linux-kernel - Re: [PATCH] vsprintf: Make "null" pointer dereference more robust

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Date:   Fri, 02 Mar 2018 16:17:34 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     "Tobin C . Harding" <me@...in.cc>, linux@...musvillemoes.dk,
        Joe Perches <joe@...ches.com>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH] vsprintf: Make "null" pointer dereference more robust

On Fri, 2018-03-02 at 13:53 +0100, Petr Mladek wrote:
> %p has many modifiers where the pointer is dereferenced. An invalid
> pointer might cause kernel to crash silently.
> 
> Note that printk() formats the string 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.
> 
> In general, we should do our best to get useful message from printk().
> All pointers to the first memory page must be invalid. Let's prevent
> the dereference and print "(null)" in this case. This is already done
> in many other situations, including "%s" format handling and many
> page fault handlers.
> 


With such explanation it makes at least clear for the reader why it's
done.

Thanks!

Would you be okay if I take this one as a first in my series and
resubmit the series based on it?

> Signed-off-by: Petr Mladek <pmladek@...e.com>
> ---
>  lib/vsprintf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index d7a708f82559..5c2d1f44218a 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -1849,7 +1849,7 @@ char *pointer(const char *fmt, char *buf, char
> *end, void *ptr,
>  {
>  	const int default_width = 2 * sizeof(void *);
>  
> -	if (!ptr && *fmt != 'K' && *fmt != 'x') {
> +	if ((unsigned long)ptr < PAGE_SIZE && *fmt != 'K' && *fmt !=
> 'x') {
>  		/*
>  		 * Print (null) with the same width as a pointer so
> it makes
>  		 * tabular output look nice.

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