linux-kernel - Re: [PATCH] drivers/base: Fix length checks in create_syslog_header()/dev_vprintk

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Date:	Mon, 13 Jan 2014 10:52:21 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Ben Hutchings <ben@...adent.org.uk>
Cc:	LKML <linux-kernel@...r.kernel.org>, Joe Perches <joe@...ches.com>,
	Kay Sievers <kay@...y.org>
Subject: Re: [PATCH] drivers/base: Fix length checks in
 create_syslog_header()/dev_vprintk_emit()

On Fri, Dec 27, 2013 at 08:10:27PM +0100, Ben Hutchings wrote:
> On Fri, 2013-12-27 at 10:47 -0800, Greg Kroah-Hartman wrote:
> > On Fri, Dec 27, 2013 at 06:18:18PM +0100, Ben Hutchings wrote:
> > > snprintf() returns the number of bytes that could have been written
> > > (excluding the null), not the actual number of bytes written.  Given a
> > > long enough subsystem or device name, these functions will advance
> > > beyond the end of the on-stack buffer in dev_vprintk_exit(), resulting
> > > in an information leak or stack corruption.  I don't know whether such
> > > a long name is currently possible.
> > > 
> > > In case snprintf() returns a value >= the buffer size, do not add
> > > structured logging information.  Also WARN the first time this
> > > happens, so we can fix the driver or increase the buffer size.
> > > 
> > > Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
> > > ---
> > >  drivers/base/core.c | 9 +++++++++
> > >  1 file changed, 9 insertions(+)
> > > 
> > > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > > index 67b180d..989a93c 100644
> > > --- a/drivers/base/core.c
> > > +++ b/drivers/base/core.c
> > > @@ -2022,6 +2022,8 @@ create_syslog_header(const struct device *dev, char *hdr, size_t hdrlen)
> > >  		return 0;
> > >  
> > >  	pos += snprintf(hdr + pos, hdrlen - pos, "SUBSYSTEM=%s", subsys);
> > > +	if (pos >= hdrlen)
> > > +		goto overflow;
> > >  
> > >  	/*
> > >  	 * Add device identifier DEVICE=:
> > > @@ -2053,7 +2055,14 @@ create_syslog_header(const struct device *dev, char *hdr, size_t hdrlen)
> > >  				"DEVICE=+%s:%s", subsys, dev_name(dev));
> > >  	}
> > >  
> > > +	if (pos >= hdrlen)
> > > +		goto overflow;
> > > +
> > >  	return pos;
> > > +
> > > +overflow:
> > > +	dev_WARN_ONCE(dev, 1, "device/subsystem name too long");
> > 
> > Why only warn once?  Any device/subsystem mix should be complained
> > about, if for only that we should be really annoying about it to get it
> > resolved.
> 
> This would expand the volume of logging for the problem device by a
> factor of ~50 so it doesn't seem like a good failure mode.

I want that failure mode to be really obvious so that the user tells the
driver author to fix their code.  A single error message usually just
gets ignored.

So please fail really loudly.

thanks,

greg k-h
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