Re: [PATCH 9/9] MODSIGN: Allow the "db" UEFI variable to be suppressed

Ard Biesheuvel
2016-11-21T16:19:02Z

On 16 November 2016 at 18:11, David Howells <dhowells@redhat.com> wrote:
> From: Josh Boyer <jwboyer@fedoraproject.org>
>
> If a user tells shim to not use the certs/hashes in the UEFI db variable
> for verification purposes, shim will set a UEFI variable called
> MokIgnoreDB. Have the uefi import code look for this and ignore the db
> variable if it is found.
>

Similar concern as in the previous patch: it appears to me that you
can DoS a machine by setting MokIgnoreDB if, e.g., its modules are
signed against a cert that resides in db, and shim/mokmanager are not
being used.

> Signed-off-by: Josh Boyer <jwboyer@fedoraproject.org>
> Signed-off-by: David Howells <dhowells@redhat.com>
> ---
>
> certs/load_uefi.c | 44 ++++++++++++++++++++++++++++++++++----------
> 1 file changed, 34 insertions(+), 10 deletions(-)
>
> diff --git a/certs/load_uefi.c b/certs/load_uefi.c
> index b44e464c3ff4..3d8845986019 100644
> --- a/certs/load_uefi.c
> +++ b/certs/load_uefi.c
> @@ -13,6 +13,26 @@ static __initdata efi_guid_t efi_cert_x509_sha256_guid = EFI_CERT_X509_SHA256_GU
> static __initdata efi_guid_t efi_cert_sha256_guid = EFI_CERT_SHA256_GUID;
>
> /*
> + * Look to see if a UEFI variable called MokIgnoreDB exists and return true if
> + * it does.
> + *
> + * This UEFI variable is set by the shim if a user tells the shim to not use
> + * the certs/hashes in the UEFI db variable for verification purposes. If it
> + * is set, we should ignore the db variable also and the true return indicates
> + * this.
> + */
> +static __init bool uefi_check_ignore_db(void)
> +{
> + efi_status_t status;
> + unsigned int db = 0;
> + unsigned long size = sizeof(db);
> + efi_guid_t guid = EFI_SHIM_LOCK_GUID;
> +
> + status = efi.get_variable(L"MokIgnoreDB", &guid, NULL, &size, &db);
> + return status == EFI_SUCCESS;
> +}
> +
> +/*
> * Get a certificate list blob from the named EFI variable.
> */
> static __init void *get_cert_list(efi_char16_t *name, efi_guid_t *guid,
> @@ -113,7 +133,9 @@ static __init efi_element_handler_t get_handler_for_dbx(const efi_guid_t *sig_ty
> }
>
> /*
> - * Load the certs contained in the UEFI databases
> + * Load the certs contained in the UEFI databases into the secondary trusted
> + * keyring and the UEFI blacklisted X.509 cert SHA256 hashes into the blacklist
> + * keyring.
> */
> static int __init load_uefi_certs(void)
> {
> @@ -129,15 +151,17 @@ static int __init load_uefi_certs(void)
> /* Get db, MokListRT, and dbx. They might not exist, so it isn't
> * an error if we can't get them.
> */
> - db = get_cert_list(L"db", &secure_var, &dbsize);
> - if (!db) {
> - pr_err("MODSIGN: Couldn't get UEFI db list\n");
> - } else {
> - rc = parse_efi_signature_list("UEFI:db",
> - db, dbsize, get_handler_for_db);
> - if (rc)
> - pr_err("Couldn't parse db signatures: %d\n", rc);
> - kfree(db);
> + if (!uefi_check_ignore_db()) {
> + db = get_cert_list(L"db", &secure_var, &dbsize);
> + if (!db) {
> + pr_err("MODSIGN: Couldn't get UEFI db list\n");
> + } else {
> + rc = parse_efi_signature_list("UEFI:db",
> + db, dbsize, get_handler_for_db);
> + if (rc)
> + pr_err("Couldn't parse db signatures: %d\n", rc);
> + kfree(db);
> + }
> }
>
> mok = get_cert_list(L"MokListRT", &mok_var, &moksize);
>
> --
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