From: Marco Elver <elver@google.com>
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>
Subject: Re: [PATCH 3/3] [v4] lib/vsprintf: no_hash_pointers prints all addresses as unhashed
Date: Tue, 2 Mar 2021 13:45:07 +0100 [thread overview]
Message-ID: <CANpmjNNm-4s16_KQ1_NqFN4XOESJh4_=33LHQzt+p4V0Cy=Xzw@mail.gmail.com> (raw)
In-Reply-To: <CAMuHMdULKZCJevVJcp7TxzLdWLjsQPhE8hqxhnztNi9bjT_cEw@mail.gmail.com>
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
next prev parent reply other threads:[~2021-03-02 15:09 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-14 16:13 [PATCH 0/3][v4] add support for never printing hashed addresses Timur Tabi
2021-02-14 16:13 ` [PATCH 1/3] [v4] lib: use KSTM_MODULE_GLOBALS macro in kselftest drivers Timur Tabi
2021-02-14 16:13 ` [PATCH 2/3] [v4] kselftest: add support for skipped tests Timur Tabi
2021-02-14 16:13 ` [PATCH 3/3] [v4] lib/vsprintf: no_hash_pointers prints all addresses as unhashed Timur Tabi
2021-03-02 11:51 ` Geert Uytterhoeven
2021-03-02 12:45 ` Marco Elver [this message]
2021-03-02 12:51 ` Geert Uytterhoeven
2021-03-02 13:29 ` Petr Mladek
2021-03-02 13:37 ` Vlastimil Babka
2021-03-02 13:49 ` Geert Uytterhoeven
2021-03-02 14:08 ` Steven Rostedt
2021-03-02 14:26 ` Marco Elver
2021-03-02 14:35 ` Matthew Wilcox
2021-03-02 14:40 ` Marco Elver
2021-03-02 14:55 ` Geert Uytterhoeven
2021-03-02 14:57 ` Marco Elver
2021-03-02 14:28 ` Geert Uytterhoeven
2021-03-02 15:16 ` Rasmus Villemoes
2021-03-02 15:29 ` Andy Shevchenko
2021-03-02 17:53 ` Petr Mladek
2021-09-11 2:25 ` Xiaoming Ni
2021-09-11 2:39 ` Tetsuo Handa
2021-02-14 16:18 ` [PATCH 0/3][v4] add support for never printing hashed addresses Timur Tabi
2021-02-15 11:08 ` Petr Mladek
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