Re: [RFC 2/4] lib/strncpy_from_user: Remove redundant user space pointer range check

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Jan 14, 2020 at 01:22:07PM -0800, Linus Torvalds wrote:

> The fact is, copying a string from user space is *very* different from
> copying a fixed number of bytes, and that whole dance with
> 
>         max_addr = user_addr_max();
> 
> is absolutely required and necessary.
> 
> You completely broke string copying.

BTW, a quick grep through the callers has found something odd -
static ssize_t kmemleak_write(struct file *file, const char __user *user_buf,
                              size_t size, loff_t *ppos)
{
        char buf[64];
        int buf_size;
        int ret;

        buf_size = min(size, (sizeof(buf) - 1));
        if (strncpy_from_user(buf, user_buf, buf_size) < 0)
                return -EFAULT;
        buf[buf_size] = 0;

What the hell?  If somebody is calling write(fd, buf, n) they'd
better be ready to see any byte from buf[0] up to buf[n - 1]
fetched, and if something is unmapped - deal with -EFAULT.
Is something really doing that and if so, why does kmemleak
try to accomodate that idiocy?

The same goes for several more ->write() instances - mtrr_write(),
armada_debugfs_crtc_reg_write() and cio_ignore_write(); IMO that's
seriously misguided (and cio one ought use vmemdup_user() instead
of what it's doing)...