Re: [PATCH 2/2 v3] efi: print appropriate status message when loading certificates

From: Ard Biesheuvel
Date: Fri May 03 2019 - 10:24:33 EST

On Fri, 3 May 2019 at 10:59, joeyli <jlee@xxxxxxxx> wrote:
>
> On Fri, May 03, 2019 at 10:07:59AM +0200, Ard Biesheuvel wrote:
> > On Fri, 3 May 2019 at 09:18, joeyli <jlee@xxxxxxxx> wrote:
> > >
> > > Hi Ard,
> > >
> > > On Thu, May 02, 2019 at 11:04:34AM +0200, Ard Biesheuvel wrote:
> > > > On Thu, 2 May 2019 at 06:04, Lee, Chun-Yi <joeyli.kernel@xxxxxxxxx> wrote:
> > > > >
> > > > > When loading certificates list from UEFI variable, the original error
> > > > > message direct shows the efi status code from UEFI firmware. It looks
> > > > > ugly:
> > > > >
> > > > > [ 2.335031] Couldn't get size: 0x800000000000000e
> > > > > [ 2.335032] Couldn't get UEFI MokListRT
> > > > > [ 2.339985] Couldn't get size: 0x800000000000000e
> > > > > [ 2.339987] Couldn't get UEFI dbx list
> > > > >
> > > > > So, this patch shows the status string instead of status code.
> > > > >
> > > > > On the other hand, the "Couldn't get UEFI" message doesn't need
> > > > > to be exposed when db/dbx/mok variable do not exist. So, this
> > > > > patch set the message level to debug.
> > > > >
> > > > > v3.
> > > > > - Print messages similar to db/mok when loading dbx hash to blacklist:
> > > > > [ 1.500952] EFI: Blacklisting hash of an executable: UEFI:dbx
> > > > > [ 1.501773] blacklist: Loaded blacklisting hash
> > > > > 'bin:80b4d96931bf0d02fd91a61e19d14f1da452e66db2408ca8604d411f92659f0a'
> > > > >
> > > > > - Setting messages for the existence of db/mok/dbx lists to debug level.
> > > > >
> > > > > v2.
> > > > > Setting the MODSIGN messages level to debug.
> > > > >
> > > > > Link:
> > > > > https://forums.opensuse.org/showthread.php/535324-MODSIGN-Couldn-t-get-UEFI-db-list?p=2897516#post2897516
> > > > > Cc: James Morris <jmorris@xxxxxxxxx>
> > > > > Cc: Serge E. Hallyn" <serge@xxxxxxxxxx>
> > > > > Cc: David Howells <dhowells@xxxxxxxxxx>
> > > > > Cc: Nayna Jain <nayna@xxxxxxxxxxxxx>
> > > > > Cc: Josh Boyer <jwboyer@xxxxxxxxxxxxxxxxx>
> > > > > Cc: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> > > > > Signed-off-by: "Lee, Chun-Yi" <jlee@xxxxxxxx>
> > > > > ---
> > > > > certs/blacklist.c | 3 +-
> > > > > security/integrity/platform_certs/load_uefi.c | 40 +++++++++++++++++++--------
> > > > > 2 files changed, 31 insertions(+), 12 deletions(-)
> > > > >
> > > > > diff --git a/certs/blacklist.c b/certs/blacklist.c
> > > > > index 3a507b9e2568..f91437e39e44 100644
> > > > > --- a/certs/blacklist.c
> > > > > +++ b/certs/blacklist.c
> > > > > @@ -100,7 +100,8 @@ int mark_hash_blacklisted(const char *hash)
> > > > > if (IS_ERR(key)) {
> > > > > pr_err("Problem blacklisting hash (%ld)\n", PTR_ERR(key));
> > > > > return PTR_ERR(key);
> > > > > - }
> > > > > + } else
> > > > > + pr_notice("Loaded blacklisting hash '%s'\n", hash);
> > > > > return 0;
> > > > > }
> > > > >
> > > > > diff --git a/security/integrity/platform_certs/load_uefi.c b/security/integrity/platform_certs/load_uefi.c
> > > > > index 81b19c52832b..6b6996e5bc27 100644
> > > > > --- a/security/integrity/platform_certs/load_uefi.c
> > > > > +++ b/security/integrity/platform_certs/load_uefi.c
> > > > > @@ -1,5 +1,7 @@
> > > > > // SPDX-License-Identifier: GPL-2.0
> > > > >
> > > > > +#define pr_fmt(fmt) "EFI: "fmt
> > > > > +
> > > > > #include <linux/kernel.h>
> > > > > #include <linux/sched.h>
> > > > > #include <linux/cred.h>
> > > > > @@ -35,6 +37,18 @@ static __init bool uefi_check_ignore_db(void)
> > > > > return status == EFI_SUCCESS;
> > > > > }
> > > > >
> > > > > +static void str16_to_str(efi_char16_t *str16, char *str, int str_size)
> > > > > +{
> > > > > + int i = 0;
> > > > > +
> > > > > + while (str16[i] != '\0' && i < (str_size - 1)) {
> > > > > + str[i] = str16[i];
> > > > > + i++;
> > > > > + }
> > > > > +
> > > > > + str[i] = '\0';
> > > > > +}
> > > > > +
> > > > > /*
> > > > > * Get a certificate list blob from the named EFI variable.
> > > > > */
> > > > > @@ -44,13 +58,20 @@ static __init void *get_cert_list(efi_char16_t *name, efi_guid_t *guid,
> > > > > efi_status_t status;
> > > > > unsigned long lsize = 4;
> > > > > unsigned long tmpdb[4];
> > > > > + char namestr[16];
> > > > > void *db;
> > > > >
> > > > > + str16_to_str(name, namestr, ARRAY_SIZE(namestr));
> > > >
> > > > Please drop this (and the function above) - instead, just return NULL
> > > > if the variable is not found (without reporting an error).
