| Message ID | 20190325220954.29054-28-matthewgarrett@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | None | expand |
On Mon, 2019-03-25 at 15:09 -0700, Matthew Garrett wrote: > Systems in lockdown mode should block the kexec of untrusted kernels. > For x86 and ARM we can ensure that a kernel is trustworthy by validating > a PE signature, but this isn't possible on other architectures. On those > platforms we can use IMA digital signatures instead. Add a function to > determine whether IMA has or will verify signatures for a given event type, > and if so permit kexec_file() even if the kernel is otherwise locked down. > This is restricted to cases where CONFIG_INTEGRITY_TRUSTED_KEYRING is set > in order to prevent an attacker from loading additional keys at runtime. > > Signed-off-by: Matthew Garrett <mjg59@google.com> > Cc: Mimi Zohar <zohar@linux.ibm.com> > Cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com> > Cc: linux-integrity@vger.kernel.org Acked-by: Mimi Zohar <zohar@linux.ibm.com> > --- > include/linux/ima.h | 9 ++++++ > kernel/kexec_file.c | 7 +++- > security/integrity/ima/ima.h | 2 ++ > security/integrity/ima/ima_main.c | 2 +- > security/integrity/ima/ima_policy.c | 50 +++++++++++++++++++++++++++++ > 5 files changed, 68 insertions(+), 2 deletions(-) > > diff --git a/include/linux/ima.h b/include/linux/ima.h > index b5e16b8c50b7..05921227d700 100644 > --- a/include/linux/ima.h > +++ b/include/linux/ima.h > @@ -127,4 +127,13 @@ static inline int ima_inode_removexattr(struct dentry *dentry, > return 0; > } > #endif /* CONFIG_IMA_APPRAISE */ > + > +#if defined(CONFIG_IMA_APPRAISE) && defined(CONFIG_INTEGRITY_TRUSTED_KEYRING) > +extern bool ima_appraise_signature(enum kernel_read_file_id func); > +#else > +static inline bool ima_appraise_kexec_signature(enum kernel_read_file_id func) > +{ > + return false; > +} > +#endif /* CONFIG_IMA_APPRAISE && CONFIG_INTEGRITY_TRUSTED_KEYRING */ > #endif /* _LINUX_IMA_H */ > diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c > index 0cfe4f6f7f85..8ffa4b75c620 100644 > --- a/kernel/kexec_file.c > +++ b/kernel/kexec_file.c > @@ -240,7 +240,12 @@ kimage_file_prepare_segments(struct kimage *image, int kernel_fd, int initrd_fd, > > ret = 0; > > - if (kernel_is_locked_down(reason)) { > + /* If IMA is guaranteed to appraise a signature on the kexec > + * image, permit it even if the kernel is otherwise locked > + * down. > + */ > + if (!ima_appraise_signature(READING_KEXEC_IMAGE) && > + kernel_is_locked_down(reason)) { > ret = -EPERM; > goto out; > } > diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h > index cc12f3449a72..fe03cc6f1ca4 100644 > --- a/security/integrity/ima/ima.h > +++ b/security/integrity/ima/ima.h > @@ -115,6 +115,8 @@ struct ima_kexec_hdr { > u64 count; > }; > > +extern const int read_idmap[]; > + > #ifdef CONFIG_HAVE_IMA_KEXEC > void ima_load_kexec_buffer(void); > #else > diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c > index 4ffac4f5c647..106f06dee9d1 100644 > --- a/security/integrity/ima/ima_main.c > +++ b/security/integrity/ima/ima_main.c > @@ -442,7 +442,7 @@ int ima_read_file(struct file *file, enum kernel_read_file_id read_id) > return 0; > } > > -static const int read_idmap[READING_MAX_ID] = { > +const int read_idmap[READING_MAX_ID] = { > [READING_FIRMWARE] = FIRMWARE_CHECK, > [READING_FIRMWARE_PREALLOC_BUFFER] = FIRMWARE_CHECK, > [READING_MODULE] = MODULE_CHECK, > diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c > index 122797023bdb..f8f1cdb74a4f 100644 > --- a/security/integrity/ima/ima_policy.c > +++ b/security/integrity/ima/ima_policy.c > @@ -1341,3 +1341,53 @@ int ima_policy_show(struct seq_file *m, void *v) > return 0; > } > #endif /* CONFIG_IMA_READ_POLICY */ > + > +#if defined(CONFIG_IMA_APPRAISE) && defined(CONFIG_INTEGRITY_TRUSTED_KEYRING) > +/* > + * ima_appraise_signature: whether IMA will appraise a given function using > + * an IMA digital signature. This is restricted to cases where the kernel > + * has a set of built-in trusted keys in order to avoid an attacker simply > + * loading additional keys. > + */ > +bool ima_appraise_signature(enum kernel_read_file_id id) > +{ > + struct ima_rule_entry *entry; > + bool found = false; > + enum ima_hooks func; > + > + if (id >= READING_MAX_ID) > + return false; > + > + func = read_idmap[id] ?: FILE_CHECK; > + > + rcu_read_lock(); > + list_for_each_entry_rcu(entry, ima_rules, list) { > + if (entry->action != APPRAISE) > + continue; > + > + /* > + * A generic entry will match, but otherwise require that it > + * match the func we're looking for > + */ > + if (entry->func && entry->func != func) > + continue; > + > + /* > + * We require this to be a digital signature, not a raw IMA > + * hash. > + */ > + if (entry->flags & IMA_DIGSIG_REQUIRED) > + found = true; > + > + /* > + * We've found a rule that matches, so break now even if it > + * didn't require a digital signature - a later rule that does > + * won't override it, so would be a false positive. > + */ > + break; > + } > + > + rcu_read_unlock(); > + return found; > +} > +#endif /* CONFIG_IMA_APPRAISE && CONFIG_INTEGRITY_TRUSTED_KEYRING */
