Re: [PATCH 1/3] s390/ctcm: Fix return type of ctc{mp,}m_tx()

Kees Cook
2022-11-02T19:09:51Z

On Wed, Nov 02, 2022 at 09:32:50AM -0700, Nathan Chancellor wrote:
> With clang's kernel control flow integrity (kCFI, CONFIG_CFI_CLANG),
> indirect call targets are validated against the expected function
> pointer prototype to make sure the call target is valid to help mitigate
> ROP attacks. If they are not identical, there is a failure at run time,
> which manifests as either a kernel panic or thread getting killed. A
> proposed warning in clang aims to catch these at compile time, which
> reveals:
>
> drivers/s390/net/ctcm_main.c:1064:21: error: incompatible function pointer types initializing 'netdev_tx_t (*)(struct sk_buff *, struct net_device *)' (aka 'enum netdev_tx (*)(struct sk_buff *, struct net_device *)') with an expression of type 'int (struct sk_buff *, struct net_device *)' [-Werror,-Wincompatible-function-pointer-types-strict]
> .ndo_start_xmit = ctcm_tx,
> ^~~~~~~
> drivers/s390/net/ctcm_main.c:1072:21: error: incompatible function pointer types initializing 'netdev_tx_t (*)(struct sk_buff *, struct net_device *)' (aka 'enum netdev_tx (*)(struct sk_buff *, struct net_device *)') with an expression of type 'int (struct sk_buff *, struct net_device *)' [-Werror,-Wincompatible-function-pointer-types-strict]
> .ndo_start_xmit = ctcmpc_tx,
> ^~~~~~~~~
>
> ->ndo_start_xmit() in 'struct net_device_ops' expects a return type of
> 'netdev_tx_t', not 'int'. Adjust the return type of ctc{mp,}m_tx() to
> match the prototype's to resolve the warning and potential CFI failure,
> should s390 select ARCH_SUPPORTS_CFI_CLANG in the future.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1750
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook