linux-kernel - RE: [PATCH] printk: introduce kptr

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Date:   Thu, 6 Oct 2016 13:23:42 +0000
From:   "Roberts, William C" <william.c.roberts@...el.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
CC:     "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        "corbet@....net" <corbet@....net>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] printk: introduce kptr_restrict level 3



> -----Original Message-----
> From: Rasmus Villemoes [mailto:linux@...musvillemoes.dk]
> Sent: Wednesday, October 5, 2016 4:53 PM
> To: Roberts, William C <william.c.roberts@...el.com>
> Cc: kernel-hardening@...ts.openwall.com; corbet@....net; linux-
> doc@...r.kernel.org; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH] printk: introduce kptr_restrict level 3
> 
> On Wed, Oct 05 2016, william.c.roberts@...el.com wrote:
> 
> > From: William Roberts <william.c.roberts@...el.com>
> >
> > Some out-of-tree modules do not use %pK and just use %p, as it's the
> > common C paradigm for printing pointers. Because of this,
> > kptr_restrict has no affect on the output and thus, no way to contain
> > the kernel address leak.
> >
> > Introduce kptr_restrict level 3 that causes the kernel to treat %p as
> > if it was %pK and thus always prints zeros.
> >
> > Sample Output:
> > kptr_restrict == 2:
> > p: 00000000604369f4
> > pK: 0000000000000000
> >
> > kptr_restrict == 3:
> > p: 0000000000000000
> > pK: 0000000000000000
> >
> > Signed-off-by: William Roberts <william.c.roberts@...el.com>
> > ---
> >  Documentation/sysctl/kernel.txt |   3 ++
> >  kernel/sysctl.c                 |   3 +-
> >  lib/vsprintf.c                  | 107 ++++++++++++++++++++++++----------------
> 
> That's a lot of changed lines. Why isn't this just

I moved the nested case into a static local function, I thought it was easier to read than the existing
nested switches. The other reason was so we didn't have kptr_restrict littering that code
and it was contained within the default and K values of the switch.

> 
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -1719,6 +1719,8 @@ char *pointer(const char *fmt, char *buf, char *end,
> void *ptr,
>         case 'G':
>                 return flags_string(buf, end, ptr, fmt);
>         }
> +       if (kptr_restrict == 3)
> +               ptr = NULL;
>         spec.flags |= SMALL;
>         if (spec.field_width == -1) {
>                 spec.field_width = default_width;
> 
> ?