linux-kernel - Re: [PATCH] Fix faulty logic in the case of recursive printk

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Date:	Tue, 26 Aug 2014 14:56:20 +0200
From:	Jan Kara <jack@...e.cz>
To:	Patrick Palka <patrick@...cs.ath.cx>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	pmladek@...e.cz, jack@...e.cz, rostedt@...dmis.org
Subject: Re: [PATCH] Fix faulty logic in the case of recursive printk

On Tue 26-08-14 07:23:22, Patrick Palka wrote:
> Thanks for reviewing!  I have put back the call to strlen() and adjusted
> the commit message accordingly.
> 
> Patrick
> 
> -- >8 --
> 
> We shouldn't set text_len in the code path that detects printk recursion
> because text_len corresponds to the length of the string inside textbuf.
> A few lines down from the line
> 
>     text_len = strlen(recursion_msg);
> 
> is the line
> 
>     text_len += vscnprintf(text + text_len, ...);
> 
> So if printk detects recursion, it sets text_len to 29 (the length of
> recursion_msg) and logs an error.  Then the message supplied by the
> caller of printk is stored inside textbuf but offset by 29 bytes.  This
> means that the output of the recursive call to printk will contain 29
> bytes of garbage in front of it.
> 
> This defect is caused by commit 458df9fd ("printk: remove separate
> printk_sched buffers and use printk buf instead") which turned the line
> 
>     text_len = vscnprintf(text, ...);
> 
> into
> 
>     text_len += vscnprintf(text + text_len, ...);
> 
> To fix this, this patch avoids setting text_len when logging the printk
> recursion error.  This patch also marks unlikely() the branch leading
> up to this code.
> 
> Signed-off-by: Patrick Palka <patrick@...cs.ath.cx>
  The patch looks good. You can add:
Reviewed-by: Jan Kara <jack@...e.cz>

  Plus you could also add tags:
Fixes: 458df9fd4815b47809875d57f42e16401674b621
CC: stable@...r.kernel.org

								Honza
> ---
>  kernel/printk/printk.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index e04c455..1ce7706 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1665,15 +1665,15 @@ asmlinkage int vprintk_emit(int facility, int level,
>  	raw_spin_lock(&logbuf_lock);
>  	logbuf_cpu = this_cpu;
>  
> -	if (recursion_bug) {
> +	if (unlikely(recursion_bug)) {
>  		static const char recursion_msg[] =
>  			"BUG: recent printk recursion!";
>  
>  		recursion_bug = 0;
> -		text_len = strlen(recursion_msg);
>  		/* emit KERN_CRIT message */
>  		printed_len += log_store(0, 2, LOG_PREFIX|LOG_NEWLINE, 0,
> -					 NULL, 0, recursion_msg, text_len);
> +					 NULL, 0, recursion_msg,
> +					 strlen(recursion_msg));
>  	}
>  
>  	/*
> -- 
> 2.1.0
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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