[PATCH v2 06/17] efi_loader: image_loader: add a check against certificate type of authenticode

[AKASHI Takahiro <takahiro.akashi@linaro.org>]

Heinrich,

On Fri, Jul 03, 2020 at 12:56:55PM +0200, Heinrich Schuchardt wrote:
> On 09.06.20 07:09, AKASHI Takahiro wrote:
> > UEFI specification requires that we shall support three type of
> > certificates of authenticode in PE image:
> >   WIN_CERT_TYPE_EFI_GUID with the guid, EFI_CERT_TYPE_PCKS7_GUID
> >   WIN_CERT_TYPE_PKCS_SIGNED_DATA
> >   WIN_CERT_TYPE_EFI_PKCS1_15
> >
> > As EDK2 does, we will support the first two that are pkcs7 SignedData.
> >
> > Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
> > ---
> >  lib/efi_loader/efi_image_loader.c | 56 ++++++++++++++++++++++++-------
> >  1 file changed, 44 insertions(+), 12 deletions(-)
> >
> > diff --git a/lib/efi_loader/efi_image_loader.c b/lib/efi_loader/efi_image_loader.c
> > index 5b00fea2f113..38b2c24ab1d6 100644
> > --- a/lib/efi_loader/efi_image_loader.c
> > +++ b/lib/efi_loader/efi_image_loader.c
> > @@ -484,7 +484,8 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
> >  	struct efi_signature_store *db = NULL, *dbx = NULL;
> >  	struct x509_certificate *cert = NULL;
> >  	void *new_efi = NULL;
> > -	size_t new_efi_size;
> > +	u8 *auth, *wincerts_end;
> > +	size_t new_efi_size, auth_size;
> >  	bool ret = false;
> >
> >  	if (!efi_secure_boot_enabled())
> > @@ -533,21 +534,52 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
> >  	}
> >
> >  	/* go through WIN_CERTIFICATE list */
> > -	for (wincert = wincerts;
> > -	     (void *)wincert < (void *)wincerts + wincerts_len;
> > -	     wincert = (void *)wincert + ALIGN(wincert->dwLength, 8)) {
> > -		if (wincert->dwLength < sizeof(*wincert)) {
> > -			EFI_PRINT("%s: dwLength too small: %u < %zu\n",
> > -				  __func__, wincert->dwLength,
> > -				  sizeof(*wincert));
> > -			goto err;
> > +	for (wincert = wincerts, wincerts_end = (u8 *)wincerts + wincerts_len;
> > +	     (u8 *)wincert < wincerts_end;
> 
> Shouldn't you compare the end of the current certificate to wincerts_end?

No.

>     ((u8 *)wincert + ALIGN(wincert->dwLength, 8))) <= wincerts_end
> 
> But you doing such a comparison anyway two lines below. So there is no
> need to duplicate the test here.
> 
> > +	     wincert = (WIN_CERTIFICATE *)
> > +			((u8 *)wincert + ALIGN(wincert->dwLength, 8))) {
> > +		if ((u8 *)wincert + sizeof(*wincert) >= wincerts_end)
> > +			break;

Is this the line that you mentioned as "two lines below"?
If so, the check is not the exact same.

As an image is provided by a *user*, we have to be as careful
as possible in handling data in an image.
In this case, the value of size in a header may not always be
correct (or at least, can be corrupted). So we will make sure
that we have enough data for accessing a header at the beginning.

> Why is this >= and not >?

Because "<start> + <size>" would be exclusive.

So I will keep my code unchanged.

-Takahiro Akashi


> Best regards
> 
> Heinrich
> 
> > +
> > +		if (wincert->dwLength <= sizeof(*wincert)) {
> > +			EFI_PRINT("dwLength too small: %u < %zu\n",
> > +				  wincert->dwLength, sizeof(*wincert));
> > +			continue;
> > +		}
> > +
> > +		EFI_PRINT("WIN_CERTIFICATE_TYPE: 0x%x\n",
> > +			  wincert->wCertificateType);
> > +
> > +		auth = (u8 *)wincert + sizeof(*wincert);
> > +		auth_size = wincert->dwLength - sizeof(*wincert);
> > +		if (wincert->wCertificateType == WIN_CERT_TYPE_EFI_GUID) {
> > +			if (auth + sizeof(efi_guid_t) >= wincerts_end)
> > +				break;
> > +
> > +			if (auth_size <= sizeof(efi_guid_t)) {
> > +				EFI_PRINT("dwLength too small: %u < %zu\n",
> > +					  wincert->dwLength, sizeof(*wincert));
> > +				continue;
> > +			}
> > +			if (guidcmp(auth, &efi_guid_cert_type_pkcs7)) {
> > +				EFI_PRINT("Certificate type not supported: %pUl\n",
> > +					  auth);
> > +				continue;
> > +			}
> > +
> > +			auth += sizeof(efi_guid_t);
> > +			auth_size -= sizeof(efi_guid_t);
> > +		} else if (wincert->wCertificateType
> > +				!= WIN_CERT_TYPE_PKCS_SIGNED_DATA) {
> > +			EFI_PRINT("Certificate type not supported\n");
> > +			continue;
> >  		}
> > -		msg = pkcs7_parse_message((void *)wincert + sizeof(*wincert),
> > -					  wincert->dwLength - sizeof(*wincert));
> > +
> > +		msg = pkcs7_parse_message(auth, auth_size);
> >  		if (IS_ERR(msg)) {
> >  			EFI_PRINT("Parsing image's signature failed\n");
> >  			msg = NULL;
> > -			goto err;
> > +			continue;
> >  		}
> >
> >  		/* try black-list first */
> >
>