diff --git a/include/linux/ima.h b/include/linux/ima.h index b5e16b8c50b7..05921227d700 100644 --- a/include/linux/ima.h +++ b/include/linux/ima.h @@ -127,4 +127,13 @@ static inline int ima_inode_removexattr(struct dentry *dentry, return 0; } #endif /* CONFIG_IMA_APPRAISE */ + +#if defined(CONFIG_IMA_APPRAISE) && defined(CONFIG_INTEGRITY_TRUSTED_KEYRING) +extern bool ima_appraise_signature(enum kernel_read_file_id func); +#else +static inline bool ima_appraise_kexec_signature(enum kernel_read_file_id func) +{ + return false; +} +#endif /* CONFIG_IMA_APPRAISE && CONFIG_INTEGRITY_TRUSTED_KEYRING */ #endif /* _LINUX_IMA_H */ diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c index 0cfe4f6f7f85..8ffa4b75c620 100644 --- a/kernel/kexec_file.c +++ b/kernel/kexec_file.c @@ -240,7 +240,12 @@ kimage_file_prepare_segments(struct kimage *image, int kernel_fd, int initrd_fd, ret = 0; - if (kernel_is_locked_down(reason)) { + /* If IMA is guaranteed to appraise a signature on the kexec + * image, permit it even if the kernel is otherwise locked + * down. + */ + if (!ima_appraise_signature(READING_KEXEC_IMAGE) && + kernel_is_locked_down(reason)) { ret = -EPERM; goto out; } diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h index cc12f3449a72..fe03cc6f1ca4 100644 --- a/security/integrity/ima/ima.h +++ b/security/integrity/ima/ima.h @@ -115,6 +115,8 @@ struct ima_kexec_hdr { u64 count; }; +extern const int read_idmap[]; + #ifdef CONFIG_HAVE_IMA_KEXEC void ima_load_kexec_buffer(void); #else diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c index 4ffac4f5c647..106f06dee9d1 100644 --- a/security/integrity/ima/ima_main.c +++ b/security/integrity/ima/ima_main.c @@ -442,7 +442,7 @@ int ima_read_file(struct file *file, enum kernel_read_file_id read_id) return 0; } -static const int read_idmap[READING_MAX_ID] = { +const int read_idmap[READING_MAX_ID] = { [READING_FIRMWARE] = FIRMWARE_CHECK, [READING_FIRMWARE_PREALLOC_BUFFER] = FIRMWARE_CHECK, [READING_MODULE] = MODULE_CHECK, diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c index 122797023bdb..f8f1cdb74a4f 100644 --- a/security/integrity/ima/ima_policy.c +++ b/security/integrity/ima/ima_policy.c @@ -1341,3 +1341,53 @@ int ima_policy_show(struct seq_file *m, void *v) return 0; } #endif /* CONFIG_IMA_READ_POLICY */ + +#if defined(CONFIG_IMA_APPRAISE) && defined(CONFIG_INTEGRITY_TRUSTED_KEYRING) +/* + * ima_appraise_signature: whether IMA will appraise a given function using + * an IMA digital signature. This is restricted to cases where the kernel + * has a set of built-in trusted keys in order to avoid an attacker simply + * loading additional keys. + */ +bool ima_appraise_signature(enum kernel_read_file_id id) +{ + struct ima_rule_entry *entry; + bool found = false; + enum ima_hooks func; + + if (id >= READING_MAX_ID) + return false; + + func = read_idmap[id] ?: FILE_CHECK; + + rcu_read_lock(); + list_for_each_entry_rcu(entry, ima_rules, list) { + if (entry->action != APPRAISE) + continue; + + /* + * A generic entry will match, but otherwise require that it + * match the func we're looking for + */ + if (entry->func && entry->func != func) + continue; + + /* + * We require this to be a digital signature, not a raw IMA + * hash. + */ + if (entry->flags & IMA_DIGSIG_REQUIRED) + found = true; + + /* + * We've found a rule that matches, so break now even if it + * didn't require a digital signature - a later rule that does + * won't override it, so would be a false positive. + */ + break; + } + + rcu_read_unlock(); + return found; +} +#endif /* CONFIG_IMA_APPRAISE && CONFIG_INTEGRITY_TRUSTED_KEYRING */
Systems in lockdown mode should block the kexec of untrusted kernels. For x86 and ARM we can ensure that a kernel is trustworthy by validating a PE signature, but this isn't possible on other architectures. On those platforms we can use IMA digital signatures instead. Add a function to determine whether IMA has or will verify signatures for a given event type, and if so permit kexec_file() even if the kernel is otherwise locked down. This is restricted to cases where CONFIG_INTEGRITY_TRUSTED_KEYRING is set in order to prevent an attacker from loading additional keys at runtime. Signed-off-by: Matthew Garrett <mjg59@google.com> Cc: Mimi Zohar <zohar@linux.ibm.com> Cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com> Cc: linux-integrity@vger.kernel.org --- include/linux/ima.h | 9 ++++++ kernel/kexec_file.c | 7 +++- security/integrity/ima/ima.h | 2 ++ security/integrity/ima/ima_main.c | 2 +- security/integrity/ima/ima_policy.c | 50 +++++++++++++++++++++++++++++ 5 files changed, 68 insertions(+), 2 deletions(-)