> > > >
> > >
> > > This name string is for printing debug level message, not error message.
> > > This function already returns NULL when EFI_NOT_FOUND be returned by
> > > firmware.
> > >
> > > > > status = efi.get_variable(name, guid, NULL, &lsize, &tmpdb);
> > > > > if (status != EFI_BUFFER_TOO_SMALL) {
> > > > > - pr_err("Couldn't get size: 0x%lx\n", status);
> > > > > + if (status == EFI_NOT_FOUND)
> > > > > + pr_debug("UEFI %s list doesn't exist\n", namestr);
> > > > > + else
> > > > > + pr_err("Couldn't get size for UEFI %s list: %s\n",
> > > > > + namestr, efi_status_to_str(status));
> > > > > return NULL;
> > >
> > > here returns NULL when EFI_NOT_FOUND. The message of existence is for
> > > debugging.
> > >
> >
> > I understand that. But I don't think we need it.
> >
>
> OK. I will remove the debug message.
>
> > > > > }
> > > > > + pr_debug("UEFI %s list exists\n", namestr);
> > > > >
> > > > > db = kmalloc(lsize, GFP_KERNEL);
> > > > > if (!db)
> > > > > @@ -59,7 +80,8 @@ static __init void *get_cert_list(efi_char16_t *name, efi_guid_t *guid,
> > > > > status = efi.get_variable(name, guid, NULL, &lsize, db);
> > > > > if (status != EFI_SUCCESS) {
> > > > > kfree(db);
> > > > > - pr_err("Error reading db var: 0x%lx\n", status);
> > > > > + pr_err("Error reading UEFI %s list: %s\n",
> > > > > + namestr, efi_status_to_str(status));
> > > > > return NULL;
> > > > > }
> > > > >
> > > > > @@ -95,6 +117,7 @@ static __init void uefi_blacklist_hash(const char *source, const void *data,
> > > > > static __init void uefi_blacklist_x509_tbs(const char *source,
> > > > > const void *data, size_t len)
> > > > > {
> > > > > + pr_info("Blacklisting X.509 TBS hash: %s\n", source);
> > > > > uefi_blacklist_hash(source, data, len, "tbs:", 4);
> > > > > }
> > > > >
> > > > > @@ -104,6 +127,7 @@ static __init void uefi_blacklist_x509_tbs(const char *source,
> > > > > static __init void uefi_blacklist_binary(const char *source,
> > > > > const void *data, size_t len)
> > > > > {
> > > > > + pr_info("Blacklisting hash of an executable: %s\n", source);
> > > > > uefi_blacklist_hash(source, data, len, "bin:", 4);
> > > > > }
> > > > >
> > > >
> > > > These are separate changes - I don't have an opinion whether they are
> > > > appropriate or not, but they should be in a separate patch.
> > > >
> > >
> > > I will move the message of blacklising hash to other patch. Thanks!
> > >
> > > > > @@ -154,9 +178,7 @@ static int __init load_uefi_certs(void)
> > > > > */
> > > > > 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 {
> > > > > + if (db) {
> > > > > rc = parse_efi_signature_list("UEFI:db",
> > > > > db, dbsize, get_handler_for_db);
> > > > > if (rc)
> > > > > @@ -167,9 +189,7 @@ static int __init load_uefi_certs(void)
> > > > > }
> > > > >
> > > > > mok = get_cert_list(L"MokListRT", &mok_var, &moksize);
> > > > > - if (!mok) {
> > > > > - pr_info("Couldn't get UEFI MokListRT\n");
> > > > > - } else {
> > > > > + if (mok) {
> > > > > rc = parse_efi_signature_list("UEFI:MokListRT",
> > > > > mok, moksize, get_handler_for_db);
> > > > > if (rc)
> > > > > @@ -178,9 +198,7 @@ static int __init load_uefi_certs(void)
> > > > > }
> > > > >
> > > > > dbx = get_cert_list(L"dbx", &secure_var, &dbxsize);
> > > > > - if (!dbx) {
> > > > > - pr_info("Couldn't get UEFI dbx list\n");
> > > > > - } else {
> > > > > + if (dbx) {
> > > > > rc = parse_efi_signature_list("UEFI:dbx",
> > > > > dbx, dbxsize,
> > > > > get_handler_for_dbx);
> > > > > --
> > > > > 2.16.4
> > > > >
> > > >
> > > > I think we should consider carefully what it means if some of these
> > > > variables don't exist:
> > > > - if secure boot is enabled, db and dbx must exist, so if they don't,
> > > > something is wrong
> > >
> > > The existence of db/dbx is not related to secure boot. If manufacturer/user
> > > enrolled certificate/hash to db or dbx, then the variable will be created.
> > > If user didn't enroll anything to db/dbx, then variables will not show up.
> > >
> >
> > Yes, but if secure boot is enabled and db is empty, how could you have
> > booted in the first place?
> >
>
> I agree. When secure boot enabled, kernel can not be booted without db.
>
> > And what about the converse case: if secure boot is not enabled, why
> > should we trust the contents of db?
> >
>
> The db and dbx are authenticated variables that it protected by KEK.
> So it can be trusted even secure boot is disabled. Unless manufacturer
> or user's KEK is leaked.
>

