Re: [PATCH] qemu_fw_cfg: Make fw_cfg_rev_attr a proper kobj_attribute

[Kees Cook]

On Thu, Feb 11, 2021 at 12:42:58PM -0700, Nathan Chancellor wrote:
> fw_cfg_showrev() is called by an indirect call in kobj_attr_show(),
> which violates clang's CFI checking because fw_cfg_showrev()'s second
> parameter is 'struct attribute', whereas the ->show() member of 'struct
> kobj_structure' expects the second parameter to be of type 'struct
> kobj_attribute'.
> 
> $ cat /sys/firmware/qemu_fw_cfg/rev
> 3
> 
> $ dmesg | grep "CFI failure"
> [   26.016832] CFI failure (target: fw_cfg_showrev+0x0/0x8):
> 
> Fix this by converting fw_cfg_rev_attr to 'struct kobj_attribute' where
> this would have been caught automatically by the incompatible pointer
> types compiler warning. Update fw_cfg_showrev() accordingly.
> 
> Fixes: 75f3e8e47f38 ("firmware: introduce sysfs driver for QEMU's fw_cfg device")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1299
> Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>

Ah, nice, yes.

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

Michael, are you able to take this? I can snag it if needed.

-Kees

> ---
>  drivers/firmware/qemu_fw_cfg.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
> index 0078260fbabe..172c751a4f6c 100644
> --- a/drivers/firmware/qemu_fw_cfg.c
> +++ b/drivers/firmware/qemu_fw_cfg.c
> @@ -299,15 +299,13 @@ static int fw_cfg_do_platform_probe(struct platform_device *pdev)
>  	return 0;
>  }
>  
> -static ssize_t fw_cfg_showrev(struct kobject *k, struct attribute *a, char *buf)
> +static ssize_t fw_cfg_showrev(struct kobject *k, struct kobj_attribute *a,
> +			      char *buf)
>  {
>  	return sprintf(buf, "%u\n", fw_cfg_rev);
>  }
>  
> -static const struct {
> -	struct attribute attr;
> -	ssize_t (*show)(struct kobject *k, struct attribute *a, char *buf);
> -} fw_cfg_rev_attr = {
> +static const struct kobj_attribute fw_cfg_rev_attr = {
>  	.attr = { .name = "rev", .mode = S_IRUSR },
>  	.show = fw_cfg_showrev,
>  };
> 
> base-commit: 92bf22614b21a2706f4993b278017e437f7785b3
> -- 
> 2.30.1
> 

-- 
Kees Cook