linux-kernel - Re: [PATCH] iio: dac: ad3552r-hs: fix out-of-bound write in ad3552r_hs_write_data

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Message-ID: <aQDXF-AIF6wNIo76@smile.fi.intel.com>
Date: Tue, 28 Oct 2025 16:45:43 +0200
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Nuno Sá <noname.nuno@...il.com>
Cc: Miaoqian Lin <linmq006@...il.com>, Markus Burri <markus.burri@...com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Michael Hennerich <Michael.Hennerich@...log.com>,
	Jonathan Cameron <jic23@...nel.org>,
	David Lechner <dlechner@...libre.com>,
	Nuno Sá <nuno.sa@...log.com>,
	Andy Shevchenko <andy@...nel.org>,
	Angelo Dureghello <adureghello@...libre.com>,
	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] iio: dac: ad3552r-hs: fix out-of-bound write in
 ad3552r_hs_write_data_source

On Tue, Oct 28, 2025 at 12:31:04PM +0000, Nuno Sá wrote:
> On Tue, 2025-10-28 at 11:07 +0200, Andy Shevchenko wrote:
> > On Tue, Oct 28, 2025 at 10:19:27AM +0200, Andy Shevchenko wrote:
> > > On Tue, Oct 28, 2025 at 10:18:05AM +0200, Andy Shevchenko wrote:
> > > > On Mon, Oct 27, 2025 at 11:07:13PM +0800, Miaoqian Lin wrote:

...

> > > > > +	if (count >= sizeof(buf))
> > > > > +		return -ENOSPC;
> > > > 
> > > > But this makes the validation too strict now.
> > > > 
> > > > >  	ret = simple_write_to_buffer(buf, sizeof(buf) - 1, ppos,
> > > > > userbuf,
> > > > >  				     count);
> > > > 
> > > > You definitely failed to read the code that implements the above.
> > > > 
> > > > >  	if (ret < 0)
> > > > >  		return ret;
> > > 
> > > > > -	buf[count] = '\0';
> > > > > +	buf[ret] = '\0';
> > > 
> > > Maybe this line is what we might need, but I haven't checked deeper if it's
> > > a
> > > problem.
> > 
> > So, copy_to_user() and copy_from_user() are always inlined macros.
> > The simple_write_to_buffer() is not. The question here is how
> > the __builit_object_size() will behave on the address given as a parameter to
> > copy_from_user() in simple_write_to_buffer().
> > 
> > If it may detect reliably that the buffer is the size it has. I believe it's
> > easy for the byte arrays on stack.
> 
> I think the above does not make sense (unless I'm missing your point which might
> very well be).

It seems I stand corrected. I was staring too much at copy_from_user() without
retrieving the validation logic behind simple_write_to_buffer().

> So, __builit_object_size() is for things known at compile time.
> Moreover, simple_write_to_buffer() already has all of the gymnastics to make
> sure copy_from_user() has the proper parameters. The thing is that it does it in
> a "silent" way which means that if your buffer is not big enough you'll get a
> concatenated string. Sure, you'll likely get an error down the road (due to an
> invalid value) but I do see some value in returning back the root cause of the
> issue.
> 
> So, the preliminary check while not being a big deal, it's also not completely
> useless IMO. I do not have any strong feeling though. However, I think the below
> is very much needed...
> 
> > That said, without proof that compiler is unable to determine the destination
> > buffer size, this patch and the one by Markus are simple noise which actually
> > changes an error code on the overflow condition.
> > 
> > The only line that assigns NUL character might be useful in some cases
> > (definitely when buffer comes through indirect calls from a heap, etc).
> 
> I think you can easily pass a string >= than 64 bytes (from userspace). AFAIR,
> you don't really set a size into debugfs files. For sure you can mess things
> with zero sized binary attributes so I have some confidence you have the same
> with debugfs.
> 
> And even if all the above is not reproducible I'm still of the opinion that
> 
> buf[ret] = '\0';
> 
> is semantically the correct code.

Yes, but it should either be explained as just making code robust vs. real bugfix.
For the latter I want to see the real traceback and a reproducer. I also wonder why
we never had reports from syzkaller on this. It has non-zero chance to stumble over
the issue here (if there is an issue to begin with).

-- 
With Best Regards,
Andy Shevchenko