Re: [PATCH 1/5] debugfs: Prevent NULL dereference reading from string property

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, May 16, 2023 at 07:04:42PM +0100, Richard Fitzgerald wrote:
> On 16/5/23 18:43, Greg KH wrote:
> > On Tue, May 16, 2023 at 06:29:52PM +0100, Richard Fitzgerald wrote:
> > > On 16/5/23 17:33, Greg KH wrote:
> > > > On Tue, May 16, 2023 at 05:07:49PM +0100, Richard Fitzgerald wrote:
> > > > > Check in debugfs_read_file_str() if the string pointer is NULL.
> > > > > 
> > > > > It is perfectly reasonable that a driver may wish to export a string
> > > > > to debugfs that can have the value NULL to indicate empty/unused/ignore.
> > > > 
> > > > Does any in-kernel driver do this today?
> > > 
> > > I don't know. The history here is that I was using debugfs_create_str()
> > > to add a debugfs to a driver and made these improvements along the way.
> > > Ultimately I had a reason to use a custom reader implementation.
> > > But as I'd already written these patches I thought I'd send them.
> > > 
> > > > 
> > > > If not, why not fix up the driver instead?
> > > > 
> > > 
> > > Well... could do. Though it seems a bit odd to me that a driver
> > > design should be forced by the debugfs API, instead of the debugfs API
> > > fitting normal code design. It's pretty standard and idiomatic for code
> > > to use if (!str) { /* bail */ } type logic, so why shouldn't the debugfs
> > > API handle that?
> > > 
> > > > > 
> > > > > Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
> > > > > ---
> > > > >    fs/debugfs/file.c | 3 +++
> > > > >    1 file changed, 3 insertions(+)
> > > > > 
> > > > > diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
> > > > > index 1f971c880dde..2c085ab4e800 100644
> > > > > --- a/fs/debugfs/file.c
> > > > > +++ b/fs/debugfs/file.c
> > > > > @@ -878,6 +878,9 @@ ssize_t debugfs_read_file_str(struct file *file, char __user *user_buf,
> > > > >    		return ret;
> > > > >    	str = *(char **)file->private_data;
> > > > > +	if (!str)
> > > > > +		return simple_read_from_buffer(user_buf, count, ppos, "\n", 1);
> > > > 
> > > > Why not print "(NULL)"?
> > > > 
> > > 
> > > Again, could do. My thought here is that a debugfs can be piped into
> > > tools and having to insert a catch for "(NULL)" in the pipeline is a
> > > nuisance. This is a bit different from a dmesg print, which is less
> > > likely to be used this way or to guarantee machine-parsing.
> > > However, I don't mind changing to "(NULL)" if you prefer.
> > 
> > If a driver wants an "empty" string, they should provide an empty
> > string.  We don't do empty values for any other type of pointer, right?
> > 
> > Actually we really should just bail out with an error if this is NULL,
> > let's not paper over bad drivers like this.
> > 
> 
> I don't understand this comment.
> I think you'll find there is a very large amount of kernel code that
> uses a NULL value in a pointer to mean ignore/unspecified in
> some way. This has always been accepted C coding style.
> 
> The whole idea that a driver is "bad" for signalling some state
> by a pointer being NULL makes no sense.

The whole idea of passing a NULL pointer to debugfs makes no sense :)

If a driver does this, then they deserve the crash, let's just say "do
not do that" and leave it at that please.

> Please ignore this patch chain. I really don't feel like writing
> non-idiomatic C code just to work around badly designed debugfs APIs.
> Better to write a custom read().

Let's fix the badly designed debugfs apis please, it's not good to have
code that is impossible to use correctly.

thanks,

greg k-h