Re: [PATCH] lib: vsprintf: Avoid 32-bit truncation in vsscanf number parsing

[Richard Fitzgerald]

On 13/11/2020 14:00, Petr Mladek wrote:
> On Thu 2020-11-12 12:04:27, Steven Rostedt wrote:
> > On Thu, 12 Nov 2020 15:46:46 +0000
> > Richard Fitzgerald <rf@xxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > > See this thread from 2014 where the field width problem was raised and
> > > explained:
> > > http://lkml.iu.edu/hypermail/linux/kernel/1401.1/03443.html
> > >
> > > and the reply from Linus Torvalds that was against fixing field width
> > > handling:
> > > http://lkml.iu.edu/hypermail/linux/kernel/1401.1/03488.html
> >
> > Thanks for the pointers, but note, that references to older emails should
> > use https://lore.kernel.org/ as these links format the output really
> > horribly.
> >
> > > which I assume is why the field handling wasn't unoptimized to be
> > > strictly correct.
>
> Honestly, the handling of the number width by div does not look like
> a real optimization to me. It avoids the need of the temporary buffer
> by expensive and error-prone operation.
>
> IMHO, it is safe to assume that the width will be limited so that
> the value would never overflow.
>
> The longest supported number would be (2^64 - 1) in octal. If I am
> counting correctly, it is
>
>       01777777777777777777777
>
> and it fits into buf[24] including the trailing '\0'.
>
> We could call WARN_ON_ONCE() when the width >= 24 is higher.
> And we could add a compiler check when long long is bigger
> than 64-bit.
>
> > Yes, but perhaps its time to fix the real problem and not just add
> > band-aids. That thread is over 6 years old (the email was from Jan 14, 2014)
> >
> > $ git diff `git rev-list --before 'Jan 14 2014' HEAD --max-count=1` |
> >    grep '^+' | grep sscanf | wc -l
> > 622
> >
> > There's been over 600 new additions of sscanf(). Now is the time to just
> > fix it correctly.
>
> And the following one might suffer from this problem:
>
> drivers/soundwire/slave.c:              ret = sscanf(compat, "sdw%01x%04hx%04hx%02hhx", &sdw_version,

That's exactly the bug I have.
I'll look at reworking the code to handle number field widths properly.

> I agree with Steven that it is time to fix it properly.
>
> Best Regards,
> Petr