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 <[email protected]> --- include/linux/ima.h | 9 +++++ kernel/kexec_file.c | 9 +++-- security/integrity/ima/ima.h | 3 ++ security/integrity/ima/ima_main.c | 2 +- security/integrity/ima/ima_policy.c | 52 +++++++++++++++++++++++++++++ 5 files changed, 72 insertions(+), 3 deletions(-) diff --git a/include/linux/ima.h b/include/linux/ima.h index dc12fbcf484c..a78c04580a3c 100644 --- a/include/linux/ima.h +++ b/include/linux/ima.h @@ -132,4 +132,13 @@ 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..3598f93a731a 100644 --- a/kernel/kexec_file.c +++ b/kernel/kexec_file.c @@ -20,11 +20,11 @@ #include <linux/mutex.h> #include <linux/list.h> #include <linux/fs.h> -#include <linux/ima.h> #include <crypto/hash.h> #include <crypto/sha.h> #include <linux/elf.h> #include <linux/elfcore.h> +#include <linux/ima.h> #include <linux/kernel.h> #include <linux/syscalls.h> #include <linux/vmalloc.h> @@ -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..93a6825bfcaf 100644 --- a/security/integrity/ima/ima.h +++ b/security/integrity/ima/ima.h @@ -20,6 +20,7 @@ #include <linux/types.h> #include <linux/crypto.h> #include <linux/fs.h> +#include <linux/ima.h> #include <linux/security.h> #include <linux/hash.h> #include <linux/tpm.h> @@ -115,6 +116,8 @@ 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 357edd140c09..927fe889201a 100644 --- a/security/integrity/ima/ima_main.c +++ b/security/integrity/ima/ima_main.c @@ -473,7 +473,7 @@ 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 8bc8a1c8cb3f..bbd1d5b3d45a 100644 --- a/security/integrity/ima/ima_policy.c +++ b/security/integrity/ima/ima_policy.c @@ -21,6 +21,7 @@ #include <linux/genhd.h> #include <linux/seq_file.h> #include <linux/ima.h> +#include <linux/evm.h> #include "ima.h" @@ -1336,4 +1337,55 @@ int ima_policy_show(struct seq_file *m, void *v) seq_puts(m, "\n"); 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 */ -- 2.21.0.225.g810b269d1ac-goog _______________________________________________ kexec mailing list [email protected] http://lists.infradead.org/mailman/listinfo/kexec
