linux-kernel - Re: [PATCH 3/3] [v4] lib/vsprintf: no_hash_pointers prints all addresses as unhashed

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Date:   Tue, 2 Mar 2021 13:51:35 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Marco Elver <elver@...gle.com>
Cc:     Timur Tabi <timur@...nel.org>, Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        roman.fietze@...na.com, Kees Cook <keescook@...omium.org>,
        John Ogness <john.ogness@...utronix.de>,
        Akinobu Mita <akinobu.mita@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Pavel Machek <pavel@....cz>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH 3/3] [v4] lib/vsprintf: no_hash_pointers prints all
 addresses as unhashed

Hi Marco,

On Tue, Mar 2, 2021 at 1:45 PM Marco Elver <elver@...gle.com> wrote:
> On Tue, 2 Mar 2021 at 12:51, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > On Sun, Feb 14, 2021 at 5:17 PM Timur Tabi <timur@...nel.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@...nel.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

I guess the people who do care about kernel size do know to disable
CONFIG_DEBUG_KERNEL, so it would help them.
The everything-but-the-kitchen-sink distro people don't care about kernel
size anyway.

> 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?

That would indeed help to reduce run-time memory consumption.
It would not solve the raw kernel size increase.

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