Re: [PATCH v8 5/5] certs: Allow root user to append signed hashes to the blacklist keyring

Jarkko Sakkinen
2022-03-09T23:11:59Z

On Wed, Mar 09, 2022 at 07:36:50PM +0100, Mickaël Salaün wrote:
>
> On 09/03/2022 17:01, Jarkko Sakkinen wrote:
> > On Tue, Mar 08, 2022 at 05:02:23PM +0100, Mickaël Salaün wrote:
> > >
> > > On 08/03/2022 14:19, Jarkko Sakkinen wrote:
> > > > On Tue, Mar 08, 2022 at 01:18:28PM +0100, Mickaël Salaün wrote:
> > > > >
> > > > > On 08/03/2022 12:53, Jarkko Sakkinen wrote:
> > > > > > On Mon, Jul 12, 2021 at 07:03:13PM +0200, Mickaël Salaün wrote:
> > > > > > > From: Mickaël Salaün <mic@linux.microsoft.com>
> > > > > > >
> > > > > > > Add a kernel option SYSTEM_BLACKLIST_AUTH_UPDATE to enable the root user
> > > > > > > to dynamically add new keys to the blacklist keyring. This enables to
> > > > > > > invalidate new certificates, either from being loaded in a keyring, or
> > > > > > > from being trusted in a PKCS#7 certificate chain. This also enables to
> > > > > > > add new file hashes to be denied by the integrity infrastructure.
> > > > > > >
> > > > > > > Being able to untrust a certificate which could have normaly been
> > > > > > > trusted is a sensitive operation. This is why adding new hashes to the
> > > > > > > blacklist keyring is only allowed when these hashes are signed and
> > > > > > > vouched by the builtin trusted keyring. A blacklist hash is stored as a
> > > > > > > key description. The PKCS#7 signature of this description must be
> > > > > > > provided as the key payload.
> > > > > > >
> > > > > > > Marking a certificate as untrusted should be enforced while the system
> > > > > > > is running. It is then forbiden to remove such blacklist keys.
> > > > > > >
> > > > > > > Update blacklist keyring, blacklist key and revoked certificate access rights:
> > > > > > > * allows the root user to search for a specific blacklisted hash, which
> > > > > > > make sense because the descriptions are already viewable;
> > > > > > > * forbids key update (blacklist and asymmetric ones);
> > > > > > > * restricts kernel rights on the blacklist keyring to align with the
> > > > > > > root user rights.
> > > > > > >
> > > > > > > See help in tools/certs/print-cert-tbs-hash.sh .
> > > > > > >
> > > > > > > Cc: David Howells <dhowells@redhat.com>
> > > > > > > Cc: David Woodhouse <dwmw2@infradead.org>
> > > > > > > Cc: Eric Snowberg <eric.snowberg@oracle.com>
> > > > > > > Cc: Jarkko Sakkinen <jarkko@kernel.org>
> > > > > > > Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
> > > > > > > Link: https://lore.kernel.org/r/20210712170313.884724-6-mic@digikod.net
> > > > > > > ---
> > > > > > >
> > > > > > > Changes since v6:
> > > > > > > * Rebase on keys-cve-2020-26541-v3: commit ebd9c2ae369a ("integrity:
> > > > > > > Load mokx variables into the blacklist keyring").
> > > > > > >
> > > > > > > Changes since v5:
> > > > > > > * Rebase on keys-next, fix Kconfig conflict, and update the asymmetric
> > > > > > > key rights added to the blacklist keyring by the new
> > > > > > > add_key_to_revocation_list(): align with blacklist key rights by
> > > > > > > removing KEY_POS_WRITE as a safeguard, and add
> > > > > > > KEY_ALLOC_BYPASS_RESTRICTION to not be subject to
> > > > > > > restrict_link_for_blacklist() that only allows blacklist key types to
> > > > > > > be added to the keyring.
> > > > > > > * Change the return code for restrict_link_for_blacklist() from -EPERM
> > > > > > > to -EOPNOTSUPP to align with asymmetric key keyrings.
