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

Heinrich Schuchardt
2022-02-10T07:31:22Z

On 2/10/22 08:13, Ilias Apalodimas wrote:
> 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 amount of new code is ok.

>
>>
>>> 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?

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?

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.
>>
>> -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