Re: [PATCH linux-next] parisc: use strscpy() to instead of strncpy()

[Helge Deller]

On 12/27/22 23:43, James Bottomley wrote:
> On Tue, 2022-12-27 at 22:38 +0100, Helge Deller wrote:
> > Hi James,
> >
> > On 12/27/22 13:38, James Bottomley wrote:
> > > On Fri, 2022-12-23 at 08:55 +0100, Helge Deller wrote:
> > > > On 12/23/22 03:40, yang.yang29@xxxxxxxxxx wrote:
> > > > > From: Xu Panda <xu.panda@xxxxxxxxxx>
> > > > >
> > > > > The implementation of strscpy() is more robust and safer.
> > > > > That's now the recommended way to copy NUL-terminated strings.
> > > >
> > > > Thanks for your patch, but....
> > > >
> > > > > Signed-off-by: Xu Panda <xu.panda@xxxxxxxxxx>
> > > > > Signed-off-by: Yang Yang <yang.yang29@xxxxxxx>
> > > > > ---
> > > > >     drivers/parisc/pdc_stable.c | 9 +++------
> > > > >     1 file changed, 3 insertions(+), 6 deletions(-)
> > > > >
> > > > > diff --git a/drivers/parisc/pdc_stable.c
> > > > > b/drivers/parisc/pdc_stable.c
> > > > > index d6af5726ddf3..403bca0021c5 100644
> > > > > --- a/drivers/parisc/pdc_stable.c
> > > > > +++ b/drivers/parisc/pdc_stable.c
> > > > > @@ -274,8 +274,7 @@ pdcspath_hwpath_write(struct pdcspath_entry
> > > > > *entry, const char *buf, size_t coun
> > > > >
> > > > >           /* We'll use a local copy of buf */
> > > > >           count = min_t(size_t, count, sizeof(in)-1);
> > > > > -       strncpy(in, buf, count);
> > > > > -       in[count] = '\0';
> > > > > +       strscpy(in, buf, count + 1);
> > > >
> > > > could you resend it somewhat simplified, e.g.
> > > > strscpy(in, buf, sizeof(in));
> > >
> > > I don't think you can: count is the size of buf, if that's <
> > > sizeof(in) you've introduced a write beyond end of buffer.  In fact
> > > sysfs tends to pass pages as buffers, so there's no actual problem,
> > > but if that ever changed ...
> >
> > Huh?... he doesn't change "count", so what's wrong with the latest
> > patch?
>
> the array buf[] is actually buf[count], so if count < 64 then
> sizeof(buf) < sizeof(in) and you're copying whatever is after buf on
> the stack or wherever it comes from. The amount you copy into in[]
> truly has to be the smaller of count and sizeof(in).  These are file
> operations, so you shouldn't rely on buf[] being null terminated

Ok, the main point I missed was that buf[] might not be null terminated.
Thanks for the explanation.

Yang & Xu, no need to resend the patch. I'll take your v1 version.

Thanks!
Helge

> (kernfs ensures it is, but it's a dangerous thing to rely on in the
> face of someone trying to exploit a stack smashing attack).
>
> James