Re: [PATCH] lib/strscpy: avoid KASAN false positive

[Chris Metcalf]

On 7/18/2017 6:04 PM, Andrew Morton wrote:
> On Wed, 19 Jul 2017 00:31:36 +0300 Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:
>
> > On 07/18/2017 11:26 PM, Linus Torvalds wrote:
> > > On Tue, Jul 18, 2017 at 1:15 PM, Andrey Ryabinin
> > > <aryabinin@xxxxxxxxxxxxx> wrote:
> > > > No, it does warn about valid users. The report that Dave posted wasn't about wrong strscpy() usage
> > > > it was about reading 8-bytes from 5-bytes source string. It wasn't about buggy 'count' at all.
> > > > So KASAN will warn for perfectly valid code like this:
> > > >          char dest[16];
> > > >          strscpy(dest, "12345", sizeof(dest)):
> > > Ugh, ok, yes.
> > >
> > > > For strscpy() that would mean making the *whole* read from 'src' buffer unchecked by KASAN.
> > > So we do have that READ_ONCE_NOCHECK(), but could we perhaps have
> > > something that doesn't do a NOCHECK but a partial check and is simply
> > > ok with "this is an optimistc longer access"
> > This can be dont, I think.
> >
> > Something like this:
> > static inline unsigned long read_partial_nocheck(unsigned long *x)
> > {
> > 	unsigned long ret = READ_ONCE_NOCHECK(x);
> > 	kasan_check_partial(x, sizeof(unsigned long));
> > 	return ret;
> > }
> (Cc Chris)
>
> We could just remove all that word-at-a-time logic.  Do we have any
> evidence that this would harm anything?

The word-at-a-time logic was part of the initial commit since I wanted
to ensure that strscpy could be used to replace strlcpy or strncpy without
serious concerns about performance.  It seems unfortunate to remove it
unconditionally to support KASAN, but I haven't looked deeply at the
tradeoffs here.

--
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com