[PATCH V35 25/29] kexec: Allow kexec_file() with appropriate IMA policy when locked down
From: Matthew Garrett <hidden>
Date: 2019-07-15 20:01:32
Also in:
linux-integrity, linux-security-module, lkml
Subsystem:
extended verification module (evm), integrity measurement architecture (ima), kexec, security subsystem, the rest · Maintainers:
Mimi Zohar, Roberto Sassu, Dmitry Kasatkin, Andrew Morton, Baoquan He, Mike Rapoport, Pasha Tatashin, Pratyush Yadav, Paul Moore, James Morris, "Serge E. Hallyn", Linus Torvalds
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 <redacted> Acked-by: 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 | 12 +++++-- security/integrity/ima/ima.h | 2 ++ security/integrity/ima/ima_main.c | 2 +- security/integrity/ima/ima_policy.c | 50 +++++++++++++++++++++++++++++ 5 files changed, 72 insertions(+), 3 deletions(-)
diff --git a/include/linux/ima.h b/include/linux/ima.h
index a20ad398d260..1c37f17f7203 100644
--- a/include/linux/ima.h
+++ b/include/linux/ima.h@@ -131,4 +131,13 @@ 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_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 dd06f1070d66..13c9960a5860 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c@@ -228,9 +228,17 @@ kimage_file_prepare_segments(struct kimage *image, int kernel_fd, int initrd_fd, goto out; } - ret = security_locked_down(LOCKDOWN_KEXEC); - if (ret) + ret = 0; + + /* 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) && + security_locked_down(LOCKDOWN_KEXEC)) { + ret = -EPERM; goto out; + } break;
diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index 011b91c79351..64dcb11cf444 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h@@ -113,6 +113,8 @@ 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 584019728660..b9f57503af2c 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c@@ -502,7 +502,7 @@ 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 6df7f641ff66..827f1e33fe86 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c@@ -1456,3 +1456,53 @@ 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 */
--
2.22.0.510.g264f2c817a-goog