linux-kernel - Re: [RFC PATCH v1 01/10] s390/uaccess: Add storage key checked access to user memory

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Date:   Tue, 18 Jan 2022 16:37:39 +0100
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
Cc:     Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Nico Boehr <nrb@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 01/10] s390/uaccess: Add storage key checked
 access to user memory

Hi Janis,

Janis Schoetterl-Glausch <scgl@...ux.ibm.com> writes:

> KVM needs a mechanism to do accesses to guest memory that honor
> storage key protection.
> Since the copy_to/from_user implementation makes use of move
> instructions that support having an additional access key supplied,
> we can implement __copy_from/to_user_with_key by enhancing the
> existing implementation.
>
> Signed-off-by: Janis Schoetterl-Glausch <scgl@...ux.ibm.com>

This doesn't apply to my master branch.

> diff --git a/arch/s390/lib/uaccess.c b/arch/s390/lib/uaccess.c
> index d3a700385875..ce7a150dd93a 100644
> --- a/arch/s390/lib/uaccess.c
> +++ b/arch/s390/lib/uaccess.c
> @@ -59,11 +59,13 @@ static inline int copy_with_mvcos(void)
>  #endif
>  
>  static inline unsigned long copy_from_user_mvcos(void *x, const void __user *ptr,
> -						 unsigned long size)
> +						 unsigned long size, char key)
>  {
>  	unsigned long tmp1, tmp2;
>  	union oac spec = {
> +		.oac2.key = key,
>  		.oac2.as = PSW_BITS_AS_SECONDARY,
> +		.oac2.k = 1,
>  		.oac2.a = 1,
>  	};
>  
> @@ -94,19 +96,19 @@ static inline unsigned long copy_from_user_mvcos(void *x, const void __user *ptr
>  }
>  
>  static inline unsigned long copy_from_user_mvcp(void *x, const void __user *ptr,
> -						unsigned long size)
> +						unsigned long size, char key)

Any special reason for using 'char' as type for key here? Given the left shift
below i would prefer 'unsigned char' to avoid having to think about
whether this can overflow. The end result wouldn't look different,
so more or less a cosmetic issue.

>  {
>  	unsigned long tmp1, tmp2;
>  
>  	tmp1 = -256UL;
>  	asm volatile(
>  		"   sacf  0\n"
> -		"0: mvcp  0(%0,%2),0(%1),%3\n"
> +		"0: mvcp  0(%0,%2),0(%1),%[key]\n"
>  		"7: jz    5f\n"
>  		"1: algr  %0,%3\n"
>  		"   la    %1,256(%1)\n"
>  		"   la    %2,256(%2)\n"
> -		"2: mvcp  0(%0,%2),0(%1),%3\n"
> +		"2: mvcp  0(%0,%2),0(%1),%[key]\n"
>  		"8: jnz   1b\n"
>  		"   j     5f\n"
>  		"3: la    %4,255(%1)\n"	/* %4 = ptr + 255 */
> @@ -115,7 +117,7 @@ static inline unsigned long copy_from_user_mvcp(void *x, const void __user *ptr,
>  		"   slgr  %4,%1\n"
>  		"   clgr  %0,%4\n"	/* copy crosses next page boundary? */
>  		"   jnh   6f\n"
> -		"4: mvcp  0(%4,%2),0(%1),%3\n"
> +		"4: mvcp  0(%4,%2),0(%1),%[key]\n"
>  		"9: slgr  %0,%4\n"
>  		"   j     6f\n"
>  		"5: slgr  %0,%0\n"
> @@ -123,24 +125,36 @@ static inline unsigned long copy_from_user_mvcp(void *x, const void __user *ptr,
>  		EX_TABLE(0b,3b) EX_TABLE(2b,3b) EX_TABLE(4b,6b)
>  		EX_TABLE(7b,3b) EX_TABLE(8b,3b) EX_TABLE(9b,6b)
>  		: "+a" (size), "+a" (ptr), "+a" (x), "+a" (tmp1), "=a" (tmp2)
> -		: : "cc", "memory");
> +		: [key] "d" (key << 4)
> +		: "cc", "memory");
>  	return size;
>  }
>  

With that minor nitpick:

Reviewed-by: Sven Schnelle <svens@...ux.ibm.com>