linux-kernel - Re: [PATCH] docs: add documentation on printing kernel addresses

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Message-ID: <CAGXu5j+pAw0KSgVQ0TjNxjyQx0SLrWp3OxcWtOxPrSDJDROrRA@mail.gmail.com>
Date:   Wed, 6 Dec 2017 16:43:40 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Tobin C. Harding" <me@...in.cc>
Cc:     Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] docs: add documentation on printing kernel addresses

On Wed, Dec 6, 2017 at 4:26 PM, Tobin C. Harding <me@...in.cc> wrote:
> Hashing addresses printed with printk specifier %p was implemented
> recently. During development a number of issues were raised regarding
> leaking kernel addresses to userspace. We should update the
> documentation appropriately.
>
> Add documentation regarding printing kernel addresses.
>
> Signed-off-by: Tobin C. Harding <me@...in.cc>

Acked-by: Kees Cook <keescook@...omium.org>

> ---
>
> Is there a proffered method for subscripts in sphinx kernel docs? Here
> we use '[*]'

Great question... I can't find an answer to this. :P

>
> thanks,
> Tobin.
>
>  Documentation/security/self-protection.rst | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/Documentation/security/self-protection.rst b/Documentation/security/self-protection.rst
> index 60c8bd8b77bf..e711280cfdd7 100644
> --- a/Documentation/security/self-protection.rst
> +++ b/Documentation/security/self-protection.rst
> @@ -270,6 +270,20 @@ attacks, it is important to defend against exposure of both kernel memory
>  addresses and kernel memory contents (since they may contain kernel
>  addresses or other sensitive things like canary values).
>
> +Kernel addresses
> +----------------
> +
> +Printing kernel addresses to userspace leaks sensitive information about
> +the kernel memory layout. Care should be exercised when using any printk
> +specifier that prints the raw address, currently %px, %p[ad], (and %p[sSb]
> +in certain circumstances [*]).  Any file written to using one of these
> +specifiers should be readable only by privileged processes.
> +
> +Kernels 4.14 and older printed the raw address using %p. As of 4.15-rc1
> +addresses printed with the specifier %p are hashed before printing.
> +
> +[*] If symbol lookup fails, the raw address is currently printed.

Is there a plan to adjust this case?

Thanks!

-Kees

> +
>  Unique identifiers
>  ------------------
>
> --
> 2.7.4
>



-- 
Kees Cook
Pixel Security