Subject: Re: [PATCH 02/11] mm: call import_iovec() instead of rw_copy_check_uvector() in process_vm_rw()

From: Al Viro <[email-masked]>
Date: Mon, 21 Sep 2020 16:02:11 +0100

On Mon, Sep 21, 2020 at 04:34:25PM +0200, Christoph Hellwig wrote:
> From: David Laight <[email-masked]>
> 
> This is the only direct call of rw_copy_check_uvector().  Removing it
> will allow rw_copy_check_uvector() to be inlined into import_iovec(),
> while only paying a minor price by setting up an otherwise unused
> iov_iter in the process_vm_readv/process_vm_writev syscalls that aren't
> in a super hot path.

> @@ -443,7 +443,7 @@ void iov_iter_init(struct iov_iter *i, unsigned int direction,
>  			const struct iovec *iov, unsigned long nr_segs,
>  			size_t count)
>  {
> -	WARN_ON(direction & ~(READ | WRITE));
> +	WARN_ON(direction & ~(READ | WRITE | CHECK_IOVEC_ONLY));
>  	direction &= READ | WRITE;

Ugh...

> -	rc = rw_copy_check_uvector(CHECK_IOVEC_ONLY, rvec, riovcnt, UIO_FASTIOV,
> -				   iovstack_r, &iov_r);
> +	rc = import_iovec(CHECK_IOVEC_ONLY, rvec, riovcnt, UIO_FASTIOV, &iov_r,
> +			  &iter_r);
>  	if (rc <= 0)
>  		goto free_iovecs;
>  
> -	rc = process_vm_rw_core(pid, &iter, iov_r, riovcnt, flags, vm_write);
> +	rc = process_vm_rw_core(pid, &iter_l, iter_r.iov, iter_r.nr_segs,
> +				flags, vm_write);

... and ugh^2, since now you are not only setting a meaningless iov_iter,
you are creating a new place that pokes directly into struct iov_iter
guts.

Sure, moving rw_copy_check_uvector() over to lib/iov_iter.c makes sense.
But I would rather split the access_ok()-related checks out of that thing
and bury CHECK_IOVEC_ONLY.

Step 1: move the damn thing to lib/iov_iter.c (same as you do, but without
making it static)

Step 2: split it in two:

ssize_t rw_copy_check_uvector(const struct iovec __user * uvector,
                              unsigned long nr_segs, unsigned long fast_segs,
                              struct iovec *fast_pointer,
                              struct iovec **ret_pointer)
{
	unsigned long seg;
	ssize_t ret;
	struct iovec *iov = fast_pointer;

	*ret_pointer = fast_pointer;
	/*
	 * SuS says "The readv() function *may* fail if the iovcnt argument
	 * was less than or equal to 0, or greater than {IOV_MAX}.  Linux has
	 * traditionally returned zero for zero segments, so...
	 */
	if (nr_segs == 0)
		return 0;

	/*
	 * First get the "struct iovec" from user memory and
	 * verify all the pointers
	 */
	if (nr_segs > UIO_MAXIOV)
		return -EINVAL;

	if (nr_segs > fast_segs) {
		iov = kmalloc_array(nr_segs, sizeof(struct iovec), GFP_KERNEL);
		if (!iov)
			return -ENOMEM;
		*ret_pointer = iov;
	}
	if (copy_from_user(iov, uvector, nr_segs*sizeof(*uvector)))
		return -EFAULT;

	/*
	 * According to the Single Unix Specification we should return EINVAL
	 * if an element length is < 0 when cast to ssize_t or if the
	 * total length would overflow the ssize_t return value of the
	 * system call.
	 *
	 * Linux caps all read/write calls to MAX_RW_COUNT, and avoids the
	 * overflow case.
	 */
	ret = 0;
	for (seg = 0; seg < nr_segs; seg++) {
		void __user *buf = iov[seg].iov_base;
		ssize_t len = (ssize_t)iov[seg].iov_len;

		/* see if we we're about to use an invalid len or if
		 * it's about to overflow ssize_t */
		if (len < 0)
			return -EINVAL;
		if (len > MAX_RW_COUNT - ret) {
			len = MAX_RW_COUNT - ret;
			iov[seg].iov_len = len;
		}
		ret += len;
	}
	return ret;
}

/*
 *  This is merely an early sanity check; we do _not_ rely upon
 *  it when we get to the actual memory accesses.
 */
static bool check_iovecs(const struct iovec *iov, int nr_segs)
{
        for (seg = 0; seg < nr_segs; seg++) {
                void __user *buf = iov[seg].iov_base;
                ssize_t len = (ssize_t)iov[seg].iov_len;

                if (unlikely(!access_ok(buf, len)))
                        return false;
        }
	return true;
}

ssize_t import_iovec(int type, const struct iovec __user * uvector,
                 unsigned nr_segs, unsigned fast_segs,
                 struct iovec **iov, struct iov_iter *i)
{
	struct iovec *p;
	ssize_t n;

	n = rw_copy_check_uvector(uvector, nr_segs, fast_segs, *iov, &p);
	if (n > 0 && !check_iovecs(p, nr_segs))
		n = -EFAULT;
	if (n < 0) {
		if (p != *iov)
			kfree(p);
		*iov = NULL;
		return n;
	}
	iov_iter_init(i, type, p, nr_segs, n);
	*iov = p == *iov ? NULL : p;
	return n;
}

kill CHECK_IOVEC_ONLY and use rw_copy_check_uvector() without the type
argument in mm/process_vm_access.c

Saner that way, IMO...