/dev/mem: Add bounce buffer for copy-out

As done for /proc/kcore in

commit df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext data")

this adds a bounce buffer when reading memory via /dev/mem. This
is needed to allow kernel text memory to be read out when built with
CONFIG_HARDENED_USERCOPY (which refuses to read out kernel text) and
without CONFIG_STRICT_DEVMEM (which would have refused to read any RAM
contents at all).

Since this build configuration isn't common (most systems with
CONFIG_HARDENED_USERCOPY also have CONFIG_STRICT_DEVMEM), this also tries
to inform Kconfig about the recommended settings.

This patch is modified from Brad Spengler/PaX Team's changes to /dev/mem
code in the last public patch of grsecurity/PaX based on my understanding
of the code. Changes or omissions from the original code are mine and
don't reflect the original grsecurity/PaX code.

Reported-by: Michael Holzheu <[e-mail]>
Fixes: f5509cc18daa ("mm: Hardened usercopy")
Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/char/mem.c

···
 	phys_addr_t p = *ppos;
 	ssize_t read, sz;
 	void *ptr;
+	char *bounce;
+	int err;
 
 	if (p != *ppos)
 		return 0;
···
 	}
 #endif
 
+	bounce = kmalloc(PAGE_SIZE, GFP_KERNEL);
+	if (!bounce)
+		return -ENOMEM;
+
 	while (count > 0) {
 		unsigned long remaining;
 		int allowed;
 
 		sz = size_inside_page(p, count);
 
+		err = -EPERM;
 		allowed = page_is_allowed(p >> PAGE_SHIFT);
 		if (!allowed)
-			return -EPERM;
+			goto failed;
+
+		err = -EFAULT;
 		if (allowed == 2) {
 			/* Show zeros for restricted memory. */
 			remaining = clear_user(buf, sz);
···
 			 */
 			ptr = xlate_dev_mem_ptr(p);
 			if (!ptr)
-				return -EFAULT;
+				goto failed;
 
-			remaining = copy_to_user(buf, ptr, sz);
-
+			err = probe_kernel_read(bounce, ptr, sz);
 			unxlate_dev_mem_ptr(p, ptr);
+			if (err)
+				goto failed;
+
+			remaining = copy_to_user(buf, bounce, sz);
 		}
 
 		if (remaining)
-			return -EFAULT;
+			goto failed;
 
 		buf += sz;
 		p += sz;
 		count -= sz;
 		read += sz;
 	}
+	kfree(bounce);
 
 	*ppos += read;
 	return read;
+
+failed:
+	kfree(bounce);
+	return err;
 }
 
 static ssize_t write_mem(struct file *file, const char __user *buf,

security/Kconfig

···
 	bool "Harden memory copies between kernel and userspace"
 	depends on HAVE_HARDENED_USERCOPY_ALLOCATOR
 	select BUG
+	imply STRICT_DEVMEM
 	help
 	  This option checks for obviously wrong memory regions when
 	  copying memory to/from the kernel (via copy_to_user() and