linux-kernel - Re: [PATCH v2] vsprintf: don't obfuscate NULL and error pointers

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Message-ID: <CAHp75Vf24yNeLEweq_70AUzKwbdyurB6ze9739Qy9djA9dSefg@mail.gmail.com>
Date:   Wed, 19 Feb 2020 19:37:29 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Ilya Dryomov <idryomov@...il.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        "Tobin C . Harding" <me@...in.cc>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v2] vsprintf: don't obfuscate NULL and error pointers

On Wed, Feb 19, 2020 at 7:13 PM Ilya Dryomov <idryomov@...il.com> wrote:
>
> I don't see what security concern is addressed by obfuscating NULL
> and IS_ERR() error pointers, printed with %p/%pK.  Given the number
> of sites where %p is used (over 10000) and the fact that NULL pointers
> aren't uncommon, it probably wouldn't take long for an attacker to
> find the hash that corresponds to 0.  Although harder, the same goes
> for most common error values, such as -1, -2, -11, -14, etc.
>
> The NULL part actually fixes a regression: NULL pointers weren't
> obfuscated until commit 3e5903eb9cff ("vsprintf: Prevent crash when
> dereferencing invalid pointers") which went into 5.2.  I'm tacking
> the IS_ERR() part on here because error pointers won't leak kernel
> addresses and printing them as pointers shouldn't be any different
> from e.g. %d with PTR_ERR_OR_ZERO().  Obfuscating them just makes
> debugging based on existing pr_debug and friends excruciating.
>
> Note that the "always print 0's for %pK when kptr_restrict == 2"
> behaviour which goes way back is left as is.
>
> Example output with the patch applied:
>
>                             ptr         error-ptr              NULL
> %p:            0000000001f8cc5b  fffffffffffffff2  0000000000000000
> %pK, kptr = 0: 0000000001f8cc5b  fffffffffffffff2  0000000000000000
> %px:           ffff888048c04020  fffffffffffffff2  0000000000000000
> %pK, kptr = 1: ffff888048c04020  fffffffffffffff2  0000000000000000
> %pK, kptr = 2: 0000000000000000  0000000000000000  0000000000000000

...

> +/*
> + * NULL pointers aren't hashed.
> + */
>  static void __init
>  null_pointer(void)
>  {
> -       test_hashed("%p", NULL);
> +       test(ZEROS "00000000", "%p", NULL);
>         test(ZEROS "00000000", "%px", NULL);
>         test("(null)", "%pE", NULL);
>  }
>
> +/*
> + * Error pointers aren't hashed.
> + */
> +static void __init
> +error_pointer(void)
> +{
> +       test(ONES "fffffff5", "%p", ERR_PTR(-EAGAIN));
> +       test(ONES "fffffff5", "%px", ERR_PTR(-EAGAIN));

> +       test("(efault)", "%pE", ERR_PTR(-EAGAIN));

Hmm... Is capital E on purpose here?
Maybe we may use something else ('%ph'?) for sake of deviation?

> +}

-- 
With Best Regards,
Andy Shevchenko