> > > > > > >
> > > > > > > Changes since v3:
> > > > > > > * Update commit message for print-cert-tbs-hash.sh .
> > > > > > >
> > > > > > > Changes since v2:
> > > > > > > * Add comment for blacklist_key_instantiate().
> > > > > > > ---
> > > > > > > certs/Kconfig | 10 +++++
> > > > > > > certs/blacklist.c | 96 ++++++++++++++++++++++++++++++++++++-----------
> > > > > > > 2 files changed, 85 insertions(+), 21 deletions(-)
> > > > > > >
> > > > > > > diff --git a/certs/Kconfig b/certs/Kconfig
> > > > > > > index 0fbe184ceca5..e0e524b7eff9 100644
> > > > > > > --- a/certs/Kconfig
> > > > > > > +++ b/certs/Kconfig
> > > > > > > @@ -103,4 +103,14 @@ config SYSTEM_REVOCATION_KEYS
> > > > > > > containing X.509 certificates to be included in the default blacklist
> > > > > > > keyring.
> > > > > > > +config SYSTEM_BLACKLIST_AUTH_UPDATE
> > > > > > > + bool "Allow root to add signed blacklist keys"
> > > > > > > + depends on SYSTEM_BLACKLIST_KEYRING
> > > > > > > + depends on SYSTEM_DATA_VERIFICATION
> > > > > > > + help
> > > > > > > + If set, provide the ability to load new blacklist keys at run time if
> > > > > > > + they are signed and vouched by a certificate from the builtin trusted
> > > > > > > + keyring. The PKCS#7 signature of the description is set in the key
> > > > > > > + payload. Blacklist keys cannot be removed.
> > > > > > > +
> > > > > > > endmenu
> > > > > > > diff --git a/certs/blacklist.c b/certs/blacklist.c
> > > > > > > index b254c87ceb3a..486ce0dd8e9c 100644
> > > > > > > --- a/certs/blacklist.c
> > > > > > > +++ b/certs/blacklist.c
> > > > > > > @@ -15,6 +15,7 @@
> > > > > > > #include <linux/err.h>
> > > > > > > #include <linux/seq_file.h>
> > > > > > > #include <linux/uidgid.h>
> > > > > > > +#include <linux/verification.h>
> > > > > > > #include <keys/system_keyring.h>
> > > > > > > #include "blacklist.h"
> > > > > > > #include "common.h"
> > > > > > > @@ -26,6 +27,9 @@
> > > > > > > */
> > > > > > > #define MAX_HASH_LEN 128
> > > > > > > +#define BLACKLIST_KEY_PERM (KEY_POS_SEARCH | KEY_POS_VIEW | \
> > > > > > > + KEY_USR_SEARCH | KEY_USR_VIEW)
> > > > > > > +
> > > > > > > static const char tbs_prefix[] = "tbs";
> > > > > > > static const char bin_prefix[] = "bin";
> > > > > > > @@ -80,19 +84,51 @@ static int blacklist_vet_description(const char *desc)
> > > > > > > return 0;
> > > > > > > }
> > > > > > > -/*
> > > > > > > - * The hash to be blacklisted is expected to be in the description. There will
> > > > > > > - * be no payload.
> > > > > > > - */
> > > > > > > -static int blacklist_preparse(struct key_preparsed_payload *prep)
> > > > > > > +static int blacklist_key_instantiate(struct key *key,
> > > > > > > + struct key_preparsed_payload *prep)
> > > > > > > {
> > > > > > > - if (prep->datalen > 0)
> > > > > > > - return -EINVAL;
> > > > > > > - return 0;
> > > > > > > +#ifdef CONFIG_SYSTEM_BLACKLIST_AUTH_UPDATE
> > > > > > > + int err;
> > > > > > > +#endif
> > > > > > > +
> > > > > > > + /* Sets safe default permissions for keys loaded by user space. */
> > > > > > > + key->perm = BLACKLIST_KEY_PERM;
> > > > > > > +
> > > > > > > + /*
> > > > > > > + * Skips the authentication step for builtin hashes, they are not
> > > > > > > + * signed but still trusted.
