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Date: Wed, 11 Nov 2020 19:58:30 -0500
From: Mimi Zohar <zohar@...ux.ibm.com>
To: Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
Cc: tusharsu@...ux.microsoft.com, linux-integrity@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ima: select ima-buf template for buffer measurement
Hi Lakshmi,
On Wed, 2020-11-11 at 12:59 -0800, Lakshmi Ramasubramanian wrote:
> The default IMA template for measuring buffer should be 'ima-buf' - so
> that the measured buffer is correctly included in the IMA measurement
> log entry. But the default IMA template used for all policy rules is
> the value set for CONFIG_IMA_DEFAULT_TEMPLATE if the policy rule does
> not specify a template.
The second sentence defines the current status. The first sentence
describes the problem. I would reverse the sentence order.
^measuring buffer -> buffer measurements
> IMA does not take into account the template
> requirements of different rules when choosing a default template for
> a given policy rule. This breaks the buffer measurement if the template
> is not provided as part of the rule because the default template could
> be different than 'ima-buf'.
Does the above paragraph add anything new? Instead describe the
problem. Perhaps something like:
With the default template format, buffer measurements are added to the
measurement list, but do not include the buffer data, making it
difficult, if not impossible, to validate. Including "ima-buf"
template records in the measurement list by default, should not impact
existing attestation servers without "ima-buf" template support.
>
> For example, the following IMA policy rule enables measuring
> the command line arguments passed to the new kernel on kexec system call.
>
> measure func=KEXEC_CMDLINE
>
> The IMA template selected should be 'ima-buf' to have the measured
> command line arguments included in the IMA measurement log entry.
> Instead the default IMA template is selected, which could be different
> than 'ima-buf'.
When upstreaming a new type of measurement, including an example
provides how to validate the new template data. Not every patch
description requires an example.
>
> Initialize a global 'ima-buf' template and select that template,
> by default, for measuring buffer.
Good.
>
> Signed-off-by: Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
> ---
> security/integrity/ima/ima.h | 1 +
> security/integrity/ima/ima_main.c | 17 +++++------------
> security/integrity/ima/ima_policy.c | 2 +-
> security/integrity/ima/ima_template.c | 25 +++++++++++++++++++++++++
> 4 files changed, 32 insertions(+), 13 deletions(-)
>
> diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
> index 6ebefec616e4..8e8b1e3cb847 100644
> --- a/security/integrity/ima/ima.h
> +++ b/security/integrity/ima/ima.h
> @@ -156,6 +156,7 @@ int template_desc_init_fields(const char *template_fmt,
> const struct ima_template_field ***fields,
> int *num_fields);
> struct ima_template_desc *ima_template_desc_current(void);
> +struct ima_template_desc *ima_template_desc_buf(void);
> struct ima_template_desc *lookup_template_desc(const char *name);
> bool ima_template_has_modsig(const struct ima_template_desc *ima_template);
> int ima_restore_measurement_entry(struct ima_template_entry *entry);
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index a962b23e0429..3646ae763ba9 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -413,7 +413,7 @@ int ima_file_mmap(struct file *file, unsigned long prot)
> */
> int ima_file_mprotect(struct vm_area_struct *vma, unsigned long prot)
> {
> - struct ima_template_desc *template;
> + struct ima_template_desc *template = NULL;
> struct file *file = vma->vm_file;
> char filename[NAME_MAX];
> char *pathbuf = NULL;
> @@ -802,7 +802,7 @@ void process_buffer_measurement(struct inode *inode, const void *buf, int size,
> .filename = eventname,
> .buf = buf,
> .buf_len = size};
> - struct ima_template_desc *template = NULL;
> + struct ima_template_desc *template = ima_template_desc_buf();
> struct {
> struct ima_digest_data hdr;
> char digest[IMA_MAX_DIGEST_SIZE];
> @@ -833,16 +833,9 @@ void process_buffer_measurement(struct inode *inode, const void *buf, int size,
> pcr = CONFIG_IMA_MEASURE_PCR_IDX;
>
> if (!template) {
> - template = lookup_template_desc("ima-buf");
> - ret = template_desc_init_fields(template->fmt,
> - &(template->fields),
> - &(template->num_fields));
> - if (ret < 0) {
> - pr_err("template %s init failed, result: %d\n",
> - (strlen(template->name) ?
> - template->name : template->fmt), ret);
> - return;
> - }
> + ret = -EINVAL;
> + audit_cause = "ima_template_desc_buf";
> + goto out;
Normally a test follows the variable assignment, but in this case, the
check is being deferred in case there isn't a policy rule.
> }
>
> iint.ima_hash = &hash.hdr;
> diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> index 9b5adeaa47fc..823a0c1379cb 100644
> --- a/security/integrity/ima/ima_policy.c
> +++ b/security/integrity/ima/ima_policy.c
> @@ -628,7 +628,7 @@ int ima_match_policy(struct inode *inode, const struct cred *cred, u32 secid,
> struct ima_rule_entry *entry;
> int action = 0, actmask = flags | (flags << 1);
>
> - if (template_desc)
> + if (template_desc && !*template_desc)
> *template_desc = ima_template_desc_current();
>
> rcu_read_lock();
> diff --git a/security/integrity/ima/ima_template.c b/security/integrity/ima/ima_template.c
> index 1e89e2d3851f..e53fce2c1610 100644
> --- a/security/integrity/ima/ima_template.c
> +++ b/security/integrity/ima/ima_template.c
> @@ -55,6 +55,7 @@ static const struct ima_template_field supported_fields[] = {
> #define MAX_TEMPLATE_NAME_LEN sizeof("d-ng|n-ng|sig|buf|d-modisg|modsig")
>
> static struct ima_template_desc *ima_template;
> +static struct ima_template_desc *ima_buf_template;
>
> /**
> * ima_template_has_modsig - Check whether template has modsig-related fields.
> @@ -252,6 +253,30 @@ struct ima_template_desc *ima_template_desc_current(void)
> return ima_template;
> }
>
> +struct ima_template_desc *ima_template_desc_buf(void)
> +{
> + struct ima_template_desc *template = NULL;
> + int ret = 0;
> +
> + if (ima_buf_template)
> + return ima_buf_template;
> +
> + ima_init_template_list();
> + template = lookup_template_desc("ima-buf");
> + if (!template)
> + return NULL;
> +
> + ret = template_desc_init_fields(template->fmt,
> + &(template->fields),
> + &(template->num_fields));
> + if (ret)
> + return NULL;
Instead of initializing the fields here, maybe it should be done in
ima_init_template()? That would remove the deferred !template test in
process_buffer_measurement() and would also simplify this function.
thanks,
Mimi
> +
> + ima_buf_template = template;
> +
> + return ima_buf_template;
> +}
> +
> int __init ima_init_template(void)
> {
> struct ima_template_desc *template = ima_template_desc_current();
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