Re: [PATCH v3 1/3] dm: Add verity helpers for LoadPin

[Matthias Kaehlcke <mka@chromium.org>]

On Fri, May 13, 2022 at 03:15:53PM -0700, Kees Cook wrote:
> 
> 
> On May 4, 2022 12:54:17 PM PDT, Matthias Kaehlcke <mka@chromium.org> wrote:
> >LoadPin limits loading of kernel modules, firmware and certain
> >other files to a 'pinned' file system (typically a read-only
> >rootfs). To provide more flexibility LoadPin is being extended
> >to also allow loading these files from trusted dm-verity
> >devices. For that purpose LoadPin can be provided with a list
> >of verity root digests that it should consider as trusted.
> >
> >Add a bunch of helpers to allow LoadPin to check whether a DM
> >device is a trusted verity device. The new functions broadly
> >fall in two categories: those that need access to verity
> >internals (like the root digest), and the 'glue' between
> >LoadPin and verity. The new file dm-verity-loadpin.c contains
> >the glue functions.
> >
> >Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> > [...]
> >diff --git a/drivers/md/dm-verity-loadpin.c b/drivers/md/dm-verity-loadpin.c
> >new file mode 100644
> >index 000000000000..972ca93a2231
> >--- /dev/null
> >+++ b/drivers/md/dm-verity-loadpin.c
> >@@ -0,0 +1,80 @@
> >+// SPDX-License-Identifier: GPL-2.0-only
> >+
> >+#include <linux/list.h>
> >+#include <linux/kernel.h>
> >+#include <linux/dm-verity-loadpin.h>
> >+
> >+#include "dm.h"
> >+#include "dm-verity.h"
> >+
> >+static struct list_head *trusted_root_digests;
> 
> Does this need to exist in two places? (i.e. why can't dm and loadpin share
> this instead of needing dm_verity_loadpin_set_trusted_digests()?)

We could share it. Probably it should then be defined here, since this is
the first patch of the series, we could add an extern declaration to
dm-verity-loadpin.h.

> >+
> >+/*
> >+ * Sets the root digests of verity devices which LoadPin considers as trusted.
> >+ *
> >+ * This function must only be called once.
> >+ */
> >+void dm_verity_loadpin_set_trusted_root_digests(struct list_head *digests)
> >+{
> >+	if (!trusted_root_digests)
> >+		trusted_root_digests = digests;
> >+	else
> >+		pr_warn("verity root digests trusted by LoadPin are already set!!!\n");
> >+}
> >+
> >+static bool is_trusted_verity_target(struct dm_target *ti)
> >+{
> >+	u8 *root_digest;
> >+	unsigned int digest_size;
> >+	struct trusted_root_digest *trd;
> >+	bool trusted = false;
> >+
> >+	if (!dm_is_verity_target(ti))
> >+		return false;
> >+
> >+	if (dm_verity_get_root_digest(ti, &root_digest, &digest_size))
> >+		return false;
> >+
> >+	list_for_each_entry(trd, trusted_root_digests, node) {
> >+		if ((trd->len == digest_size) &&
> >+		    !memcmp(trd->data, root_digest, digest_size)) {
> >+			trusted = true;
> >+			break;
> >+		}
> >+	}
> >+
> >+	kfree(root_digest);
> >+
> >+	return trusted;
> >+}
> >+
> >+/*
> >+ * Determines whether a mapped device is a verity device that is trusted
> >+ * by LoadPin.
> >+ */
> >+bool dm_verity_loadpin_is_md_trusted(struct mapped_device *md)
> >+{
> >+	int srcu_idx;
> >+	struct dm_table *table;
> >+	unsigned int num_targets;
> >+	bool trusted = false;
> >+	int i;
> >+
> >+	if (!trusted_root_digests || list_empty(trusted_root_digests))
> >+		return false;
> >+
> >+	table = dm_get_live_table(md, &srcu_idx);
> >+	num_targets = dm_table_get_num_targets(table);
> >+	for (i = 0; i < num_targets; i++) {
> >+		struct dm_target *ti = dm_table_get_target(table, i);
> >+
> >+		if (is_trusted_verity_target(ti)) {
> >+			trusted = true;
> >+			break;
> >+		}
> >+	}
> 
> Pardon my lack of dm vocabulary, but what is "target" vs "table" here?
> I was only thinking of "whole device", so I must not understand what this is
> examining.

'targets' are different types of DM mappings like 'linear' or 'verity'. A
device mapper table contains has one or more targets that define the mapping
of the blocks of the mapped device.

Having spelled that out I realize that the above check is wrong. It would
consider a device like this trusted:

0 10000000 linear 8:1
10000000 10001000 verity <params>

In the above case only a small part of the DM device would be backed by verity.

I think we want a table with a single entry that is a verity target.

> > [...]
> >diff --git a/include/linux/dm-verity-loadpin.h b/include/linux/dm-verity-loadpin.h
> >new file mode 100644
> >index 000000000000..12a86911d05a
> >--- /dev/null
> >+++ b/include/linux/dm-verity-loadpin.h
> >@@ -0,0 +1,27 @@
> >+/* SPDX-License-Identifier: GPL-2.0 */
> >+
> >+#ifndef __LINUX_DM_VERITY_LOADPIN_H
> >+#define __LINUX_DM_VERITY_LOADPIN_H
> >+
> >+#include <linux/list.h>
> >+
> >+struct mapped_device;
> >+
> >+struct trusted_root_digest {
> >+	u8 *data;
> >+	unsigned int len;
> >+	struct list_head node;
> >+};
> 
> To avoid the double-alloc in patch 2 (and save 1 pointer size of memory), this could just be:
> 
> struct trusted_root_digest {
> 	struct list_head node;
> 	unsigned int len;
> 	u8 data[];
> };

Looks good to me, will change

> Otherwise, looks good to me!

Excellent, thanks for the review!