Re: [PATCH v2] ima: instantiate the bprm_creds_for_exec() hook

Mickaël Salaün
2024-12-04T10:16:02Z

On Tue, Dec 03, 2024 at 06:34:24PM -0500, Mimi Zohar wrote:
> Like direct file execution (e.g. ./script.sh), indirect file exection
> (e.g. sh script.sh) needs to be measured and appraised. Instantiate
> the new security_bprm_creds_for_exec() hook to measure and verify the
> indirect file's integrity. Unlike direct file execution, indirect file
> execution is optionally enforced by the interpreter.
>
> Differentiate kernel and userspace enforced integrity audit messages.
>

I guess there is a missing tag:

Co-developed-by: Roberto Sassu <roberto.sassu@huawei.com>

> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

With some minor comments, this looks good to me. I'll include this patch
or the next one in my patch series. Thanks!

> ---
> Changelog v2:
> - Mickael: Use same audit messages with new audit message number
> - Stefan Berger: Return boolean from is_bprm_creds_for_exec()
>
> include/uapi/linux/audit.h | 1 +
> security/integrity/ima/ima_appraise.c | 28 +++++++++++++++++++++++++--
> security/integrity/ima/ima_main.c | 22 +++++++++++++++++++++
> 3 files changed, 49 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/audit.h b/include/uapi/linux/audit.h
> index 75e21a135483..826337905466 100644
> --- a/include/uapi/linux/audit.h
> +++ b/include/uapi/linux/audit.h
> @@ -161,6 +161,7 @@
> #define AUDIT_INTEGRITY_RULE 1805 /* policy rule */
> #define AUDIT_INTEGRITY_EVM_XATTR 1806 /* New EVM-covered xattr */
> #define AUDIT_INTEGRITY_POLICY_RULE 1807 /* IMA policy rules */
> +#define AUDIT_INTEGRITY_DATA_CHECK 1808 /* Userspace enforced data integrity */
>
> #define AUDIT_KERNEL 2000 /* Asynchronous audit record. NOT A REQUEST. */
>
> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> index 656c709b974f..144e0b39fbcd 100644
> --- a/security/integrity/ima/ima_appraise.c
> +++ b/security/integrity/ima/ima_appraise.c
> @@ -8,6 +8,7 @@
> #include <linux/module.h>
> #include <linux/init.h>
> #include <linux/file.h>
> +#include <linux/binfmts.h>
> #include <linux/fs.h>
> #include <linux/xattr.h>
> #include <linux/magic.h>
> @@ -469,6 +470,18 @@ int ima_check_blacklist(struct ima_iint_cache *iint,
> return rc;
> }
>
> +static bool is_bprm_creds_for_exec(enum ima_hooks func, struct file *file)
> +{
> + struct linux_binprm *bprm = NULL;
> +
> + if (func == BPRM_CHECK) {

struct linux_binprm *bprm;

> + bprm = container_of(&file, struct linux_binprm, file);
> + if (bprm->is_check)
> + return true;

return bprm->is_check;

> + }
> + return false;
> +}
> +
> /*
> * ima_appraise_measurement - appraise file measurement
> *
> @@ -483,6 +496,7 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> int xattr_len, const struct modsig *modsig)
> {
> static const char op[] = "appraise_data";
> + int audit_msgno = AUDIT_INTEGRITY_DATA;
> const char *cause = "unknown";
> struct dentry *dentry = file_dentry(file);
> struct inode *inode = d_backing_inode(dentry);
> @@ -494,6 +508,16 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> if (!(inode->i_opflags & IOP_XATTR) && !try_modsig)
> return INTEGRITY_UNKNOWN;
>
> + /*
> + * Unlike any of the other LSM hooks where the kernel enforces file
> + * integrity, enforcing file integrity for the bprm_creds_for_exec()
> + * LSM hook with the AT_EXECVE_CHECK flag is left up to the discretion
> + * of the script interpreter(userspace). Differentiate kernel and
> + * userspace enforced integrity audit messages.
> + */
> + if (is_bprm_creds_for_exec(func, file))
> + audit_msgno = AUDIT_INTEGRITY_DATA_CHECK;
> +
> /* If reading the xattr failed and there's no modsig, error out. */
> if (rc <= 0 && !try_modsig) {
> if (rc && rc != -ENODATA)
> @@ -569,7 +593,7 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> (iint->flags & IMA_FAIL_UNVERIFIABLE_SIGS))) {
> status = INTEGRITY_FAIL;
> cause = "unverifiable-signature";
> - integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, filename,
> + integrity_audit_msg(audit_msgno, inode, filename,
> op, cause, rc, 0);
> } else if (status != INTEGRITY_PASS) {
> /* Fix mode, but don't replace file signatures. */
> @@ -589,7 +613,7 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> status = INTEGRITY_PASS;
> }
>
> - integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, filename,
> + integrity_audit_msg(audit_msgno, inode, filename,
> op, cause, rc, 0);
> } else {
> ima_cache_flags(iint, func);
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 06132cf47016..f0830e6d0cda 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -554,6 +554,27 @@ static int ima_bprm_check(struct linux_binprm *bprm)
> MAY_EXEC, CREDS_CHECK);
> }
>
> +/**
> + * ima_bprm_creds_for_exec - collect/store/appraise measurement.
> + * @bprm: contains the linux_binprm structure
> + *
> + * Based on the IMA policy and the execvat(2) AT_CHECK flag, measure and

AT_EXECVE_CHECK

> + * appraise the integrity of a file to be executed by script interpreters.
> + * Unlike any of the other LSM hooks where the kernel enforces file integrity,
> + * enforcing file integrity is left up to the discretion of the script
> + * interpreter (userspace).
> + *
> + * On success return 0. On integrity appraisal error, assuming the file
> + * is in policy and IMA-appraisal is in enforcing mode, return -EACCES.
> + */
> +static int ima_bprm_creds_for_exec(struct linux_binprm *bprm)
> +{

We could have a comment explaining that ima_bprm_check() will not be
called a second time bi the bprm_check_security hook if bprm->is_check
is true because this hook would then not be called. This would not be a
security issue anyway, just a useless call.

> + if (!bprm->is_check)
> + return 0;
> +
> + return ima_bprm_check(bprm);
> +}
> +
> /**
> * ima_file_check - based on policy, collect/store measurement.
> * @file: pointer to the file to be measured
> @@ -1177,6 +1198,7 @@ static int __init init_ima(void)
>
> static struct security_hook_list ima_hooks[] __ro_after_init = {
> LSM_HOOK_INIT(bprm_check_security, ima_bprm_check),
> + LSM_HOOK_INIT(bprm_creds_for_exec, ima_bprm_creds_for_exec),
> LSM_HOOK_INIT(file_post_open, ima_file_check),
> LSM_HOOK_INIT(inode_post_create_tmpfile, ima_post_create_tmpfile),
> LSM_HOOK_INIT(file_release, ima_file_free),
> --
> 2.47.0
>
>