Re: [PATCH 2/2] s390/fpu: make use of __uninitialized macro

[Kees Cook <keescook@chromium.org>]

On Mon, Feb 05, 2024 at 04:48:44PM +0100, Heiko Carstens wrote:
> Code sections in s390 specific kernel code which use floating point or
> vector registers all come with a 520 byte stack variable to save already in
> use registers, if required.
> 
> With INIT_STACK_ALL_PATTERN or INIT_STACK_ALL_ZERO enabled this variable
> will always be initialized on function entry in addition to saving register
> contents, which contradicts the intend (performance improvement) of such
> code sections.
> 
> Therefore provide a DECLARE_KERNEL_FPU_ONSTACK() macro which provides
> struct kernel_fpu variables with an __uninitialized attribute, and convert
> all existing code to use this.
> 
> This way only this specific type of stack variable will not be initialized,
> regardless of config options.
> 
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
> ---
>  arch/s390/crypto/chacha-glue.c    | 2 +-
>  arch/s390/crypto/crc32-vx.c       | 2 +-
>  arch/s390/include/asm/fpu/types.h | 3 +++
>  arch/s390/kernel/sysinfo.c        | 2 +-
>  lib/raid6/s390vx.uc               | 4 ++--
>  5 files changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/s390/crypto/chacha-glue.c b/arch/s390/crypto/chacha-glue.c
> index ed9959e6f714..a823132fc563 100644
> --- a/arch/s390/crypto/chacha-glue.c
> +++ b/arch/s390/crypto/chacha-glue.c
> @@ -22,7 +22,7 @@ static void chacha20_crypt_s390(u32 *state, u8 *dst, const u8 *src,
>  				unsigned int nbytes, const u32 *key,
>  				u32 *counter)
>  {
> -	struct kernel_fpu vxstate;
> +	DECLARE_KERNEL_FPU_ONSTACK(vxstate);
>  
>  	kernel_fpu_begin(&vxstate, KERNEL_VXR);
>  	chacha20_vx(dst, src, nbytes, key, counter);
> diff --git a/arch/s390/crypto/crc32-vx.c b/arch/s390/crypto/crc32-vx.c
> index 017143e9cef7..6ae3e3ff5b0a 100644
> --- a/arch/s390/crypto/crc32-vx.c
> +++ b/arch/s390/crypto/crc32-vx.c
> @@ -49,8 +49,8 @@ u32 crc32c_le_vgfm_16(u32 crc, unsigned char const *buf, size_t size);
>  	static u32 __pure ___fname(u32 crc,				    \
>  				unsigned char const *data, size_t datalen)  \
>  	{								    \
> -		struct kernel_fpu vxstate;				    \
>  		unsigned long prealign, aligned, remaining;		    \
> +		DECLARE_KERNEL_FPU_ONSTACK(vxstate);			    \
>  									    \
>  		if (datalen < VX_MIN_LEN + VX_ALIGN_MASK)		    \
>  			return ___crc32_sw(crc, data, datalen);		    \
> diff --git a/arch/s390/include/asm/fpu/types.h b/arch/s390/include/asm/fpu/types.h
> index d889e9436865..b1afa13c07b7 100644
> --- a/arch/s390/include/asm/fpu/types.h
> +++ b/arch/s390/include/asm/fpu/types.h
> @@ -35,4 +35,7 @@ struct kernel_fpu {
>  	};
>  };
>  
> +#define DECLARE_KERNEL_FPU_ONSTACK(name)	\
> +	struct kernel_fpu name __uninitialized

Are there cases when struct kernel_fpu should be initialized? e.g.
should the attribute just be added to the struct definition instead of
marking each use?

-Kees

> +
>  #endif /* _ASM_S390_FPU_TYPES_H */
> diff --git a/arch/s390/kernel/sysinfo.c b/arch/s390/kernel/sysinfo.c
> index f6f8f498c9be..b439f17516eb 100644
> --- a/arch/s390/kernel/sysinfo.c
> +++ b/arch/s390/kernel/sysinfo.c
> @@ -426,9 +426,9 @@ subsys_initcall(create_proc_service_level);
>   */
>  void s390_adjust_jiffies(void)
>  {
> +	DECLARE_KERNEL_FPU_ONSTACK(fpu);
>  	struct sysinfo_1_2_2 *info;
>  	unsigned long capability;
> -	struct kernel_fpu fpu;
>  
>  	info = (void *) get_zeroed_page(GFP_KERNEL);
>  	if (!info)
> diff --git a/lib/raid6/s390vx.uc b/lib/raid6/s390vx.uc
> index cd0b9e95f499..7b0b355e1a26 100644
> --- a/lib/raid6/s390vx.uc
> +++ b/lib/raid6/s390vx.uc
> @@ -81,7 +81,7 @@ static inline void COPY_VEC(int x, int y)
>  
>  static void raid6_s390vx$#_gen_syndrome(int disks, size_t bytes, void **ptrs)
>  {
> -	struct kernel_fpu vxstate;
> +	DECLARE_KERNEL_FPU_ONSTACK(vxstate);
>  	u8 **dptr, *p, *q;
>  	int d, z, z0;
>  
> @@ -114,7 +114,7 @@ static void raid6_s390vx$#_gen_syndrome(int disks, size_t bytes, void **ptrs)
>  static void raid6_s390vx$#_xor_syndrome(int disks, int start, int stop,
>  					size_t bytes, void **ptrs)
>  {
> -	struct kernel_fpu vxstate;
> +	DECLARE_KERNEL_FPU_ONSTACK(vxstate);
>  	u8 **dptr, *p, *q;
>  	int d, z, z0;
>  
> -- 
> 2.40.1
> 

-- 
Kees Cook