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SubjectRe: [PATCH 2/2 v3] efi: print appropriate status message when loading certificates
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@gmail.com> 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@namei.org>
> > Cc: Serge E. Hallyn" <serge@hallyn.com>
> > Cc: David Howells <dhowells@redhat.com>
> > Cc: Nayna Jain <nayna@linux.ibm.com>
> > Cc: Josh Boyer <jwboyer@fedoraproject.org>
> > Cc: Mimi Zohar <zohar@linux.ibm.com>
> > Signed-off-by: "Lee, Chun-Yi" <jlee@suse.com>
> > ---
> > 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.

> > }
> > + 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.

> - secure boot might be enabled but we may be booting without shim.

Shim always creates MokListRT no matter secure boot enabled or disabled.

> - 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.

> Tweaking the severity of error messages without having a clear idea of
> the policy we are aiming to implement is likely to cause trouble down
> the road, so perhaps someone could explain what this code does, and
> how it should behave in the above circumstances.

Thanks
Joey Lee

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