Re: [PATCH v1 1/4] lib: introduce copy_struct_from_user() helper

On Wed, Sep 25, 2019 at 10:48:31AM -0700, Linus Torvalds wrote:

On Wed, Sep 25, 2019 at 10:21 AM Aleksa Sarai [off-list ref] wrote:

quoted

Just to make sure I understand, the following diff would this solve the
problem? If so, I'll apply it, and re-send in a few hours.

Actually, looking at it more, it's still buggy.

That final "size smaller than unsigned long" doesn't correctly handle
the case of (say) a single byte in the middle of a 8-byte word.

So you need to do something like this:

    int is_zeroed_user(const void __user *from, size_t size)
  {
        unsigned long val, mask, align;

        if (unlikely(!size))
                return true;

        if (!user_access_begin(from, size))
                return -EFAULT;

        align = (uintptr_t) from % sizeof(unsigned long);
        from -= align;
        size += align;

        mask = ~aligned_byte_mask(align);

        while (size >= sizeof(unsigned long)) {
                unsafe_get_user(val, (unsigned long __user *) from, err_fault);
                val &= mask;
                if (unlikely(val))
                        goto done;
                mask = ~0ul;
                from += sizeof(unsigned long);
                size -= sizeof(unsigned long);
        }

        if (size) {
                /* (@from + @size) is unaligned. */
                unsafe_get_user(val, (unsigned long __user *) from, err_fault);
                mask &= aligned_byte_mask(size);
                val &= mask;
        }

IMO it's better to lift reading the first word out of the loop, like this:

	align = (uintptr_t) from % sizeof(unsigned long);
	from -= align;

	unsafe_get_user(val, (unsigned long __user *) from, err_fault);
	if (align) {
		size += align;
		val &= ~aligned_byte_mask(align);
	}

	while (size > sizeof(unsigned long)) {
		if (unlikely(val))
			goto done;
		from += sizeof(unsigned long);
		size -= sizeof(unsigned long);
		unsafe_get_user(val, (unsigned long __user *) from, err_fault);
	}

	if (size != size(unsigned long))
		val &= aligned_byte_mask(size);
done:

Do you see any problems with that variant?