On Mon, May 16, 2022 at 11:17:44AM -0700, Matthias Kaehlcke wrote:

On Fri, May 13, 2022 at 03:36:26PM -0700, Kees Cook wrote:

quoted

On May 4, 2022 12:54:18 PM PDT, Matthias Kaehlcke [off-list ref] wrote:

quoted

Extend LoadPin to allow loading of kernel files from trusted dm-verity [1]
devices.

This change adds the concept of trusted verity devices to LoadPin. LoadPin
maintains a list of root digests of verity devices it considers trusted.
Userspace can populate this list through an ioctl on the new LoadPin
securityfs entry 'dm-verity'. The ioctl receives a file descriptor of
a file with verity digests as parameter. Verity reads the digests from
this file after confirming that the file is located on the pinned root.
The list of trusted digests can only be set up once, which is typically
done at boot time.

When a kernel file is read LoadPin first checks (as usual) whether the file
is located on the pinned root, if so the file can be loaded. Otherwise, if
the verity extension is enabled, LoadPin determines whether the file is
located on a verity backed device and whether the root digest of that

I think this should be "... on an already trusted device ..."

It's not entirely clear which part you want me to substitute. 'an already
trusted device' makes me wonder whether you are thinking about reading the
list of digests, and not the general case of reading a kernel file, which
this paragraph intends to describe.

Sorry, I think I confused myself while reading what you'd written. I
think it's fine as is. I think I had skipped around in my mind thinking
about the trusted verity hashes file coming from the pinned root, but
you basically already said that. :) Nevermind!

quoted

quoted

+static int read_trusted_verity_root_digests(unsigned int fd)
+{
+	struct fd f;
+	void *data;

Probably easier if this is u8 *?

Maybe slightly, it would then require a cast when passing it to
kernel_read_file()

Oh, good point. That is a kinda weird API.

quoted

quoted

+	int rc;
+	char *p, *d;
+
+	/* The list of trusted root digests can only be set up once */
+	if (!list_empty(&trusted_verity_root_digests))
+		return -EPERM;
+
+	f = fdget(fd);
+	if (!f.file)
+		return -EINVAL;
+
+	data = kzalloc(SZ_4K, GFP_KERNEL);
+	if (!data) {
+		rc = -ENOMEM;
+		goto err;
+	}
+
+	rc = kernel_read_file(f.file, 0, &data, SZ_4K - 1, NULL, READING_POLICY);
+	if (rc < 0)
+		goto err;

So maybe, here, you could do:

	p = data;
	p[rc] '\0';
	p = strim(p);

etc... (the void * -> char * cast in the assignment should be accepted
without warning?)

quoted

quoted

+
+	((char *)data)[rc] = '\0';
+
+	p = strim(data);
+	while ((d = strsep(&p, ",")) != NULL) {

Maybe be flexible and add newline as a separator too?

Sure, I can add that. I'd also be fine with just allowing a newline as
separator, which seems a reasonable format for a sysfs file.

Yeah, that was my thinking too. And easier to parse for command line
tools, etc. Not a requirement at all, but might make testing easier,
etc.

-- 
Kees Cook