Merge patch series "boot: fit: authenticate the dm-verity roothash"

Daniel Golle <[email protected]> says:

A signed FIT configuration can delegate the integrity of a (potentially
large) root filesystem image to the kernel's dm-verity instead of having
U-Boot hash the whole payload at boot: the FIT carries a "dm-verity"
subnode with the roothash, salt and block parameters, U-Boot passes the
roothash to Linux through the dm-mod.create bootargs, and dm-verity then
validates the filesystem block by block against it.

For that to be safe the roothash has to be trusted, and in a signed
configuration the only thing that establishes trust is the configuration
signature. The roothash was not covered by it. fit_config_add_hash()
collected the image node, its hash subnodes and its cipher subnode into
the signed region, but not the dm-verity subnode, so the roothash, the
sole integrity anchor for the filesystem, was left unsigned.

The result is a verified-boot bypass for the root filesystem: an
attacker who can rewrite the boot medium can replace the filesystem,
recompute a matching dm-verity tree, write the new roothash into the
unsigned dm-verity subnode, and the configuration signature still
verifies. dm-verity then faithfully validates the malicious filesystem
against the attacker's roothash.

This series closes the gap.

Link: https://lore.kernel.org/r/[email protected]

1 files changed, 23 insertions, 0 deletions

diff --git a/include/image.h b/include/image.h
index 4149ebbcce9..6edcb1995bf 100644
--- a/include/image.h
+++ b/include/image.h
@@ -1897,6 +1897,29 @@ struct image_region *fit_region_make_list(const void *fit,
 		struct fdt_region *fdt_regions, int count,
 		struct image_region *region);
 
+/**
+ * fit_config_get_signed_nodes() - Build the list of nodes covered by a config
+ *				   signature
+ *
+ * Collects the paths of the nodes that the configuration signature is
+ * computed over: the root node, the configuration node, and for each image
+ * referenced by the configuration its node, its hash subnodes and its cipher
+ * and dm-verity subnodes. The result is the same node list used when creating
+ * and verifying the signature, and is suitable for passing to
+ * fdt_find_regions().
+ *
+ * @fit:	FIT blob
+ * @conf_noffset: Configuration node offset
+ * @node_inc:	Array to fill with pointers to packed path strings
+ * @max_nodes:	Number of entries in @node_inc
+ * @buf:	Buffer for the packed null-terminated path strings
+ * @buf_len:	Size of @buf
+ * Return: number of entries written to @node_inc, or -ve on error
+ */
+int fit_config_get_signed_nodes(const void *fit, int conf_noffset,
+				char **node_inc, int max_nodes,
+				char *buf, int buf_len);
+
 static inline int fit_image_check_target_arch(const void *fdt, int node)
 {
 #ifndef USE_HOSTCC