> > > > > > > + */
> > > > > > > + if (key->flags & (1 << KEY_FLAG_BUILTIN))
> > > > > > > + goto out;
> > > > > > > +
> > > > > > > +#ifdef CONFIG_SYSTEM_BLACKLIST_AUTH_UPDATE
> > > > > > > + /*
> > > > > > > + * Verifies the description's PKCS#7 signature against the builtin
> > > > > > > + * trusted keyring.
> > > > > > > + */
> > > > > > > + err = verify_pkcs7_signature(key->description,
> > > > > > > + strlen(key->description), prep->data, prep->datalen,
> > > > > > > + NULL, VERIFYING_UNSPECIFIED_SIGNATURE, NULL, NULL);
> > > > > > > + if (err)
> > > > > > > + return err;
> > > > > > > +#else
> > > > > > > + /*
> > > > > > > + * It should not be possible to come here because the keyring doesn't
> > > > > > > + * have KEY_USR_WRITE and the only other way to call this function is
> > > > > > > + * for builtin hashes.
> > > > > > > + */
> > > > > > > + WARN_ON_ONCE(1);
> > > > > > > + return -EPERM;
> > > > > > > +#endif
> > > > > > > +
> > > > > > > +out:
> > > > > > > + return generic_key_instantiate(key, prep);
> > > > > > > }
> > > > > > > -static void blacklist_free_preparse(struct key_preparsed_payload *prep)
> > > > > > > +static int blacklist_key_update(struct key *key,
> > > > > > > + struct key_preparsed_payload *prep)
> > > > > > > {
> > > > > > > + return -EPERM;
> > > > > > > }
> > > > > > > static void blacklist_describe(const struct key *key, struct seq_file *m)
> > > > > > > @@ -103,9 +139,8 @@ static void blacklist_describe(const struct key *key, struct seq_file *m)
> > > > > > > static struct key_type key_type_blacklist = {
> > > > > > > .name = "blacklist",
> > > > > > > .vet_description = blacklist_vet_description,
> > > > > > > - .preparse = blacklist_preparse,
> > > > > > > - .free_preparse = blacklist_free_preparse,
> > > > > > > - .instantiate = generic_key_instantiate,
> > > > > > > + .instantiate = blacklist_key_instantiate,
> > > > > > > + .update = blacklist_key_update,
> > > > > > > .describe = blacklist_describe,
> > > > > > > };
> > > > > > > @@ -154,8 +189,7 @@ static int mark_raw_hash_blacklisted(const char *hash)
> > > > > > > hash,
> > > > > > > NULL,
> > > > > > > 0,
> > > > > > > - ((KEY_POS_ALL & ~KEY_POS_SETATTR) |
> > > > > > > - KEY_USR_VIEW),
> > > > > > > + BLACKLIST_KEY_PERM,
> > > > > > > KEY_ALLOC_NOT_IN_QUOTA |
> > > > > > > KEY_ALLOC_BUILT_IN);
> > > > > > > if (IS_ERR(key)) {
> > > > > > > @@ -232,8 +266,10 @@ int add_key_to_revocation_list(const char *data, size_t size)
> > > > > > > NULL,
> > > > > > > data,
> > > > > > > size,
> > > > > > > - ((KEY_POS_ALL & ~KEY_POS_SETATTR) | KEY_USR_VIEW),
> > > > > > > - KEY_ALLOC_NOT_IN_QUOTA | KEY_ALLOC_BUILT_IN);
> > > > > > > + KEY_POS_VIEW | KEY_POS_READ | KEY_POS_SEARCH
> > > > > > > + | KEY_USR_VIEW,
> > > > > > > + KEY_ALLOC_NOT_IN_QUOTA | KEY_ALLOC_BUILT_IN
> > > > > > > + | KEY_ALLOC_BYPASS_RESTRICTION);
> > > > > > > if (IS_ERR(key)) {
> > > > > > > pr_err("Problem with revocation key (%ld)\n", PTR_ERR(key));
> > > > > > > @@ -260,25 +296,43 @@ int is_key_on_revocation_list(struct pkcs7_message *pkcs7)
> > > > > > > }
> > > > > > > #endif
> > > > > > > +static int restrict_link_for_blacklist(struct key *dest_keyring,
> > > > > > > + const struct key_type *type, const union key_payload *payload,
> > > > > > > + struct key *restrict_key)
> > > > > > > +{
> > > > > > > + if (type == &key_type_blacklist)
> > > > > > > + return 0;
> > > > > > > + return -EOPNOTSUPP;
> > > > > > > +}
> > > > > > > +
> > > > > > > /*
> > > > > > > * Initialise the blacklist
> > > > > > > */
