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From:   Marco Elver <elver@google.com>
Date:   Tue, 2 Mar 2021 13:45:07 +0100
Message-ID: <CANpmjNNm-4s16_KQ1_NqFN4XOESJh4_=33LHQzt+p4V0Cy=Xzw@mail.gmail.com>
Subject: Re: [PATCH 3/3] [v4] lib/vsprintf: no_hash_pointers prints all
 addresses as unhashed
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Timur Tabi <timur@kernel.org>, Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        roman.fietze@magna.com, Kees Cook <keescook@chromium.org>,
        John Ogness <john.ogness@linutronix.de>,
        Akinobu Mita <akinobu.mita@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Pavel Machek <pavel@ucw.cz>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>
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On Tue, 2 Mar 2021 at 12:51, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> Hi Timur,
>
> On Sun, Feb 14, 2021 at 5:17 PM Timur Tabi <timur@kernel.org> wrote:
> > If the no_hash_pointers command line parameter is set, then
> > printk("%p") will print pointers as unhashed, which is useful for
> > debugging purposes.  This change applies to any function that uses
> > vsprintf, such as print_hex_dump() and seq_buf_printf().
> >
> > A large warning message is displayed if this option is enabled.
> > Unhashed pointers expose kernel addresses, which can be a security
> > risk.
> >
> > Also update test_printf to skip the hashed pointer tests if the
> > command-line option is set.
> >
> > Signed-off-by: Timur Tabi <timur@kernel.org>
>
> Thanks for your patch, which is now commit 5ead723a20e0447b
> ("lib/vsprintf: no_hash_pointers prints all addresses as unhashed") in
> v5.12-rc1.
>
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> > @@ -2090,6 +2090,32 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
> >         return widen_string(buf, buf - buf_start, end, spec);
> >  }
> >
> > +/* Disable pointer hashing if requested */
> > +bool no_hash_pointers __ro_after_init;
> > +EXPORT_SYMBOL_GPL(no_hash_pointers);
> > +
> > +static int __init no_hash_pointers_enable(char *str)
> > +{
> > +       no_hash_pointers = true;
> > +
> > +       pr_warn("**********************************************************\n");
> > +       pr_warn("**   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **\n");
> > +       pr_warn("**                                                      **\n");
> > +       pr_warn("** This system shows unhashed kernel memory addresses   **\n");
> > +       pr_warn("** via the console, logs, and other interfaces. This    **\n");
> > +       pr_warn("** might reduce the security of your system.            **\n");
> > +       pr_warn("**                                                      **\n");
> > +       pr_warn("** If you see this message and you are not debugging    **\n");
> > +       pr_warn("** the kernel, report this immediately to your system   **\n");
> > +       pr_warn("** administrator!                                       **\n");
> > +       pr_warn("**                                                      **\n");
> > +       pr_warn("**   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **\n");
> > +       pr_warn("**********************************************************\n");
> > +
> > +       return 0;
> > +}
> > +early_param("no_hash_pointers", no_hash_pointers_enable);
>
> While bloat-o-meter is not smart enough to notice the real size impact,
> this does add more than 500 bytes of string data to the kernel.
> Do we really need such a large message?
> Perhaps the whole no_hash_pointers machinery should be protected by
> "#ifdef CONFIG_DEBUG_KERNEL"?

We recently stumbled across this, and it appears an increasing number
of production kernels enable CONFIG_DEBUG_KERNEL [1], so it likely
isn't the solution (we tried to use CONFIG_DEBUG_KERNEL in similar
way, and it wasn't reliable). Having no_hash_pointers frees us of
having to rely on CONFIG_DEBUG_KERNEL. (Perhaps somebody else will
comment, but I believe there were strong objections to making the
pointer hashing dependent on more Kconfig options.)

[1] https://lkml.kernel.org/r/20210223082043.1972742-1-elver@google.com

Would placing the strings into an __initconst array help?

Thanks,
-- Marco