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

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Date:	Fri, 27 Dec 2013 20:10:27 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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, 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.

Ben.

-- 
Ben Hutchings
Computers are not intelligent.	They only think they are.

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