Re: [PATCH] xfs: Use scnprintf() for avoiding potential buffer overflow

Darrick J. Wong
2020-03-13T15:52:55Z

On Fri, Mar 13, 2020 at 08:18:42AM +0100, Takashi Iwai wrote:
> On Fri, 13 Mar 2020 06:00:00 +0100,
> Dave Chinner wrote:
> >
> > On Thu, Mar 12, 2020 at 03:43:42PM -0700, Darrick J. Wong wrote:
> > > On Thu, Mar 12, 2020 at 03:27:01PM -0700, Dave Chinner wrote:
> > > >
> > > > I'm annoyed that every time a fundamental failing or technical debt
> > > > is uncovered in the kernel, nobody takes responsibility to fix the
> > > > problem completely, for everyone, for ever.
> > > >
> > > > As Thomas said recently: correctness first.
> > > >
> > > > https://lwn.net/ml/linux-kernel/87v9nc63io.fsf@nanos.tec.linutronix.de/
> > > >
> > > > This is not "good enough" - get rid of snprintf() altogether.
> > >
> > > $ git grep snprintf | wc -l
> > > 8534
> > >
> > > That's somebody's 20 year project... :/
> >
> > Or half an hour with sed.
> >
> > Indeed, not all of them are problematic:
> >
> > $ git grep "= snprintf" |wc -l
> > 1744
> > $ git grep "return snprintf"|wc -l
> > 1306
> >
> > Less than half of them use the return value.
> >
> > Anything that calls snprintf() without checking the return
> > value (just to prevent formatting overruning the buffer) can be
> > converted by search and replace because the behaviour is the
> > same for both functions in this case.
> >
> > Further, code written properly to catch a snprintf overrun will also
> > correctly pick up scnprintf filling the buffer. However, code that
> > overruns with snprintf()s return value is much more likely to work
> > correctly with scnprintf as the calculated buffer length won't
> > overrun into memory beyond the buffer.
> >
> > And that's likely all of the snprintf() calls dealt with in half an
> > hour. Now snprintf can be removed.
> >
> > What's more scary is this:
> >
> > $ git grep "+= sprintf" |wc -l
> > 1834
> >
> > which is indicative of string formatting iterating over buffers with
> > no protection against the formatting overwriting the end of the
> > buffer. Those are much more dangerous (i.e. potential buffer
> > overflows) than the snprintf problem being fixed here, and those
> > will need to be checked and fixed manually to use scnprintf().
> > That's where the really nasty technical debt lies, not snprintf...
>
> Right, that's how I started looking through the whole tree and
> submitting patches like this. I've submitted to per-subsystem patches
> and many of them have been already covered; after my tons of patches:
>
> % git grep '+= snprintf' | wc -l
> 147
>
> The remaining codes are either doing right or it's a user-space code
> that have no scnprintf() available. For other snprintf() usages can
> be converted to scnprintf() easily as you mentioned.
>
> An open question is what we should do for the code that uses
> snprintf() in a right way. snprintf() is useful to predict the
> non-fitted formatted string. Some warns if such a situation happens.
> Replacing with scnprintf(), this would never hit, so you'll lose the
> way of message truncation there.
>
> Maybe we may keep snprintf() but put a checkpatch warning for any new
> usage?
>
> In anyway, if you prefer, I'll resubmit the patch to convert all
> snprintf() calls in xfs.

I already put the first patch in -next, so send a second patch to
convert the rest, please.

--D

>
> thanks,
>
> Takashi