Re: [PATCH 3/4] KEYS: CA link restriction

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On 3/9/22 12:12, Stefan Berger wrote:


On 3/8/22 13:02, Eric Snowberg wrote:


On Mar 8, 2022, at 5:45 AM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:

On Mon, 2022-03-07 at 21:31 -0500, Stefan Berger wrote:

On 3/7/22 18:38, Eric Snowberg wrote:


On Mar 7, 2022, at 4:01 PM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:

On Mon, 2022-03-07 at 18:06 +0000, Eric Snowberg wrote:

diff --git a/crypto/asymmetric_keys/restrict.c b/crypto/asymmetric_keys/restrict.c
index 6b1ac5f5896a..49bb2ea7f609 100644
--- a/crypto/asymmetric_keys/restrict.c
+++ b/crypto/asymmetric_keys/restrict.c
@@ -108,6 +108,49 @@ int restrict_link_by_signature(struct key *dest_keyring,
    return ret;
}
+/**
+ * restrict_link_by_ca - Restrict additions to a ring of CA keys
+ * @dest_keyring: Keyring being linked to.
+ * @type: The type of key being added.
+ * @payload: The payload of the new key.
+ * @trust_keyring: Unused.
+ *
+ * Check if the new certificate is a CA. If it is a CA, then mark the new
+ * certificate as being ok to link.

CA = root CA here, right?

Yes, I’ll update the comment

Updating the comment is not enough.  There's an existing function named "x509_check_for_self_signed()" which determines whether the certificate
is self-signed.

Originally I tried using that function.  However when the restrict link code is called, all the necessary x509 information is no longer available.   The code in restrict_link_by_ca is basically doing the equivalent to x509_check_for_self_signed. After verifying the cert has the CA flag set, the call to public_key_verify_signature
validates the cert is self signed.

Isn't x509_cert_parse() being called as part of parsing the certificate?
If so, it seems to check for a self-signed certificate every time. You
could add something like the following to x509_check_for_self_signed(cert):
pub->x509_self_signed = cert->self_signed = true;

This could then reduce the function in 3/4 to something like:

return payload->data[asym_crypto]->x509_self_signed;

When I was studying the restriction code, before writing this patch, it looked like it was written from the standpoint to be as generic as possible.  All code contained within it works on either a public_key_signature or a public_key.  I had assumed it was written this way to be used with different asymmetrical key types now and in the future. I called the public_key_verify_signature function instead of interrogating the x509 payload to keep in line with what I thought was the original design. Let me
know if I should be carrying x509 code in here to make the change above.

It does not seem right if there were two functions trying to determine whether an x509 cert is self-signed. The existing is invoked as part of loading a key onto the machine keyring from what I can see. It has access to more data about the cert and therefore can do stronger tests, yours doesn't have access to the data. So I guess I would remember in a boolean in the public key structure that the x509 cert it comes from was self signed following the existing test. Key in your function may be that that payload->data[] array is guaranteed to be from the x509 cert as set in x509_key_preparse().

https://elixir.bootlin.com/linux/v5.17-rc7/source/crypto/asymmetric_keys/x509_public_key.c#L236

    Stefan

Sorry for the mess in the response. The first version is the good one :-)



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