On Thu, 10 Feb 2022 at 10:01, AKASHI Takahiro takahiro.akashi@linaro.org wrote:
On Thu, Feb 10, 2022 at 09:55:20AM +0200, Ilias Apalodimas wrote:
On Thu, Feb 10, 2022 at 04:41:15PM +0900, AKASHI Takahiro wrote:
On Thu, Feb 10, 2022 at 09:33:46AM +0200, Ilias Apalodimas wrote:
> > msg = pkcs7_parse_message(auth, auth_size);
[...]
> > @@ -717,32 +665,32 @@ static bool efi_image_authenticate(void *efi, size_t efi_size) > > */ > > /* try black-list first */ > > if (efi_signature_verify_one(regs, msg, dbx)) { > > + ret = false; > > EFI_PRINT("Signature was rejected by "dbx"\n"); > > - continue; > > + goto out; > > If we go to "out" here, we have no chance to verify some cases: > 1) An image has two signatures, for instance, one signed by SHA1 cert > and the other signed by SHA256 cert. A user wants to reject SHA1 cert > and put the cert in dbx.
I am not sure I am following, what does he gain be rejecting the SHA1 portion only? Avoid potential collisions?
If an image has a SHA1 and a SHA256 signature attached and SHA1 *or* SHA256 is in dbx, we must reject the image. Don't expect a dbx entry for each of the hashes. - But isn't this what your are doing here: for all signatures of the image look for one hit in dbx?
Yes exactly. Any match on dbx of any certificate or sha256 of a certificate or a sha256 of the executable will reject the image.
But we believe that SHA256-based signature is still valid even if we don't trust SHA1.
UEFI spec 2.9 page 1715 describes exaclty what we propose here as a change. The SHAxxx choise is irrelevant, any potential match should reject the image.
Regards /Ilias
Best regards
Heinrich
> But this image can and should yet be verified by SHA256 cert.
Why should it be verified? My understanding of the EFI spec is that any match in dbx of any certificate in the signing chain of the signature being verified means reject the image.
> 2) A user knows that a given image is safe for some reason even though > he or she doesn't trust the certficate which is used for signing > the image.
Then he should resign his image with a proper certificate.
No, I don't think so. The hash-based verification is for that.
If an image is rejected by a corresponding x509 in dbx or a shaxxx of the certificate, execution should be denied. I am not really sure what you are trying to describe here.
Regards /Ilias
-Takahiro Akashi
Regards /Ilias
What do you think of this case?
-Takahiro Akashi
> -Takahiro Akashi > > > } > > > > if (!efi_signature_check_signers(msg, dbx)) { > > + ret = false; > > EFI_PRINT("Signer(s) in "dbx"\n"); > > - continue; > > + goto out; > > } > > > > /* try white-list */ > > if (efi_signature_verify(regs, msg, db, dbx)) { > > ret = true; > > - break; > > + continue; > > } > > > > EFI_PRINT("Signature was not verified by "db"\n"); > > + } > > > > - if (efi_signature_lookup_digest(regs, db, false)) { > > - ret = true; > > - break; > > - } > > > > - EFI_PRINT("Image's digest was not found in "db" or "dbx"\n"); > > - } > > + /* last resort try the image sha256 hash in db */ > > + if (!ret && efi_signature_lookup_digest(regs, db, false)) > > + ret = true; > > > > -err: > > +out: > > efi_sigstore_free(db); > > efi_sigstore_free(dbx); > > pkcs7_free_message(msg); > > -- > > 2.32.0 > >
Thanks /Ilias