linux-kernel - Re: [PATCH v5 09/11] vsprintf: Prevent crash when dereferencing invalid pointers

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Date:   Wed, 25 Apr 2018 18:10:54 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Petr Mladek <pmladek@...e.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Tobin C . Harding" <me@...in.cc>, Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 09/11] vsprintf: Prevent crash when dereferencing
 invalid pointers

On Wed, 2018-04-25 at 13:12 +0200, Petr Mladek wrote:
> We already prevent crash when dereferencing some obviously broken
> pointers. But the handling is not consistent. Sometimes we print
> "(null)"
> only for pure NULL pointer, sometimes for pointers in the first
> page and sometimes also for pointers in the last page (error codes).
> 
> Note that printk() call this code under logbuf_lock. Any recursive
> printks are redirected to the printk_safe implementation and the
> messages
> are stored into per-CPU buffers. These buffers might be eventually
> flushed
> in printk_safe_flush_on_panic() but it is not guaranteed.
> 
> This patch adds a check using probe_kernel_read(). It is not a full-
> proof
> test. But it should help to see the error message in 99% situations
> where
> the kernel would silently crash otherwise.
> 
> Also it makes the error handling unified for "%s" and the many %p*
> specifiers that need to read the data from a given address. We print:
> 
>    + (null)   when accessing data on pure pure NULL address
>    + (efault) when accessing data on an invalid address
> 
> It does not affect the %p* specifiers that just print the given
> address
> in some form, namely %pF, %pf, %pS, %ps, %pB, %pK, %px, and plain %p.
> 
> Note that we print (efault) from security reasons. In fact, the real
> address can be seen only by %px or eventually %pK.


> +static const char *check_pointer_access(const void *ptr)
> +{
> +	char byte;
> +
> +	if (!ptr)
> +		return "(null)";
> +
> +	if (probe_kernel_address(ptr, byte))
> +		return "(efault)";
> +
> +	return NULL;
> +}
> +
> +static bool valid_pointer_access(char **buf, char *end, const void
> *ptr,
> +				 struct printf_spec spec)
> +{
> +	const char *err_msg;
> +
> +	err_msg = check_pointer_access(ptr);
> +	if (err_msg) {
> +		*buf = valid_string(*buf, end, err_msg, spec);
> +		return false;
> +	}
> +
> +	return true;
> +}

I would preserve similar style of buf pointer handling, i.e.

static char *valid_pointer_access(char **buf, char *end,
				  const void *ptr, struct printf_spec spec)
{
	const char *err_msg;

	err_msg = check_pointer_access(ptr);
	if (err_msg)
		return = valid_string(*buf, end, err_msg, spec);

	return NULL;
}

-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy