Commit 7228b66a authored by Mat Martineau's avatar Mat Martineau Committed by James Morris

KEYS: Add documentation for asymmetric keyring restrictions

Provide more specific examples of keyring restrictions as applied to
X.509 signature chain verification.
Signed-off-by: default avatarMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
Signed-off-by: default avatarJames Morris <james.l.morris@oracle.com>
parent 4f9dabfa
......@@ -10,6 +10,7 @@ Contents:
- Signature verification.
- Asymmetric key subtypes.
- Instantiation data parsers.
- Keyring link restrictions.
========
......@@ -318,7 +319,8 @@ KEYRING LINK RESTRICTIONS
=========================
Keyrings created from userspace using add_key can be configured to check the
signature of the key being linked.
signature of the key being linked. Keys without a valid signature are not
allowed to link.
Several restriction methods are available:
......@@ -327,9 +329,10 @@ Several restriction methods are available:
- Option string used with KEYCTL_RESTRICT_KEYRING:
- "builtin_trusted"
The kernel builtin trusted keyring will be searched for the signing
key. The ca_keys kernel parameter also affects which keys are used for
signature verification.
The kernel builtin trusted keyring will be searched for the signing key.
If the builtin trusted keyring is not configured, all links will be
rejected. The ca_keys kernel parameter also affects which keys are used
for signature verification.
(2) Restrict using the kernel builtin and secondary trusted keyrings
......@@ -337,8 +340,10 @@ Several restriction methods are available:
- "builtin_and_secondary_trusted"
The kernel builtin and secondary trusted keyrings will be searched for the
signing key. The ca_keys kernel parameter also affects which keys are used
for signature verification.
signing key. If the secondary trusted keyring is not configured, this
restriction will behave like the "builtin_trusted" option. The ca_keys
kernel parameter also affects which keys are used for signature
verification.
(3) Restrict using a separate key or keyring
......@@ -346,7 +351,7 @@ Several restriction methods are available:
- "key_or_keyring:<key or keyring serial number>[:chain]"
Whenever a key link is requested, the link will only succeed if the key
being linked is signed by one of the designated keys. This key may be
being linked is signed by one of the designated keys. This key may be
specified directly by providing a serial number for one asymmetric key, or
a group of keys may be searched for the signing key by providing the
serial number for a keyring.
......@@ -354,7 +359,51 @@ Several restriction methods are available:
When the "chain" option is provided at the end of the string, the keys
within the destination keyring will also be searched for signing keys.
This allows for verification of certificate chains by adding each
cert in order (starting closest to the root) to one keyring.
certificate in order (starting closest to the root) to a keyring. For
instance, one keyring can be populated with links to a set of root
certificates, with a separate, restricted keyring set up for each
certificate chain to be validated:
# Create and populate a keyring for root certificates
root_id=`keyctl add keyring root-certs "" @s`
keyctl padd asymmetric "" $root_id < root1.cert
keyctl padd asymmetric "" $root_id < root2.cert
# Create and restrict a keyring for the certificate chain
chain_id=`keyctl add keyring chain "" @s`
keyctl restrict_keyring $chain_id asymmetric key_or_keyring:$root_id:chain
# Attempt to add each certificate in the chain, starting with the
# certificate closest to the root.
keyctl padd asymmetric "" $chain_id < intermediateA.cert
keyctl padd asymmetric "" $chain_id < intermediateB.cert
keyctl padd asymmetric "" $chain_id < end-entity.cert
If the final end-entity certificate is successfully added to the "chain"
keyring, we can be certain that it has a valid signing chain going back to
one of the root certificates.
A single keyring can be used to verify a chain of signatures by
restricting the keyring after linking the root certificate:
# Create a keyring for the certificate chain and add the root
chain2_id=`keyctl add keyring chain2 "" @s`
keyctl padd asymmetric "" $chain2_id < root1.cert
# Restrict the keyring that already has root1.cert linked. The cert
# will remain linked by the keyring.
keyctl restrict_keyring $chain2_id asymmetric key_or_keyring:0:chain
# Attempt to add each certificate in the chain, starting with the
# certificate closest to the root.
keyctl padd asymmetric "" $chain2_id < intermediateA.cert
keyctl padd asymmetric "" $chain2_id < intermediateB.cert
keyctl padd asymmetric "" $chain2_id < end-entity.cert
If the final end-entity certificate is successfully added to the "chain2"
keyring, we can be certain that there is a valid signing chain going back
to the root certificate that was added before the keyring was restricted.
In all of these cases, if the signing key is found the signature of the key to
be linked will be verified using the signing key. The requested key is added
......
......@@ -894,6 +894,12 @@ The keyctl syscall functions are:
To apply a keyring restriction the process must have Set Attribute
permission and the keyring must not be previously restricted.
One application of restricted keyrings is to verify X.509 certificate
chains or individual certificate signatures using the asymmetric key type.
See Documentation/crypto/asymmetric-keys.txt for specific restrictions
applicable to the asymmetric key type.
Kernel Services
===============
......
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