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

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Date:	Mon, 19 May 2014 15:32:22 -0700
From:	Joe Perches <joe@...ches.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Tejun Heo <tj@...nel.org>, 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, 2014-05-19 at 15:25 -0700, Greg Kroah-Hartman wrote:
> 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.
[]
> > diff --git 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);

	memset(buf, 0, count)?

> Thanks, I'll go queue this up.


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