive complaints from people who care about seeing if
a pointer is cacheline-aligned or not. Fixing that may need up to 7 bits, I'm
afraid, which is a bit too much to give up.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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