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

Josh Boyer
2016-11-21T16:26:07Z

On Mon, Nov 21, 2016 at 11:18 AM, Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
> 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.

If shim/mokmanager aren't used, then you can't actually modify
MokIgnoreDB. Again, it requires physical access and a reboot into
mokmanager to actually take effect.

josh

>> 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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