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@xxxxxxxxxxxxxxxxxxx> > > > > > > > > > > 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@xxxxxxxxxx> > > > > > Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx> > > > > > Cc: Eric Snowberg <eric.snowberg@xxxxxxxxxx> > > > > > Cc: Jarkko Sakkinen <jarkko@xxxxxxxxxx> > > > > > Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx> > > > > > Link: https://lore.kernel.org/r/20210712170313.884724-6-mic@xxxxxxxxxxx > > > > > --- > > > > > > > > > > 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. > > > > 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? BR, Jarkko
