Re: [PATCH v3] net: don't call strlen on non-terminated string in dev_set_alias()

[Yury Norov]

On Thu, Jun 01, 2017 at 03:50:33PM +0200, Alexander Potapenko wrote:
> On Thu, Jun 1, 2017 at 3:47 PM, Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> wrote:
> > On Thu, Jun 01, 2017 at 02:38:29PM +0200, Alexander Potapenko wrote:
> >> KMSAN reported a use of uninitialized memory in dev_set_alias(),
> >> which was caused by calling strlcpy() (which in turn called strlen())
> >> on the user-supplied non-terminated string.
> >>
> >> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> >> ---
> >> v3: removed the multi-line comment
> >> v2: fixed an off-by-one error spotted by Dmitry Vyukov
> >
> > [...]
> >
> >> ---
> >>  net/core/dev.c | 4 +++-
> >>  1 file changed, 3 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/net/core/dev.c b/net/core/dev.c
> >> index fca407b4a6ea..3e3b29133cc9 100644
> >> --- a/net/core/dev.c
> >> +++ b/net/core/dev.c
> >> @@ -1254,7 +1254,9 @@ int dev_set_alias(struct net_device *dev, const char *alias, size_t len)
> >>               return -ENOMEM;
> >>       dev->ifalias = new_ifalias;
> >>
> >> -     strlcpy(dev->ifalias, alias, len+1);
> >> +     /* alias comes from the userspace and may not be zero-terminated. */
> >
> > So if the comment is correct, you'd use copy_from_user() instead.
> Well, the contents of |alias| have been previously copied from the
> userspace, but this is a pointer to a kernel buffer, as the function
> prototype tells.
> Do you think a confusion is possible here?

Yes, I think so... If pointer comes from userspace, it normally points to
userspace data. If you have the data already copied, just say:
+     /* alias may not be zero-terminated. */

Or say nothing, because at the first glance, using the strlcpy()
instead of simple memcpy() looks weird in this case - you know the
length of the string from the beginning, and should not recalculate it
again.

Yury

> >> +     memcpy(dev->ifalias, alias, len);
> >> +     dev->ifalias[len] = 0;
> >>       return len;
> >>  }
> >>
> >> --
> >> 2.13.0.219.gdb65acc882-goog
> 
> 
> 
> -- 
> Alexander Potapenko
> Software Engineer
> 
> Google Germany GmbH
> Erika-Mann-Straße, 33
> 80636 München
> 
> Geschäftsführer: Matthew Scott Sucherman, Paul Terence Manicle
> Registergericht und -nummer: Hamburg, HRB 86891
> Sitz der Gesellschaft: Hamburg