linux-kernel - Re: [PATCH 1/2] misc: c2port: core: Make copying name from userspace more secure

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Date:   Mon, 2 Nov 2020 13:47:29 +0000
From:   Lee Jones <lee.jones@...aro.org>
To:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
Cc:     David Laight <David.Laight@...lab.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Rodolfo Giometti <giometti@...eenne.com>,
        "Eurotech S.p.A" <info@...otech.it>,
        Geert Uytterhoeven <geert+renesas@...der.be>
Subject: Re: [PATCH 1/2] misc: c2port: core: Make copying name from userspace
 more secure

On Mon, 02 Nov 2020, gregkh@...uxfoundation.org wrote:

> On Mon, Nov 02, 2020 at 12:43:01PM +0000, Lee Jones wrote:
> > On Mon, 02 Nov 2020, gregkh@...uxfoundation.org wrote:
> > 
> > > On Mon, Nov 02, 2020 at 11:49:03AM +0000, Lee Jones wrote:
> > > > On Mon, 02 Nov 2020, David Laight wrote:
> > > > 
> > > > > From: Lee Jones
> > > > > > Sent: 02 November 2020 11:12
> > > > > > 
> > > > > > strncpy() may not provide a NUL terminator, which means that a 1-byte
> > > > > > leak would be possible *if* this was ever copied to userspace.  Ensure
> > > > > > the buffer will always be NUL terminated by using the kernel's
> > > > > > strscpy() which a) uses the destination (instead of the source) size
> > > > > > as the bytes to copy and b) is *always* NUL terminated.
> > > > > > 
> > > > > > Cc: Rodolfo Giometti <giometti@...eenne.com>
> > > > > > Cc: "Eurotech S.p.A" <info@...otech.it>
> > > > > > Reported-by: Geert Uytterhoeven <geert+renesas@...der.be>
> > > > > > Acked-by: Arnd Bergmann <arnd@...db.de>
> > > > > > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > > > > > ---
> > > > > >  drivers/misc/c2port/core.c | 2 +-
> > > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > > 
> > > > > > diff --git a/drivers/misc/c2port/core.c b/drivers/misc/c2port/core.c
> > > > > > index 80d87e8a0bea9..b96444ec94c7e 100644
> > > > > > --- a/drivers/misc/c2port/core.c
> > > > > > +++ b/drivers/misc/c2port/core.c
> > > > > > @@ -923,7 +923,7 @@ struct c2port_device *c2port_device_register(char *name,
> > > > > >  	}
> > > > > >  	dev_set_drvdata(c2dev->dev, c2dev);
> > > > > > 
> > > > > > -	strncpy(c2dev->name, name, C2PORT_NAME_LEN - 1);
> > > > > > +	strscpy(c2dev->name, name, sizeof(c2dev->name));
> > > > > 
> > > > > strscpy() doesn't zero fill so if the memory isn't zeroed
> > > > > and a 'blind' copy to user of the structure is done
> > > > > then more data is leaked.
> > > > > 
> > > > > strscpy() may be better, but rational isn't right.
> > > > 
> > > > The original patch zeroed the data too, but I was asked to remove that
> > > > part [0].  In your opinion, should it be reinstated?
> > > > 
> > > > [0] https://lore.kernel.org/patchwork/patch/1272290/
> > > 
> > > Just keep the kzalloc() part of the patch, this portion makes no sense
> > > to me.
> > 
> > Can do.
> > 
> > > But if you REALLY want to get it correct, call dev_set_name()
> > > instead please, as that is what it is there for.
> > 
> > The line above isn't setting the 'struct device' name.  It looks as
> > though 'struct c2port' has it's own member, also called 'name'.  As to
> > how they differ, I'm not currently aware.  Nor do I wish to mess
> > around with the semantics all that much.
> > 
> > Going with suggestion #1.
> 
> As the "device" already has a name, I suggest just getting rid of this
> name field anyway, no need for duplicates.

That definitely goes against the point I made above:

 "Nor do I wish to mess around with the semantics all that much."

It looks as though the device name 'c2port%d' varies greatly to the
requested name 'uc'.  I don't have enough knowledge of how user-
space expects to use the provided sysfs entries to be able to
competently merge/decide which of these should be kept and which to
discard.

Hopefully one of the authors/maintainers are reading this and can come
up with an acceptable solution.

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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