[PATCH v2 1/4] powerpc/64s: implement probe_kernel_read/write without touching AMR

[Christophe Leroy]

Hi Nick

Le 03/04/2020 à 11:35, Nicholas Piggin a écrit :
> There is no need to allow user accesses when probing kernel addresses.

You should have a look at 
https://github.com/torvalds/linux/commit/fa94111d94354de76c47fea6e1187d1ee91e23a7

At seems to implement a generic way of achieving what you are trying to 
do here.

Christophe

> 
> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
> ---
> v2:
> - Enable for all powerpc (suggested by Christophe)
> - Fold helper function together (Christophe)
> - Rename uaccess.c to maccess.c to match kernel/maccess.c.
> 
>   arch/powerpc/include/asm/uaccess.h | 25 +++++++++++++++-------
>   arch/powerpc/lib/Makefile          |  2 +-
>   arch/powerpc/lib/maccess.c         | 34 ++++++++++++++++++++++++++++++
>   3 files changed, 52 insertions(+), 9 deletions(-)
>   create mode 100644 arch/powerpc/lib/maccess.c
> 
> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> index 2f500debae21..670910df3cc7 100644
> --- a/arch/powerpc/include/asm/uaccess.h
> +++ b/arch/powerpc/include/asm/uaccess.h
> @@ -341,8 +341,8 @@ raw_copy_in_user(void __user *to, const void __user *from, unsigned long n)
>   }
>   #endif /* __powerpc64__ */
>   
> -static inline unsigned long raw_copy_from_user(void *to,
> -		const void __user *from, unsigned long n)
> +static inline unsigned long
> +raw_copy_from_user_allowed(void *to, const void __user *from, unsigned long n)
>   {
>   	unsigned long ret;
>   	if (__builtin_constant_p(n) && (n <= 8)) {
> @@ -351,19 +351,19 @@ static inline unsigned long raw_copy_from_user(void *to,
>   		switch (n) {
>   		case 1:
>   			barrier_nospec();
> -			__get_user_size(*(u8 *)to, from, 1, ret);
> +			__get_user_size_allowed(*(u8 *)to, from, 1, ret);
>   			break;
>   		case 2:
>   			barrier_nospec();
> -			__get_user_size(*(u16 *)to, from, 2, ret);
> +			__get_user_size_allowed(*(u16 *)to, from, 2, ret);
>   			break;
>   		case 4:
>   			barrier_nospec();
> -			__get_user_size(*(u32 *)to, from, 4, ret);
> +			__get_user_size_allowed(*(u32 *)to, from, 4, ret);
>   			break;
>   		case 8:
>   			barrier_nospec();
> -			__get_user_size(*(u64 *)to, from, 8, ret);
> +			__get_user_size_allowed(*(u64 *)to, from, 8, ret);
>   			break;
>   		}
>   		if (ret == 0)
> @@ -371,9 +371,18 @@ static inline unsigned long raw_copy_from_user(void *to,
>   	}
>   
>   	barrier_nospec();
> -	allow_read_from_user(from, n);
>   	ret = __copy_tofrom_user((__force void __user *)to, from, n);
> -	prevent_read_from_user(from, n);
> +	return ret;
> +}
> +
> +static inline unsigned long
> +raw_copy_from_user(void *to, const void __user *from, unsigned long n)
> +{
> +	unsigned long ret;
> +
> +	allow_read_from_user(to, n);
> +	ret = raw_copy_from_user_allowed(to, from, n);
> +	prevent_read_from_user(to, n);
>   	return ret;
>   }
>   
> diff --git a/arch/powerpc/lib/Makefile b/arch/powerpc/lib/Makefile
> index b8de3be10eb4..77af10ad0b0d 100644
> --- a/arch/powerpc/lib/Makefile
> +++ b/arch/powerpc/lib/Makefile
> @@ -16,7 +16,7 @@ CFLAGS_code-patching.o += -DDISABLE_BRANCH_PROFILING
>   CFLAGS_feature-fixups.o += -DDISABLE_BRANCH_PROFILING
>   endif
>   
> -obj-y += alloc.o code-patching.o feature-fixups.o pmem.o
> +obj-y += alloc.o code-patching.o feature-fixups.o pmem.o maccess.o
>   
>   ifndef CONFIG_KASAN
>   obj-y	+=	string.o memcmp_$(BITS).o
> diff --git a/arch/powerpc/lib/maccess.c b/arch/powerpc/lib/maccess.c
> new file mode 100644
> index 000000000000..ce5465db1e2d
> --- /dev/null
> +++ b/arch/powerpc/lib/maccess.c
> @@ -0,0 +1,34 @@
> +#include <linux/mm.h>
> +#include <linux/uaccess.h>
> +
> +/*
> + * Override the generic weak linkage functions to avoid changing KUP state via
> + * the generic user access functions, as this is accessing kernel addresses.
> + */
> +long probe_kernel_read(void *dst, const void *src, size_t size)
> +{
> +	long ret;
> +	mm_segment_t old_fs = get_fs();
> +
> +	set_fs(KERNEL_DS);
> +	pagefault_disable();
> +	ret = raw_copy_from_user_allowed(dst, (__force const void __user *)src, size);
> +	pagefault_enable();
> +	set_fs(old_fs);
> +
> +	return ret ? -EFAULT : 0;
> +}
> +
> +long probe_kernel_write(void *dst, const void *src, size_t size)
> +{
> +	long ret;
> +	mm_segment_t old_fs = get_fs();
> +
> +	set_fs(KERNEL_DS);
> +	pagefault_disable();
> +	ret = raw_copy_to_user_allowed((__force void __user *)dst, src, size);
> +	pagefault_enable();
> +	set_fs(old_fs);
> +
> +	return ret ? -EFAULT : 0;
> +}
>