Re: [PATCH v2 6/6] powerpc: Implement user_access_begin and friends

Michael Ellerman
2020-01-23T12:05:57Z

Christophe Leroy <christophe.leroy@c-s.fr> writes:
> Today, when a function like strncpy_from_user() is called,
> the userspace access protection is de-activated and re-activated
> for every word read.
>
> By implementing user_access_begin and friends, the protection
> is de-activated at the beginning of the copy and re-activated at the
> end.
>
> Implement user_access_begin(), user_access_end() and
> unsafe_get_user(), unsafe_put_user() and unsafe_copy_to_user()
>
> For the time being, we keep user_access_save() and
> user_access_restore() as nops.

That means we will run with user access enabled in a few more places, but
it's only used sparingly AFAICS:

kernel/trace/trace_branch.c: unsigned long flags = user_access_save();
lib/ubsan.c: unsigned long flags = user_access_save();
lib/ubsan.c: unsigned long ua_flags = user_access_save();
mm/kasan/common.c: unsigned long flags = user_access_save();

And we don't have objtool checking that user access enablement isn't
leaking in the first place, so I guess it's OK for us not to implement
these to begin with?

cheers

> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> index cafad1960e76..ea67bbd56bd4 100644
> --- a/arch/powerpc/include/asm/uaccess.h
> +++ b/arch/powerpc/include/asm/uaccess.h
> @@ -91,9 +91,14 @@ static inline int __access_ok(unsigned long addr, unsigned long size,
> __put_user_check((__typeof__(*(ptr)))(x), (ptr), sizeof(*(ptr)))
>
> #define __get_user(x, ptr) \
> - __get_user_nocheck((x), (ptr), sizeof(*(ptr)))
> + __get_user_nocheck((x), (ptr), sizeof(*(ptr)), true)
> #define __put_user(x, ptr) \
> - __put_user_nocheck((__typeof__(*(ptr)))(x), (ptr), sizeof(*(ptr)))
> + __put_user_nocheck((__typeof__(*(ptr)))(x), (ptr), sizeof(*(ptr)), true)
> +
> +#define __get_user_allowed(x, ptr) \
> + __get_user_nocheck((x), (ptr), sizeof(*(ptr)), false)
> +#define __put_user_allowed(x, ptr) \
> + __put_user_nocheck((__typeof__(*(ptr)))(x), (ptr), sizeof(*(ptr)), false)
>
> #define __get_user_inatomic(x, ptr) \
> __get_user_nosleep((x), (ptr), sizeof(*(ptr)))
> @@ -138,10 +143,9 @@ extern long __put_user_bad(void);
> : "r" (x), "b" (addr), "i" (-EFAULT), "0" (err))
> #endif /* __powerpc64__ */
>
> -#define __put_user_size(x, ptr, size, retval) \
> +#define __put_user_size_allowed(x, ptr, size, retval) \
> do { \
> retval = 0; \
> - allow_write_to_user(ptr, size); \
> switch (size) { \
> case 1: __put_user_asm(x, ptr, retval, "stb"); break; \
> case 2: __put_user_asm(x, ptr, retval, "sth"); break; \
> @@ -149,17 +153,26 @@ do { \
> case 8: __put_user_asm2(x, ptr, retval); break; \
> default: __put_user_bad(); \
> } \
> +} while (0)
> +
> +#define __put_user_size(x, ptr, size, retval) \
> +do { \
> + allow_write_to_user(ptr, size); \
> + __put_user_size_allowed(x, ptr, size, retval); \
> prevent_write_to_user(ptr, size); \
> } while (0)
>
> -#define __put_user_nocheck(x, ptr, size) \
> +#define __put_user_nocheck(x, ptr, size, allow) \
> ({ \
> long __pu_err; \
> __typeof__(*(ptr)) __user *__pu_addr = (ptr); \
> if (!is_kernel_addr((unsigned long)__pu_addr)) \
> might_fault(); \
> __chk_user_ptr(ptr); \
> - __put_user_size((x), __pu_addr, (size), __pu_err); \
> + if (allow) \
> + __put_user_size((x), __pu_addr, (size), __pu_err); \
> + else \
> + __put_user_size_allowed((x), __pu_addr, (size), __pu_err); \
> __pu_err; \
> })
>
> @@ -236,13 +249,12 @@ extern long __get_user_bad(void);
> : "b" (addr), "i" (-EFAULT), "0" (err))
