Re: [PATCH v2 1/2] um: net: Fix return type of uml_net_start_xmit()

[Kees Cook <keescook@chromium.org>]

On Wed, Dec 06, 2023 at 09:49:46AM -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
> warning in clang aims to catch these at compile time, which reveals:
> 
>   arch/um/drivers/net_kern.c:353:21: warning: 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 *)' [-Wincompatible-function-pointer-types-strict]
>     353 |         .ndo_start_xmit         = uml_net_start_xmit,
>         |                                   ^~~~~~~~~~~~~~~~~~
>   1 warning generated.
> 
> ->ndo_start_xmit() in 'struct net_device_ops' expects a return type of
> 'netdev_tx_t', not 'int'. Adjust the return type of uml_net_start_xmit()
> to match the prototype's to resolve the warning. While UML does not
> currently implement support for kCFI, it could in the future, which
> means this warning becomes a fatal CFI failure at run time.
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202310031340.v1vPh207-lkp@intel.com/
> Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Yes please. :)

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

-- 
Kees Cook