linux-kernel - Re: [PATCH v1] lkdtm/bugs: fix build error in lkdtm_UNSET

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Date:   Tue, 17 Dec 2019 16:14:56 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Brendan Higgins <brendanhiggins@...gle.com>
Cc:     jdike@...toit.com, richard@....at, anton.ivanov@...bridgegreys.com,
        linux-um@...ts.infradead.org, linux-kernel@...r.kernel.org,
        davidgow@...gle.com, arnd@...db.de, gregkh@...uxfoundation.org
Subject: Re: [PATCH v1] lkdtm/bugs: fix build error in lkdtm_UNSET_SMEP

On Thu, Dec 12, 2019 at 04:35:22PM -0800, Brendan Higgins wrote:
> When building ARCH=um with CONFIG_UML_X86=y and CONFIG_64BIT=y we get
> the build errors:
> 
> drivers/misc/lkdtm/bugs.c: In function ‘lkdtm_UNSET_SMEP’:
> drivers/misc/lkdtm/bugs.c:288:8: error: implicit declaration of function ‘native_read_cr4’ [-Werror=implicit-function-declaration]
>   cr4 = native_read_cr4();
>         ^~~~~~~~~~~~~~~
> drivers/misc/lkdtm/bugs.c:290:13: error: ‘X86_CR4_SMEP’ undeclared (first use in this function); did you mean ‘X86_FEATURE_SMEP’?
>   if ((cr4 & X86_CR4_SMEP) != X86_CR4_SMEP) {
>              ^~~~~~~~~~~~
>              X86_FEATURE_SMEP
> drivers/misc/lkdtm/bugs.c:290:13: note: each undeclared identifier is reported only once for each function it appears in
> drivers/misc/lkdtm/bugs.c:297:2: error: implicit declaration of function ‘native_write_cr4’; did you mean ‘direct_write_cr4’? [-Werror=implicit-function-declaration]
>   native_write_cr4(cr4);
>   ^~~~~~~~~~~~~~~~
>   direct_write_cr4
> 
> So specify that this block of code should only build when
> CONFIG_X86_64=y *AND* CONFIG_UML is unset.
> 
> Signed-off-by: Brendan Higgins <brendanhiggins@...gle.com>

Thanks for catching this! Is a similar marking needed for the recently
added lkdtm_DOUBLE_FAULT() when using UML on 32-bit?

Acked-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
> 
> This patch is part of my larger effort to get allyesconfig closer to
> working for ARCH=um. For more information about that, checkout the cover
> letter for a related patchset here:
> 
> https://lore.kernel.org/lkml/20191211192742.95699-1-brendanhiggins@google.com/
> 
> ---
>  drivers/misc/lkdtm/bugs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
> index a4fdad04809a9..6c1aab177fced 100644
> --- a/drivers/misc/lkdtm/bugs.c
> +++ b/drivers/misc/lkdtm/bugs.c
> @@ -278,7 +278,7 @@ void lkdtm_STACK_GUARD_PAGE_TRAILING(void)
>  
>  void lkdtm_UNSET_SMEP(void)
>  {
> -#ifdef CONFIG_X86_64
> +#if IS_ENABLED(CONFIG_X86_64) && !IS_ENABLED(CONFIG_UML)
>  #define MOV_CR4_DEPTH	64
>  	void (*direct_write_cr4)(unsigned long val);
>  	unsigned char *insn;
> -- 
> 2.24.1.735.g03f4e72817-goog
> 

-- 
Kees Cook