linux-kernel - Re: [PATCH driver-core-linus] sysfs: make sure read buffer is zeroed

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Date:	Mon, 19 May 2014 15:25:00 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org, Kay Sievers <kay@...y.org>,
	Ron <ron@...ian.org>, Ben Hutchings <ben@...adent.org.uk>
Subject: Re: [PATCH driver-core-linus] sysfs: make sure read buffer is zeroed

On Mon, May 19, 2014 at 03:52:10PM -0400, Tejun Heo wrote:
> 13c589d5b0ac ("sysfs: use seq_file when reading regular files")
> switched sysfs from custom read implementation to seq_file to enable
> later transition to kernfs.  After the change, the buffer passed to
> ->show() is acquired through seq_get_buf(); unfortunately, this
> introduces a subtle behavior change.  Before the commit, the buffer
> passed to ->show() was always zero as it was allocated using
> get_zeroed_page().  Because seq_file doesn't clear buffers on
> allocation and neither does seq_get_buf(), after the commit, depending
> on the behavior of ->show(), we may end up exposing uninitialized data
> to userland thus possibly altering userland visible behavior and
> leaking information.
> 
> Fix it by explicitly clearing the buffer.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reported-by: Ron <ron@...ian.org> 
> Fixes: 13c589d5b0ac ("sysfs: use seq_file when reading regular files")
> ---
>  fs/sysfs/file.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
> index 28cc1acd..e9ef59b 100644
> --- a/fs/sysfs/file.c
> +++ b/fs/sysfs/file.c
> @@ -47,12 +47,13 @@ static int sysfs_kf_seq_show(struct seq_file *sf, void *v)
>  	ssize_t count;
>  	char *buf;
>  
> -	/* acquire buffer and ensure that it's >= PAGE_SIZE */
> +	/* acquire buffer and ensure that it's >= PAGE_SIZE and clear */
>  	count = seq_get_buf(sf, &buf);
>  	if (count < PAGE_SIZE) {
>  		seq_commit(sf, -1);
>  		return 0;
>  	}
> +	memset(buf, 0, PAGE_SIZE);
>  
>  	/*
>  	 * Invoke show().  Control may reach here via seq file lseek even

Thanks, I'll go queue this up.

greg k-h
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