[RFC PATCH] efi_loader: fix uefi secure boot with intermediate certs

[Ilias Apalodimas]

On Sat, Feb 19, 2022 at 10:47:16AM +0100, Heinrich Schuchardt wrote:
> On 2/14/22 10:14, Ilias Apalodimas wrote:
> > The general rule of accepting or rejecting an image is
> >   1. Is the sha256 of the image in dbx
> >   2. Is the image signed with a certificate that's found in db and
> >      not in dbx
> >   3. The image carries a cert which is signed by a cert in db (and
> >      not in dbx) and the image can be verified against the former
> >   4. Is the sha256 of the image in db
> > 
> > For example SHIM is signed by "CN=Microsoft Windows UEFI Driver Publisher",
> > which is issued by "CN=Microsoft Corporation UEFI CA 2011", which in it's
> > turn is issued by "CN=Microsoft Corporation Third Party Marketplace Root".
> > The latter is a self-signed CA certificate and with our current implementation
> > allows shim to execute if we insert it in db.
> > 
> > However it's the CA cert in the middle of the chain which usually ends up
> > in the system's db.  pkcs7_verify_one() might or might not return the root
> > certificate for a given chain.  But when verifying executables in UEFI,  the
> > trust anchor can be in the middle of the chain, as long as that certificate
> > is present in db.  Currently we only allow this check on self-signed
> > certificates,  so let's remove that check and allow all certs to try a
> > match an entry in db.
> > 
> > Open questions:
> > - Does this break any aspect of variable authentication since
> >    efi_signature_verify() is used on those as well?
> > 
> > Signed-off-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> > ---
> >   lib/efi_loader/efi_signature.c | 11 +++++------
> >   1 file changed, 5 insertions(+), 6 deletions(-)
> > 
> > diff --git a/lib/efi_loader/efi_signature.c b/lib/efi_loader/efi_signature.c
> > index 1bd1fdc95fce..79ed077ae7dd 100644
> > --- a/lib/efi_loader/efi_signature.c
> > +++ b/lib/efi_loader/efi_signature.c
> > @@ -518,12 +518,11 @@ bool efi_signature_verify(struct efi_image_regions *regs,
> >   			goto out;
> > 
> >   		EFI_PRINT("Verifying last certificate in chain\n");
> > -		if (signer->self_signed) {
> > -			if (efi_lookup_certificate(signer, db))
> > -				if (efi_signature_check_revocation(sinfo,
> > -								   signer, dbx))
> > -					break;
> > -		} else if (efi_verify_certificate(signer, db, &root)) {
> > +		if (efi_lookup_certificate(signer, db))
> > +			if (efi_signature_check_revocation(sinfo, signer, dbx))
> 
> This line is after "EFI_PRINT("Verifying last certificate in chain\n");".
> 
> According to the UEFI 2.9 specification we have to check all
> certificates used in the chain against dbx not only the last:
> 
> p. 1715
> 
> "B. Any entry with SignatureListType of EFI_CERT_X509_SHA256,
> EFI_CERT_X509_SHA384, or EFI_CERT_X509_SHA512, with any SignatureData
> which reflects the To-Be-Signed hash included in any certificate in the
> signing chain of the signature being verified."
> 
> Best regards

Yes but imho this should be part of a larger rework.  The problem is
pkcs7_verify_one() is trying to verify the binary with the certificate and
walk up the chain of trust.  Now that function might or might not return
the root certificate, but we don't check all the certificates in the chain
anyway.  This patch is trying to fix the fact that non self signed certs
in the middle of the signing chain are valid and can authenticate EFI
binaries.

Regards
/Ilias
> 
> Heinrich
> 
> > +				break;
> > +		if (!signer->self_signed &&
> > +		    efi_verify_certificate(signer, db, &root)) {
> >   			bool check;
> > 
> >   			check = efi_signature_check_revocation(sinfo, root,
>