Re: [PATCH v16 02/16] arm64: untag user pointers in access_ok and __uaccess_mask_ptr

Kees Cook
2019-06-08T03:51:47Z

On Mon, Jun 03, 2019 at 06:55:04PM +0200, Andrey Konovalov wrote:
> This patch is a part of a series that extends arm64 kernel ABI to allow to
> pass tagged user pointers (with the top byte set to something else other
> than 0x00) as syscall arguments.
>
> copy_from_user (and a few other similar functions) are used to copy data
> from user memory into the kernel memory or vice versa. Since a user can
> provided a tagged pointer to one of the syscalls that use copy_from_user,
> we need to correctly handle such pointers.
>
> Do this by untagging user pointers in access_ok and in __uaccess_mask_ptr,
> before performing access validity checks.
>
> Note, that this patch only temporarily untags the pointers to perform the
> checks, but then passes them as is into the kernel internals.
>
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
> arch/arm64/include/asm/uaccess.h | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index e5d5f31c6d36..9164ecb5feca 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -94,7 +94,7 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
> return ret;
> }
>
> -#define access_ok(addr, size) __range_ok(addr, size)
> +#define access_ok(addr, size) __range_ok(untagged_addr(addr), size)
> #define user_addr_max get_fs
>
> #define _ASM_EXTABLE(from, to) \
> @@ -226,7 +226,8 @@ static inline void uaccess_enable_not_uao(void)
>
> /*
> * Sanitise a uaccess pointer such that it becomes NULL if above the
> - * current addr_limit.
> + * current addr_limit. In case the pointer is tagged (has the top byte set),
> + * untag the pointer before checking.
> */
> #define uaccess_mask_ptr(ptr) (__typeof__(ptr))__uaccess_mask_ptr(ptr)
> static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
> @@ -234,10 +235,11 @@ static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
> void __user *safe_ptr;
>
> asm volatile(
> - " bics xzr, %1, %2\n"
> + " bics xzr, %3, %2\n"
> " csel %0, %1, xzr, eq\n"
> : "=&r" (safe_ptr)
> - : "r" (ptr), "r" (current_thread_info()->addr_limit)
> + : "r" (ptr), "r" (current_thread_info()->addr_limit),
> + "r" (untagged_addr(ptr))
> : "cc");
>
> csdb();
> --
> 2.22.0.rc1.311.g5d7573a151-goog
>

--
Kees Cook