neighbour: Fix __randomize_layout crash in struct neighbour

[ Upstream commit 45b3fae4675dc1d4ee2d7aefa19d85ee4f891377 ]

Previously, one-element and zero-length arrays were treated as true
flexible arrays, even though they are actually "fake" flex arrays.
The __randomize_layout would leave them untouched at the end of the
struct, similarly to proper C99 flex-array members.

However, this approach changed with commit 1ee60356c2dc ("gcc-plugins:
randstruct: Only warn about true flexible arrays"). Now, only C99
flexible-array members will remain untouched at the end of the struct,
while one-element and zero-length arrays will be subject to randomization.

Fix a `__randomize_layout` crash in `struct neighbour` by transforming
zero-length array `primary_key` into a proper C99 flexible-array member.

Fixes: 1ee60356c2dc ("gcc-plugins: randstruct: Only warn about true flexible arrays")
Closes: https://lore.kernel.org/linux-hardening/20231124102458.GB1503258@e124191.cambridge.arm.com/
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/ZWJoRsJGnCPdJ3+2@work
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/include/net/neighbour.h b/include/net/neighbour.h
index 07022bb..0d28172 100644 (file)
--- 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 {