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

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Message-ID: <20201113140052.GM1602@alley>
Date:   Fri, 13 Nov 2020 15:00:52 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Richard Fitzgerald <rf@...nsource.cirrus.com>,
        sergey.senozhatsky@...il.com, linux-kernel@...r.kernel.org,
        patches@...nsource.cirrus.com
Subject: Re: [PATCH] lib: vsprintf: Avoid 32-bit truncation in vsscanf number
 parsing

On Thu 2020-11-12 12:04:27, Steven Rostedt wrote:
> On Thu, 12 Nov 2020 15:46:46 +0000
> Richard Fitzgerald <rf@...nsource.cirrus.com> 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,

I agree with Steven that it is time to fix it properly.

Best Regards,
Petr