dm verity: add root hash pkcs#7 signature verification

The verification is to support cases where the root hash is not secured
by Trusted Boot, UEFI Secureboot or similar technologies.

One of the use cases for this is for dm-verity volumes mounted after
boot, the root hash provided during the creation of the dm-verity volume
has to be secure and thus in-kernel validation implemented here will be
used before we trust the root hash and allow the block device to be
created.

The signature being provided for verification must verify the root hash
and must be trusted by the builtin keyring for verification to succeed.

The hash is added as a key of type "user" and the description is passed
to the kernel so it can look it up and use it for verification.

Adds CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG which can be turned on if root
hash verification is needed.

Kernel commandline dm_verity module parameter 'require_signatures' will
indicate whether to force root hash signature verification (for all dm
verity volumes).

Signed-off-by: Jaskaran Khurana <jaskarankhurana@linux.microsoft.com>
Tested-and-Reviewed-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 3834332f4963..1856c0fb665d 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -490,6 +490,18 @@ config DM_VERITY
 
 	  If unsure, say N.
 
+config DM_VERITY_VERIFY_ROOTHASH_SIG
+	def_bool n
+	bool "Verity data device root hash signature verification support"
+	depends on DM_VERITY
+	select SYSTEM_DATA_VERIFICATION
+	  help
+	  Add ability for dm-verity device to be validated if the
+	  pre-generated tree of cryptographic checksums passed has a pkcs#7
+	  signature file that can validate the roothash of the tree.
+
+	  If unsure, say N.
+
 config DM_VERITY_FEC
 	bool "Verity forward error correction support"
 	depends on DM_VERITY