linux-kernel - Re: [PATCH][RESEND] lib/vsprintf: make-printk-non-secret printks all addresses as unhashed

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Message-ID: <YCMFzCz8ootub9pH@elver.google.com>
Date:   Tue, 9 Feb 2021 22:59:40 +0100
From:   Marco Elver <elver@...gle.com>
To:     Timur Tabi <timur@...nel.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-kernel@...r.kernel.org, vbabka@...e.cz, linux-mm@...ck.org,
        willy@...radead.org, akpm@...ux-foundation.org,
        torvalds@...ux-foundation.org, roman.fietze@...na.com,
        keescook@...omium.org, john.ogness@...utronix.de,
        akinobu.mita@...il.com, glider@...gle.com, andreyknvl@...gle.com
Subject: Re: [PATCH][RESEND] lib/vsprintf: make-printk-non-secret printks all
 addresses as unhashed

On Tue, Feb 02, 2021 at 03:36PM -0600, Timur Tabi wrote:
> If the make-printk-non-secret command-line parameter is set, then
> printk("%p") will print addresses as unhashed.  This is useful for
> debugging purposes.
> 
> A large warning message is displayed if this option is enabled,
> because unhashed addresses, while useful for debugging, exposes
> kernel addresses which can be a security risk.
> 
> Signed-off-by: Timur Tabi <timur@...nel.org>
> ---
>  lib/vsprintf.c | 34 ++++++++++++++++++++++++++++++++--
>  1 file changed, 32 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 3b53c73580c5..b9f87084afb0 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -2090,6 +2090,30 @@ 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 */
> +static bool debug_never_hash_pointers __ro_after_init;

Would it be reasonable to make this non-static? Or somehow make it
possible to get this flag from other subsystems?

There are other places in the kernel that dump sensitive data such as
registers. We'd like to be able to use 'debug_never_hash_pointers' to
decide if our debugging tools can dump registers etc. What we really
need is info if the kernel is in debug mode and we can dump all kinds of
sensitive info; debug_never_hash_pointers is would be a good enough
proxy for that.

Thanks,
-- Marco