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Date: Fri, 24 Nov 2023 09:51:36 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Kees Cook <keescook@...omium.org>, 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,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>
Subject: Re: [BUG] Boot crash on v6.7-rc2



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.

Kees,

I think we should complement the changes in commit 1ee60356c2dc with the following
update:

diff --git a/scripts/gcc-plugins/randomize_layout_plugin.c b/scripts/gcc-plugins/randomize_layout_plugin.c
index 910bd21d08f4..746ff2d272f2 100644
--- a/scripts/gcc-plugins/randomize_layout_plugin.c
+++ b/scripts/gcc-plugins/randomize_layout_plugin.c
@@ -339,8 +339,7 @@ static int relayout_struct(tree type)

         /*
          * enforce that we don't randomize the layout of the last
-        * element of a struct if it's a 0 or 1-length array
-        * or a proper flexible array
+        * element of a struct if it's a proper flexible array
          */
         if (is_flexible_array(newtree[num_fields - 1])) {
                 has_flexarray = true;

--
Gustavo

> 
>>
>> 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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