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Date:   Wed, 2 Nov 2022 12:07:02 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Sebastian Reichel <sre@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>,
        Sami Tolvanen <samitolvanen@google.com>, llvm@lists.linux.dev,
        linux-kernel@vger.kernel.org, patches@lists.linux.dev
Subject: Re: [PATCH] HSI: ssi_protocol: Fix return type of ssip_pn_xmit()
Message-ID: <202211021207.8F7604C860@keescook>
References: <20221102160233.4042756-1-nathan@kernel.org>
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On Wed, Nov 02, 2022 at 09:02:33AM -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/hsi/clients/ssi_protocol.c:1053:20: 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 = ssip_pn_xmit,
>                             ^~~~~~~~~~~~
>   1 error generated.
> 
> ->ndo_start_xmit() in 'struct net_device_ops' expects a return type of
> 'netdev_tx_t', not 'int'. Adjust the return type of ssip_pn_xmit() to
> match the prototype's to resolve the warning and CFI failure.
> Additionally, use the enum 'NETDEV_TX_OK' instead of a raw '0' for the
> return value of ssip_pn_xmit().
> 
> 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