[PATCH v4 5/7] ima: suspend measurements during buffer copy at kexec execute

[Stefan Berger]

On 1/22/24 13:38, Tushar Sugandhi wrote:
> New measurements added to the IMA log while the log is being copied
> during the kexec 'execute' may not get copied over.  This can cause the
> measurement log to be out of sync with the IMA TPM PCR, which could

It could be out of sync with any PCR that IMA extends, most often only 
the IMA TPM PCR.

> result in breaking the integrity of the measurements after kexec soft
> reboot.
> 
> Implement and call the functions ima_measurements_suspend() and
> ima_measurements_resume() from ima_update_kexec_buffer().
> 
> Add a check in the ima_add_template_entry() function not to measure
> events when 'suspend_ima_measurements' flag is set.
> 
> This ensures the integrity of the IMA log while it is being copied over
> to the new Kernel during kexec 'execute'.
> 
> Signed-off-by: Tushar Sugandhi <tusharsu at linux.microsoft.com>

Reviewed-by: Stefan Berger <stefanb at linux.ibm.com>

> ---
>   security/integrity/ima/ima.h       |  2 ++
>   security/integrity/ima/ima_kexec.c |  7 +++++++
>   security/integrity/ima/ima_queue.c | 32 ++++++++++++++++++++++++++++++
>   3 files changed, 41 insertions(+)
> 
> diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
> index c29db699c996..49a6047dd8eb 100644
> --- a/security/integrity/ima/ima.h
> +++ b/security/integrity/ima/ima.h
> @@ -161,6 +161,8 @@ bool ima_template_has_modsig(const struct ima_template_desc *ima_template);
>   int ima_restore_measurement_entry(struct ima_template_entry *entry);
>   int ima_restore_measurement_list(loff_t bufsize, void *buf);
>   int ima_measurements_show(struct seq_file *m, void *v);
> +void ima_measurements_suspend(void);
> +void ima_measurements_resume(void);
>   unsigned long ima_get_binary_runtime_size(void);
>   int ima_init_template(void);
>   void ima_init_template_list(void);
> diff --git a/security/integrity/ima/ima_kexec.c b/security/integrity/ima/ima_kexec.c
> index 25150bfc7129..f26b5b342d84 100644
> --- a/security/integrity/ima/ima_kexec.c
> +++ b/security/integrity/ima/ima_kexec.c
> @@ -179,6 +179,7 @@ static int ima_update_kexec_buffer(struct notifier_block *self,
>   	void *buf = NULL;
>   	size_t buf_size;
>   	int ret = NOTIFY_OK;
> +	bool resume = false;
>   
>   	if (!kexec_in_progress) {
>   		pr_info("%s: No kexec in progress.\n", __func__);
> @@ -190,12 +191,15 @@ static int ima_update_kexec_buffer(struct notifier_block *self,
>   		return ret;
>   	}
>   
> +	ima_measurements_suspend();
> +
>   	ret = ima_dump_measurement_list(&buf_size, &buf,
>   					kexec_segment_size);
>   
>   	if (!buf) {
>   		pr_err("%s: Dump measurements failed. Error:%d\n",
>   		       __func__, ret);
> +		resume = true;
>   		goto out;
>   	}
>   	memcpy(ima_kexec_buffer, buf, buf_size);
> @@ -203,6 +207,9 @@ static int ima_update_kexec_buffer(struct notifier_block *self,
>   	kimage_unmap_segment(ima_kexec_buffer);
>   	ima_kexec_buffer = NULL;
>   
> +	if (resume)
> +		ima_measurements_resume();
> +
>   	return ret;
>   }
>   
> diff --git a/security/integrity/ima/ima_queue.c b/security/integrity/ima/ima_queue.c
> index 532da87ce519..5946a26a2849 100644
> --- a/security/integrity/ima/ima_queue.c
> +++ b/security/integrity/ima/ima_queue.c
> @@ -44,6 +44,11 @@ struct ima_h_table ima_htable = {
>    */
>   static DEFINE_MUTEX(ima_extend_list_mutex);
>   
> +/*
> + * Used internally by the kernel to suspend-resume ima measurements.
> + */
> +static atomic_t suspend_ima_measurements;
> +
>   /* lookup up the digest value in the hash table, and return the entry */
>   static struct ima_queue_entry *ima_lookup_digest_entry(u8 *digest_value,
>   						       int pcr)
> @@ -148,6 +153,20 @@ static int ima_pcr_extend(struct tpm_digest *digests_arg, int pcr)
>   	return result;
>   }
>   
> +void ima_measurements_suspend(void)
> +{
> +	mutex_lock(&ima_extend_list_mutex);
> +	atomic_set(&suspend_ima_measurements, 1);
> +	mutex_unlock(&ima_extend_list_mutex);
> +}
> +
> +void ima_measurements_resume(void)
> +{
> +	mutex_lock(&ima_extend_list_mutex);
> +	atomic_set(&suspend_ima_measurements, 0);
> +	mutex_unlock(&ima_extend_list_mutex);
> +}
> +
>   /*
>    * Add template entry to the measurement list and hash table, and
>    * extend the pcr.
> @@ -176,6 +195,19 @@ int ima_add_template_entry(struct ima_template_entry *entry, int violation,
>   		}
>   	}
>   
> +	/*
> +	 * suspend_ima_measurements will be set if the system is
> +	 * undergoing kexec soft boot to a new kernel.
> +	 * suspending measurements in this short window ensures the
> +	 * consistency of the IMA measurement list during copying
> +	 * of the kexec buffer.
> +	 */
> +	if (atomic_read(&suspend_ima_measurements)) {
> +		audit_cause = "measurements_suspended";
> +		audit_info = 0;
> +		goto out;
> +	}
> +
>   	result = ima_add_digest_entry(entry,
>   				      !IS_ENABLED(CONFIG_IMA_DISABLE_HTABLE));
>   	if (result < 0) {