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

AKASHI Takahiro
2022-02-10T05:14:02Z

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.

> 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?

> }
>
> 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.
But this image can and should yet be verified by SHA256 cert.
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
>