Is that true for non-secureboot capable firmware? Of course, in that
case, we cannot be sure that the kernel itself is trusted, but we
shouldn't open up another hole either.

> > > > - secure boot might be enabled but we may be booting without shim.
> > >
> > > Shim always creates MokListRT no matter secure boot enabled or disabled.
> > >
> >
> > That is not my point. What happens if you booted with secure boot
> > enabled but without the help of shim?
> >
>
> Without shim, the signed EFI stub can still be booted by EFI boot manager.
> But the MokListRT will not be created for runtime. So MOK signed kernel
> module can not be verified. (or IMA can not verify MOK signed kernel image
> for kexec...)
>

So if we opt out of using shim, we lose kexec capability as well? That
doesn't make any sense.

In general, I am very uncomfortable with the way we have painted
ourselves into a corner with shim. If I install Suse on a system that
has the Suse cert in db or KEK, why would I need shim in the first
place?

> > > > - secure boot might be disabled.
> > > >
> > >
> > > It's not about secure boot, db/dbx/MokListRT are always available at
> > > runtime if user was enrolled something to those list.
> > >
> >
> > Yes, but again, depending on whether shim was involved, and/or whether
> > secure boot was enabled or not, the way we interpret these things may
> > be very different.
> >
> > I want the reasoning to be sound before merging any patches that deal
> > with these variables.
>
> Here is a simple summary:
>
> When secure boot is enabled:
> - db/dbx: Can be trusted because they are authenticated variables.
> (unless end user doesn't want to trust db/dbx)
> - MokListRT:
> - with shim: MokListRT will be created. It can be trusted.
> - without shim: MokListRT will not be created.
> MOK protected kernel module or file can not be
> verified.
>

So who polices that MoKlistRT is not created?

> When secure boot is disabled:
> - db/dbx: Can be trusted because they are authenticated variables.
> (unless end user doesn't want to trust db/dbx)

Is this true? Does non-secure boot capable firmware still guarantee
that db/dbx can't be created?

> - MokListRT:
> - with shim: MokListRT will be created. But it can not be trusted.
> MOK protected kernel module or file can not be
> verified.
> - without shim: MokListRT will not be created.
> MOK protected kernel module or file can not be
> verified.
>

Enabling this securely involves more than connecting the dots when it
comes to db, MokListRT etc. I would like for someone to convince me as
a maintainer, as well as the IMA maintainers, that the security
requirements are still met in all cases we care about. Just copying db
and MokListRT into a kernel keychain doesn't appear to do that, and if
it does, it needs more documentation to clarify that.