> > > > > > > static int __init blacklist_init(void)
> > > > > > > {
> > > > > > > const char *const *bl;
> > > > > > > + struct key_restriction *restriction;
> > > > > > > if (register_key_type(&key_type_blacklist) < 0)
> > > > > > > panic("Can't allocate system blacklist key type\n");
> > > > > > > + restriction = kzalloc(sizeof(*restriction), GFP_KERNEL);
> > > > > > > + if (!restriction)
> > > > > > > + panic("Can't allocate blacklist keyring restriction\n");
> > > > > >
> > > > > >
> > > > > > This prevents me from taking this to my pull request. In moderns standards,
> > > > > > no new BUG_ON(), panic() etc. should never added to the kernel.
> > > > > >
> > > > > > I missed this in my review.
> > > > > >
> > > > > > This should rather be e.g.
> > > > > >
> > > > > > restriction = kzalloc(sizeof(*restriction), GFP_KERNEL);
> > > > > > if (!restriction) {
> > > > > > pr_err("Can't allocate blacklist keyring restriction\n");
> > > > > > return 0;
> > > > > > }
> > > > > >
> > > > > > Unfortunately I need to drop this patch set, because adding new panic()
> > > > > > is simply a no-go.
> > > > >
> > > > > I agree that panic() is not great in general, but I followed the other part
> > > > > of the code (just above) that do the same. This part of the kernel should
> > > > > failed if critical memory allocation failed at boot time (only). It doesn't
> > > > > impact the kernel once it is running. I don't think that just ignoring this
> > > > > error with return 0 is fine, after all it's a critical error right?
> > > >
> > > > It's not good reason enough to crash the whole kernel, even if it is a
> > > > critical error (e.g. run-time foresincs). Even WARN() is not recommended
> > > > these days [*].
> > >
> > > I think that what Greg said in this email is that WARN*() should only be
> > > used for cases that should never happen, it is definitely not deprecated,
> > > but WARN_ON_ONCE() may be a better idea though. WARN*() helps detect such
> > > thought-to-be-impossible cases, that can happen e.g. with code refactoring.
> > >
> > > A lot of initialization/boot code (e.g. without user space nor external
> > > interactions, mostly __init functions) do panic if there is unexpected and
> > > unrecoverable errors like failed memory allocations. I think handling such
> > > errors otherwise would be more complex for no benefit. Moreover, delegating
> > > such error handling to user space could create new (silent) issues.
> >
> > To crash the whole kernel, you should be able to clearly explain why it
> > makes sense in the situation.
>
> If there is no enough memory to allocate 24 bytes (struct key_restriction)
> during early boot, I really doubt the kernel can do much.
>
> >
> > > >
> > > > For the existing panic()-statements: I'm happy to review patches that
> > > > render them out. >
> > > > Not sure tho, if this fails should it be then "everything blacklisted".
> > > > Just one thing to consider.
> > >
> > > Well, if it fail it will be "nothing will work afterwards". Do you have a
> > > working and useful scenario for this kind of error?
> >
> > So you have zero chances to get a shell without blacklist just to do
> > kernel forensics?
>
> Right, I don't think the kernel can launch any process (nor continue early
> boot) if it cannot allocate 24 bytes.

initcall is just wrong layer to choose to crash the kernel. It should be
either do_initcall_level() or do_one_initcall() that should care about
this (or not care). You can print error message and return -ENODEV;

BR, Jarkko