Re: [BUG] Boot crash on v6.7-rc2

[Gustavo A. R. Silva]

On 11/25/23 12:31, Kees Cook wrote:
> On November 25, 2023 9:54:28 AM PST, "Gustavo A. R. Silva" <gustavo@xxxxxxxxxxxxxx> wrote:
> > On 11/24/23 09:28, Gustavo A. R. Silva wrote:
> > > On 11/24/23 04:24, Joey Gouly wrote:
> > > > Hi all,
> > > >
> > > > I just hit a boot crash on v6.7-rc2 (arm64, FVP model):
> > >
> > > [..]
> > >
> > > > Checking `struct neighbour`:
> > > >
> > > >      struct neighbour {
> > > >          struct neighbour __rcu    *next;
> > > >          struct neigh_table    *tbl;
> > > >      .. fields ..
> > > >          u8            primary_key[0];
> > > >      } __randomize_layout;
> > > >
> > > > Due to the `__randomize_layout`, `primary_key` field is being placed before `tbl` (actually it's the same address since it's a 0 length array). That means the memcpy() corrupts the tbl pointer.
> > > >
> > > > I think I just got unlucky with my CONFIG_RANDSTRUCT seed (I can provide it if needed), it doesn't look as if it's a new issue.
> > >
> > > It seems the issue is caused by this change that was recently added to -rc2:
> > >
> > > commit 1ee60356c2dc ("gcc-plugins: randstruct: Only warn about true flexible arrays")
> > >
> > > Previously, one-element and zero-length arrays were treated as true flexible arrays
> > > (however, they are "fake" flex arrays), and __randomize_layout would leave them
> > > untouched at the end of the struct; the same for proper C99 flex-array members. But
> > > after the commit above, that's no longer the case: Only C99 flex-array members will
> > > behave correctly (remaining untouched at end of the struct), and the other two types
> > > of arrays will be randomized.
> >
> > mmh... it seems that commit 1ee60356c2dc only prevents one-element arrays from being
> > treated as flex arrays, while the code should still keep zero-length arrays untouched:
> >
> >         if (typesize == NULL_TREE && TYPE_DOMAIN(fieldtype) != NULL_TREE &&
> >             TYPE_MAX_VALUE(TYPE_DOMAIN(fieldtype)) == NULL_TREE)
> >                 return true;
> >
> > -       if (typesize != NULL_TREE &&
> > -           (TREE_CONSTANT(typesize) && (!tree_to_uhwi(typesize) ||
> > -            tree_to_uhwi(typesize) == tree_to_uhwi(elemsize))))
> > -               return true;
> > -
>
> This should be both the 0 and 1 checks. I think the original fix is correct: switch to a true flex array.

This code is new to me and I got a bit confused. Thanks for the clarification. :)

So, it'd be nice to apply this change:

https://lore.kernel.org/linux-hardening/b6c1c3ce-3ba0-4439-b0fb-2bb0c38586e0@xxxxxxxxxxxxxx/

> > Sorry about the confusion.
> >
> > > > I couldn't reproduce directly on v6.6 (the offsets for `tbl` and `primary_key` didn't overlap).
> > > > However I tried changing the zero-length-array to a flexible one:
> > > >
> > > >      +    DECLARE_FLEX_ARRAY(u8, primary_key);
> > > >      +    u8        primary_key[0];
>
> Was this line supposed to be "-"?
>
> > > > Then the field offsets ended up overlapping, and I also got the same crash on v6.6.
> > >
> > > The right approach is to transform the zero-length array into a C99 flex-array member,
> > > like this:
> > >
> > > diff --git a/include/net/neighbour.h b/include/net/neighbour.h
> > > index 07022bb0d44d..0d28172193fa 100644
> > > --- a/include/net/neighbour.h
> > > +++ b/include/net/neighbour.h
> > > @@ -162,7 +162,7 @@ struct neighbour {
> > >           struct rcu_head         rcu;
> > >           struct net_device       *dev;
> > >           netdevice_tracker       dev_tracker;
> > > -       u8                      primary_key[0];
> > > +       u8                      primary_key[];
> > >    } __randomize_layout;
> > >
> > >    struct neigh_ops {
> >
> > In any case, I think we still should convert [0] to [ ].
>
> I would expect the above to fix the problem. If it doesn't I'll need to take a closer look at the plugin...

I think this should fix the issue. Let me go create a proper patch for this.
I'll send it out, shortly.

--
Gustavo