> #endif /* __powerpc64__ */
>
> -#define __get_user_size(x, ptr, size, retval) \
> +#define __get_user_size_allowed(x, ptr, size, retval) \
> do { \
> retval = 0; \
> __chk_user_ptr(ptr); \
> if (size > sizeof(x)) \
> (x) = __get_user_bad(); \
> - allow_read_from_user(ptr, size); \
> switch (size) { \
> case 1: __get_user_asm(x, ptr, retval, "lbz"); break; \
> case 2: __get_user_asm(x, ptr, retval, "lhz"); break; \
> @@ -250,6 +262,12 @@ do { \
> case 8: __get_user_asm2(x, ptr, retval); break; \
> default: (x) = __get_user_bad(); \
> } \
> +} while (0)
> +
> +#define __get_user_size(x, ptr, size, retval) \
> +do { \
> + allow_read_from_user(ptr, size); \
> + __get_user_size_allowed(x, ptr, size, retval); \
> prevent_read_from_user(ptr, size); \
> } while (0)
>
> @@ -260,7 +278,7 @@ do { \
> #define __long_type(x) \
> __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
>
> -#define __get_user_nocheck(x, ptr, size) \
> +#define __get_user_nocheck(x, ptr, size, allow) \
> ({ \
> long __gu_err; \
> __long_type(*(ptr)) __gu_val; \
> @@ -269,7 +287,10 @@ do { \
> if (!is_kernel_addr((unsigned long)__gu_addr)) \
> might_fault(); \
> barrier_nospec(); \
> - __get_user_size(__gu_val, __gu_addr, (size), __gu_err); \
> + if (allow) \
> + __get_user_size(__gu_val, __gu_addr, (size), __gu_err); \
> + else \
> + __get_user_size_allowed(__gu_val, __gu_addr, (size), __gu_err); \
> (x) = (__typeof__(*(ptr)))__gu_val; \
> __gu_err; \
> })
> @@ -387,6 +408,34 @@ static inline unsigned long raw_copy_to_user(void __user *to,
> return ret;
> }
>
> +static inline unsigned long
> +raw_copy_to_user_allowed(void __user *to, const void *from, unsigned long n)
> +{
> + unsigned long ret;
> + if (__builtin_constant_p(n) && (n) <= 8) {
> + ret = 1;
> +
> + switch (n) {
> + case 1:
> + __put_user_size_allowed(*(u8 *)from, (u8 __user *)to, 1, ret);
> + break;
> + case 2:
> + __put_user_size_allowed(*(u16 *)from, (u16 __user *)to, 2, ret);
> + break;
> + case 4:
> + __put_user_size_allowed(*(u32 *)from, (u32 __user *)to, 4, ret);
> + break;
> + case 8:
> + __put_user_size_allowed(*(u64 *)from, (u64 __user *)to, 8, ret);
> + break;
> + }
> + if (ret == 0)
> + return 0;
> + }
> +
> + return __copy_tofrom_user(to, (__force const void __user *)from, n);
> +}
> +
> static __always_inline unsigned long __must_check
> copy_to_user_mcsafe(void __user *to, const void *from, unsigned long n)
> {
> @@ -428,4 +477,27 @@ extern long __copy_from_user_flushcache(void *dst, const void __user *src,
> extern void memcpy_page_flushcache(char *to, struct page *page, size_t offset,
> size_t len);
>
> +static __must_check inline bool user_access_begin(const void __user *ptr, size_t len)
> +{
> + if (unlikely(!access_ok(ptr, len)))
> + return false;
> + allow_read_write_user((void __user *)ptr, ptr, len);
> + return true;
> +}
> +#define user_access_begin user_access_begin
> +
> +static inline void user_access_end(void)
> +{
> + prevent_user_access(NULL, NULL, ~0UL, KUAP_SELF);
> +}
> +#define user_access_end user_access_end
> +
> +static inline unsigned long user_access_save(void) { return 0UL; }
> +static inline void user_access_restore(unsigned long flags) { }
> +
> +#define unsafe_op_wrap(op, err) do { if (unlikely(op)) goto err; } while (0)
> +#define unsafe_get_user(x,p,e) unsafe_op_wrap(__get_user_allowed(x,p),e)
> +#define unsafe_put_user(x,p,e) unsafe_op_wrap(__put_user_allowed(x,p),e)
> +#define unsafe_copy_to_user(d,s,l,e) unsafe_op_wrap(raw_copy_to_user_allowed(d,s,l),e)
> +
> #endif /* _ARCH_POWERPC_UACCESS_H */
> --
> 2.25.0