From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
To: Joey Gouly <joey.gouly@arm.com>,
linux-hardening@vger.kernel.org, netdev@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>,
Eric Dumazet <edumazet@google.com>,
Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
linux-kernel@vger.kernel.org, Kees Cook <keescook@chromium.org>,
"Gustavo A. R. Silva" <gustavoars@kernel.org>
Subject: Re: [BUG] Boot crash on v6.7-rc2
Date: Fri, 24 Nov 2023 09:28:48 -0600 [thread overview]
Message-ID: <7086f60f-9f74-4118-a10c-d98b9c6cc8eb@embeddedor.com> (raw)
In-Reply-To: <20231124102458.GB1503258@e124191.cambridge.arm.com>
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
next prev parent reply other threads:[~2023-11-24 15:29 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-24 10:24 [BUG] Boot crash on v6.7-rc2 Joey Gouly
2023-11-24 15:28 ` Gustavo A. R. Silva [this message]
2023-11-24 15:51 ` Gustavo A. R. Silva
2023-11-25 17:54 ` Gustavo A. R. Silva
2023-11-25 18:31 ` Kees Cook
2023-11-25 18:41 ` Gustavo A. R. Silva
2023-11-26 1:14 ` Bagas Sanjaya
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