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Date: Fri, 24 Nov 2023 09:28:48 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Joey Gouly <joey.gouly@....com>, linux-hardening@...r.kernel.org,
 netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, linux-kernel@...r.kernel.org,
 Kees Cook <keescook@...omium.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>
Subject: Re: [BUG] Boot crash on v6.7-rc2



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.

> 
> 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];
> 
> 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 {

--
Gustavo

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