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Message-ID: <CAMuHMdVZWUHNEbfO7OCPQV5Q3bA41c6VpwqfvT37ZEV9AR8chw@mail.gmail.com>
Date: Tue, 5 Dec 2017 21:20:57 +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>,
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Radim Krčmář <rkrcmar@...hat.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Network Development <netdev@...r.kernel.org>,
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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 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.
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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