Patch "PKCS#7: fix certificate chain verification" has been added to the 4.15-stable tree

This is a note to let you know that I've just added the patch titled

PKCS#7: fix certificate chain verification

to the 4.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
pkcs-7-fix-certificate-chain-verification.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Eric Biggers <[email protected]>
Date: Thu, 22 Feb 2018 14:38:33 +0000
Subject: PKCS#7: fix certificate chain verification

From: Eric Biggers <[email protected]>

commit 971b42c038dc83e3327872d294fe7131bab152fc upstream.

When pkcs7_verify_sig_chain() is building the certificate chain for a SignerInfo using the certificates in the PKCS#7 message, it is passing the wrong arguments to public_key_verify_signature(). Consequently, when the next certificate is supposed to be used to verify the previous certificate, the next certificate is actually used to verify itself.

An attacker can use this bug to create a bogus certificate chain that has no cryptographic relationship between the beginning and end.

Fortunately I couldn't quite find a way to use this to bypass the overall signature verification, though it comes very close. Here's the
reasoning: due to the bug, every certificate in the chain beyond the first actually has to be self-signed (where "self-signed" here refers to the actual key and signature; an attacker might still manipulate the certificate fields such that the self_signed flag doesn't actually get set, and thus the chain doesn't end immediately). But to pass trust validation (pkcs7_validate_trust()), either the SignerInfo or one of the certificates has to actually be signed by a trusted key. Since only self-signed certificates can be added to the chain, the only way for an attacker to introduce a trusted signature is to include a self-signed trusted certificate.

But, when pkcs7_validate_trust_one() reaches that certificate, instead of trying to verify the signature on that certificate, it will actually look up the corresponding trusted key, which will succeed, and then try to verify the *previous* certificate, which will fail. Thus, disaster is narrowly averted (as far as I could tell).

---
 crypto/asymmetric_keys/pkcs7_verify.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -270,7 +270,7 @@ static int pkcs7_verify_sig_chain(struct
 				sinfo->index);
 			return 0;
 		}
-		ret = public_key_verify_signature(p->pub, p->sig);
+		ret = public_key_verify_signature(p->pub, x509->sig);
 		if (ret < 0)
 			return ret;
 		x509->signer = p;

Patches currently in stable-queue which might be from [email protected] are

queue-4.15/pkcs-7-fix-certificate-blacklisting.patch queue-4.15/x.509-fix-null-dereference-when-restricting-key-with-unsupported_sig.patch queue-4.15/x.509-fix-bug_on-when-hash-algorithm-is-unsupported.patch queue-4.15/pkcs-7-fix-certificate-chain-verification.patch