Re: [RFC PATCH 1/2] efi_loader: fix dual signed image certification

Ilias Apalodimas
2022-02-10T07:13:45Z

On Thu, Feb 10, 2022 at 02:13:48PM +0900, AKASHI Takahiro wrote:
> Hi Ilias,
>
> Thank you for reviewing the logic.
>
> On Fri, Feb 04, 2022 at 09:32:01AM +0200, Ilias Apalodimas wrote:
> > The EFI spec allows for images to carry multiple signatures. Currently
> > we don't adhere to the verification process for such images.
>
> In this patch, you're trying to do three things:
> * remove efi_image_unsigned_authenticate()
> * pull efi_signature_lookup_digest() out of a while loop
> * change the logic of authentication
>
> I'd prefer to see those changes in separate patches for better reviewing.

I tried both and the current one seemed easier to review. Heinrich any
preference?

>
> > The spec says:
> > "Multiple signatures are allowed to exist in the binary's certificate
> > table (as per PE/COFF Section "Attribute Certificate Table"). Only one
> > hash or signature is required to be present in db in order to pass
> > validation, so long as neither the SHA-256 hash of the binary nor any
> > present signature is reflected in dbx."
>
> I have some concern about what the last phrase, "neither the SHA-256 hash
> of the binary nor any present signature is reflected in dbx" means.
> See the comment below.
>
> > With our current implementation signing the image with two certificates
> > and inserting both of them in db and one of them dbx doesn't always reject
> > the image. The rejection depends on the order that the image was signed
> > and the order the certificates are read (and checked) in db.
> >
> > While at it move the sha256 hash verification outside the signature
> > checking loop, since it only needs to run once per image and get simplify
> > the logic for authenticating an unsigned imahe using sha256 hashes.
> >
> > Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> > ---
> > lib/efi_loader/efi_image_loader.c | 88 +++++++------------------------
> > 1 file changed, 18 insertions(+), 70 deletions(-)
> >
> > diff --git a/lib/efi_loader/efi_image_loader.c b/lib/efi_loader/efi_image_loader.c
> > index f41cfa4fccd5..5df35939f702 100644
> > --- a/lib/efi_loader/efi_image_loader.c
> > +++ b/lib/efi_loader/efi_image_loader.c
> > @@ -516,53 +516,6 @@ err:
> > }
> >
> > #ifdef CONFIG_EFI_SECURE_BOOT
> > -/**
> > - * efi_image_unsigned_authenticate() - authenticate unsigned image with
> > - * SHA256 hash
> > - * @regs: List of regions to be verified
> > - *
> > - * If an image is not signed, it doesn't have a signature. In this case,
> > - * its message digest is calculated and it will be compared with one of
> > - * hash values stored in signature databases.
> > - *
> > - * Return: true if authenticated, false if not
> > - */
> > -static bool efi_image_unsigned_authenticate(struct efi_image_regions *regs)
> > -{
> > - struct efi_signature_store *db = NULL, *dbx = NULL;
> > - bool ret = false;
> > -
> > - dbx = efi_sigstore_parse_sigdb(u"dbx");
> > - if (!dbx) {
> > - EFI_PRINT("Getting signature database(dbx) failed\n");
> > - goto out;
> > - }
> > -
> > - db = efi_sigstore_parse_sigdb(u"db");
> > - if (!db) {
> > - EFI_PRINT("Getting signature database(db) failed\n");
> > - goto out;
> > - }
> > -
> > - /* try black-list first */
> > - if (efi_signature_lookup_digest(regs, dbx, true)) {
> > - EFI_PRINT("Image is not signed and its digest found in \"dbx\"\n");
> > - goto out;
> > - }
> > -
> > - /* try white-list */
> > - if (efi_signature_lookup_digest(regs, db, false))
> > - ret = true;
> > - else
> > - EFI_PRINT("Image is not signed and its digest not found in \"db\" or \"dbx\"\n");
> > -
> > -out:
> > - efi_sigstore_free(db);
> > - efi_sigstore_free(dbx);
> > -
> > - return ret;
> > -}
> > -
> > /**
> > * efi_image_authenticate() - verify a signature of signed image
> > * @efi: Pointer to image
> > @@ -608,14 +561,7 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
> > if (!efi_image_parse(new_efi, efi_size, &regs, &wincerts,
> > &wincerts_len)) {
> > EFI_PRINT("Parsing PE executable image failed\n");
> > - goto err;
> > - }
> > -
> > - if (!wincerts) {
> > - /* The image is not signed */
> > - ret = efi_image_unsigned_authenticate(regs);
> > -
> > - goto err;
> > + goto out;
> > }
> >
> > /*
> > @@ -624,18 +570,18 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
> > db = efi_sigstore_parse_sigdb(u"db");
> > if (!db) {
> > EFI_PRINT("Getting signature database(db) failed\n");
> > - goto err;
> > + goto out;
> > }
> >
> > dbx = efi_sigstore_parse_sigdb(u"dbx");
> > if (!dbx) {
> > EFI_PRINT("Getting signature database(dbx) failed\n");
> > - goto err;
> > + goto out;
> > }
> >
> > if (efi_signature_lookup_digest(regs, dbx, true)) {
> > EFI_PRINT("Image's digest was found in \"dbx\"\n");
> > - goto err;
> > + goto out;
> > }
> >
> > /*
> > @@ -678,7 +624,8 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
> > if (guidcmp(auth, &efi_guid_cert_type_pkcs7)) {
> > EFI_PRINT("Certificate type not supported: %pUs\n",
> > auth);
> > - continue;
> > + ret = false;
> > + goto out;
>
> Why should we break the loop here?

We were trying to reject signature verification that we don't support,
since the equivalent cert might be in dbx. But I am not 100% sure taht's
what we want here.

>
> > }
> >
> > auth += sizeof(efi_guid_t);
> > @@ -686,7 +633,8 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
> > } else if (wincert->wCertificateType
> > != WIN_CERT_TYPE_PKCS_SIGNED_DATA) {
> > EFI_PRINT("Certificate type not supported\n");
> > - continue;
> > + ret = false;
> > + goto out;
> > }
> >
> > 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?

> 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.
>
> -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