linux-kernel - Re: [PATCH REPOST] printk: fix possible printk buffer overrun introduced with recursion check

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Message-ID: <20080213124548.GB6344@elte.hu>
Date:	Wed, 13 Feb 2008 13:45:48 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Tejun Heo <htejun@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH REPOST] printk: fix possible printk buffer overrun
	introduced with recursion check


* Tejun Heo <htejun@...il.com> wrote:

> printk recursion detection prepends message to printk_buf and offsets 
> printk_buf when actual message is printed but it forgets to trim 
> buffer length accordingly.  This can result in buffer overrun in 
> extreme cases.
> 
> While at it, make printk_recursion_bug_msg static and move static 
> variables for recursion check into vprintk().

> @@ -666,7 +664,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
>  	}
>  	/* Emit the output into the temporary buffer */
>  	printed_len += vscnprintf(printk_buf + printed_len,
> -				  sizeof(printk_buf), fmt, args);
> +				  sizeof(printk_buf) - printed_len, fmt, args);
>  
>  	/*

nice one! I missed that.

Acked-by: Ingo Molnar <mingo@...e.hu>

but i'm not sure i agree with the moving of these variables inside 
vprintk:

> -/* cpu currently holding logbuf_lock */
> -static volatile unsigned int printk_cpu = UINT_MAX;
> -
> -const char printk_recursion_bug_msg [] =
> -			KERN_CRIT "BUG: recent printk recursion!\n";
> -static int printk_recursion_bug;
> -
>  asmlinkage int vprintk(const char *fmt, va_list args)
>  {
> +	/* cpu currently holding logbuf_lock */
> +	static volatile unsigned int printk_cpu = UINT_MAX;
> +	static const char printk_recursion_bug_msg [] =
> +		KERN_CRIT "BUG: recent printk recursion!\n";
> +	static int printk_recursion_bug;
>  	static int log_level_unknown = 1;
>  	static char printk_buf[1024];

it's easy to miss a static variable inside a function local variables 
block. It would b ebetter to move log_level_unknown and printk_buf 
_outside_ the function, to the other ones. [and to mark 
printk_recursion_bug_msg static]

	Ingo
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