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Commit 29d3c1c8 authored by Matthew Garrett's avatar Matthew Garrett Committed by James Morris
Browse files

kexec: Allow kexec_file() with appropriate IMA policy when locked down 



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: default avatarMatthew Garrett <mjg59@google.com>
Acked-by: default avatarMimi Zohar <zohar@linux.ibm.com>
Cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com>
Cc: linux-integrity@vger.kernel.org
Signed-off-by: default avatarJames Morris <jmorris@namei.org>
parent b0c8fdc7

include/linux/ima.h

+9 −0
Original line number Diff line number Diff line
@@ -129,4 +129,13 @@ 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 */

kernel/kexec_file.c

+9 −1
Original line number Diff line number Diff line
@@ -208,7 +208,15 @@ kimage_validate_signature(struct kimage *image) 
			return ret;

		}



		return security_locked_down(LOCKDOWN_KEXEC);

		/* 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))

			return -EPERM;



		return 0;



		/* All other errors are fatal, including nomem, unparseable

		 * signatures and signature check failures - even if signatures

security/integrity/ima/ima.h

+2 −0
Original line number Diff line number Diff line
@@ -111,6 +111,8 @@ struct ima_kexec_hdr { 
	u64 count;

};



extern const int read_idmap[];



#ifdef CONFIG_HAVE_IMA_KEXEC

void ima_load_kexec_buffer(void);

#else

security/integrity/ima/ima_main.c

+1 −1
Original line number Diff line number Diff line
@@ -469,7 +469,7 @@ 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,

security/integrity/ima/ima_policy.c

+50 −0
Original line number Diff line number Diff line
@@ -1339,3 +1339,53 @@ 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 */