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    Subject[PATCH 27/27] 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>
    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(-)

    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 */
    --
    2.21.0.392.gf8f6787159e-goog
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