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

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Date:   Fri, 8 Dec 2017 07:39:00 +1100
From:   "Tobin C. Harding" <me@...in.cc>
To:     Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] docs: add documentation on printing kernel addresses

Please drop this patch, needs amendment (commented inline).

On Thu, Dec 07, 2017 at 11:26:38AM +1100, Tobin C. Harding 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>
> ---
> 
> Is there a proffered method for subscripts in sphinx kernel docs? Here
> we use '[*]' 
> 
> 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.

[*] If KALLSYMS is enabled and symbol lookup fails, the raw address is
currently printed. If KALLSYMS is not enabled the address is printed. 

thanks,
Tobin.