linux-kernel - Re: [PATCH v6 8/9] vsprintf: Prevent crash when dereferencing invalid pointers

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Date:   Tue, 19 Feb 2019 12:30:03 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        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 v6 8/9] vsprintf: Prevent crash when dereferencing
 invalid pointers

On (02/08/19 16:23), Petr Mladek wrote:
[..]
> + /*
> +  * This is not a fool-proof test. 99% of the time that this will fault is
> +  * due to a bad pointer, not one that crosses into bad memory. Just test
> +  * the address to make sure it doesn't fault due to a poorly added printk
> +  * during debugging.
> +  */
> +static const char *check_pointer_msg(const void *ptr)
> +{
> +	char byte;
> +
> +	if (!ptr)
> +		return "(null)";
> +
> +	if (probe_kernel_address(ptr, byte))
> +		return "(efault)";
> +
> +	return NULL;
> +}

Hmm... So the assumption here is that the target buffer always has
at least strlen("(efault)") bytes and, thus, we always can write the
error message to it.

> +static int check_pointer(char **buf, char *end, const void *ptr,
> +			     struct printf_spec spec)
> +{
> +	const char *err_msg;
> +
> +	err_msg = check_pointer_msg(ptr);
> +	if (err_msg) {
> +		*buf = string_nocheck(*buf, end, err_msg, spec);
> +		return -EFAULT;
> +	}
> +
> +	return 0;
> +}

Suppose in my driver I sprintf() pointers to 4-bytes strings and, thus,
have only 5 spare bytes in target buffer. But one of the pointers is
faulty and now sprintf() writes "(efault)" to target buffer which can
hold only 5 bytes.

	-ss