linux-kernel - Re: Printk specifiers for _

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Date:   Fri, 20 Nov 2020 13:42:42 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Printk specifiers for __user pointers

On Fri, 20 Nov 2020 11:44:12 -0500
Alan Stern <stern@...land.harvard.edu> wrote:

> To the VSPRINTF maintainers:
> 
> Documentation/core-api/printk-formats.rst lists a large number of format 
> specifiers for pointers of various sorts.  Yet as far as I can see, 
> there is no specifier meant for use with __user pointers.
> 
> The security implications of printing the true, unmangled value of a 
> __user pointer are minimal, since doing so does not leak any kernel 
> information.  So %px would work, but tools like checkpatch.pl don't like 
> it.
> 
> Should a new specifier be added?  If not, should we simply use %px?

There's currently no user of '%pu' (although there is a '%pus'. Perhaps we
should have a '%pux'?

I would even state that if it is used, that if makes sure that the value is
indeed a user space pointer (goes through the same checks as accessing user
space), before its printed, otherwise it shows "(fault)" or something.

-- Steve