Code: static ssize_t write_mem(struct file *file, const char __user *buf,
			 size_t count, loff_t *ppos)
{
	phys_addr_t p = *ppos;
	ssize_t written, sz;
	unsigned long copied;
	void *ptr;

	if (p != *ppos)
		return -EFBIG;

	if (!valid_phys_addr_range(p, count))
		return -EFAULT;

	written = 0;

#ifdef __ARCH_HAS_NO_PAGE_ZERO_MAPPED
	/* we don't have page 0 mapped on sparc and m68k.. */
	if (p < PAGE_SIZE) {
		sz = size_inside_page(p, count);
		/* Hmm. Do something? */
		buf += sz;
		p += sz;
		count -= sz;
		written += sz;
	}
 #endif
 
 	while (count > 0) {
 		sz = size_inside_page(p, count);
 
		if (!range_is_allowed(p >> PAGE_SHIFT, sz))
 			return -EPERM;
 
		/*
		 * On ia64 if a page has been mapped somewhere as uncached, then
		 * it must also be accessed uncached by the kernel or data
		 * corruption may occur.
		 */
		ptr = xlate_dev_mem_ptr(p);
		if (!ptr) {
			if (written)
				break;
			return -EFAULT;
		}
 
		copied = copy_from_user(ptr, buf, sz);
		unxlate_dev_mem_ptr(p, ptr);
		if (copied) {
			written += sz - copied;
			if (written)
				break;
			return -EFAULT;
 		}
 
 		buf += sz;
		p += sz;
		count -= sz;
		written += sz;
	}

	*ppos += written;
	return written;
}

Vulnerability Type: Bypass 
CWE ID: CWE-732
Summary: The mm subsystem in the Linux kernel through 4.10.10 does not properly enforce the CONFIG_STRICT_DEVMEM protection mechanism, which allows local users to read or write to kernel memory locations in the first megabyte (and bypass slab-allocation access restrictions) via an application that opens the /dev/mem file, related to arch/x86/mm/init.c and drivers/char/mem.c.
Commit Message: mm: Tighten x86 /dev/mem with zeroing reads

Under CONFIG_STRICT_DEVMEM, reading System RAM through /dev/mem is
disallowed. However, on x86, the first 1MB was always allowed for BIOS
and similar things, regardless of it actually being System RAM. It was
possible for heap to end up getting allocated in low 1MB RAM, and then
read by things like x86info or dd, which would trip hardened usercopy:

usercopy: kernel memory exposure attempt detected from ffff880000090000 (dma-kmalloc-256) (4096 bytes)

This changes the x86 exception for the low 1MB by reading back zeros for
System RAM areas instead of blindly allowing them. More work is needed to
extend this to mmap, but currently mmap doesn't go through usercopy, so
hardened usercopy won't Oops the kernel.

Reported-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Tested-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Signed-off-by: Kees Cook <keescook@chromium.org>