On Thu, Apr 26, 2018 at 5:47 PM, Catalin Marinas
<catalin.marinas@xxxxxxx> wrote:
> On Wed, Apr 18, 2018 at 08:53:12PM +0200, Andrey Konovalov wrote:
>> @@ -238,12 +239,15 @@ static inline void uaccess_enable_not_uao(void)
>> /*
>> * Sanitise a uaccess pointer such that it becomes NULL if above the
>> * current addr_limit.
>> + * Also untag user pointers that have the top byte tag set.
>> */
>> #define uaccess_mask_ptr(ptr) (__typeof__(ptr))__uaccess_mask_ptr(ptr)
>> static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
>> {
>> void __user *safe_ptr;
>>
>> + ptr = untagged_addr(ptr);
>> +
>> asm volatile(
>> " bics xzr, %1, %2\n"
>> " csel %0, %1, xzr, eq\n"
>
> First of all, passing a tagged user pointer throughout the kernel is
> safe with uaccess routines but not suitable for find_vma() etc.
>
> With this change, we may have an inconsistent behaviour on the tag
> masking, depending on whether the entry code uses __uaccess_mask_ptr()
> or not. We could preserve the tag with something like:
>
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index e66b0fca99c2..ed15bfcbd797 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -244,10 +244,11 @@ 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();
Just to make sure I understood this assembly snippet correctly, this
change will result in checking untagged address against addr_limit,
and returning the original tagged address if the check passes. Sure,
sounds good, I'll do that.
