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    Subject[PATCH V40 25/29] kexec: Allow kexec_file() with appropriate IMA policy when locked down
    From
    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>
    Acked-by: Mimi Zohar <zohar@linux.ibm.com>
    Cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com>
    Cc: linux-integrity@vger.kernel.org
    Signed-off-by: James Morris <jmorris@namei.org>
    ---
    include/linux/ima.h | 9 ++++++
    kernel/kexec_file.c | 10 +++++-
    security/integrity/ima/ima.h | 2 ++
    security/integrity/ima/ima_main.c | 2 +-
    security/integrity/ima/ima_policy.c | 50 +++++++++++++++++++++++++++++
    5 files changed, 71 insertions(+), 2 deletions(-)

    diff --git a/include/linux/ima.h b/include/linux/ima.h
    index 00036d2f57c3..8e2f324fb901 100644
    --- a/include/linux/ima.h
    +++ b/include/linux/ima.h
    @@ -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 */
    diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
    index 43109ef4d6bf..7f4a618fc8c1 100644
    --- a/kernel/kexec_file.c
    +++ b/kernel/kexec_file.c
    @@ -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
    diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
    index ca10917b5f89..874bd77d3b91 100644
    --- a/security/integrity/ima/ima.h
    +++ b/security/integrity/ima/ima.h
    @@ -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
    diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
    index 1cffda4412b7..1747bc7bcb60 100644
    --- a/security/integrity/ima/ima_main.c
    +++ b/security/integrity/ima/ima_main.c
    @@ -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,
    diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
    index 7b53f2ca58e2..b8773f05f9da 100644
    --- a/security/integrity/ima/ima_policy.c
    +++ b/security/integrity/ima/ima_policy.c
    @@ -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 */
    --
    2.23.0.rc1.153.gdeed80330f-goog
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