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SubjectRe: [PATCH v17 01/15] arm64: untag user pointers in access_ok and __uaccess_mask_ptr
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On 12/06/2019 12:43, 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: Kees Cook <keescook@chromium.org>
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>

> ---
> arch/arm64/include/asm/uaccess.h | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index e5d5f31c6d36..df729afca0ba 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -73,6 +73,8 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
> {
> unsigned long ret, limit = current_thread_info()->addr_limit;
>
> + addr = untagged_addr(addr);
> +
> __chk_user_ptr(addr);
> asm volatile(
> // A + B <= C + 1 for all A,B,C, in four easy steps:
> @@ -226,7 +228,8 @@ 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 +237,11 @@ 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();
>

--
Regards,
Vincenzo

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