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

[Jarkko Sakkinen <jarkko@kernel.org>]

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 [*].

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.

> Calling panic() seems OK here. Is there a better way to stop the kernel for
> such critical error? If the kernel cannot allocate memory at this time, it
> would be useless to try continuing booting.

[*] https://lore.kernel.org/linux-sgx/YA0tvOGp%2FshchVhu@kroah.com/

BR, Jarkko