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

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

[Mickaël Salaün]

For reference, here is the base patch series:
https://lore.kernel.org/all/20241112191858.162021-1-mic@digikod.net/

CCing audit@

On Wed, Nov 27, 2024 at 04:02:34PM -0500, Mimi Zohar wrote:
> Like direct file execution (e.g. ./script.sh), indirect file execution
> (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 integrity is optionally enforced by the interpreter.
> 
> Update the audit messages to differentiate between kernel and userspace
> enforced integrity.

I'm not sure to see the full picture.  What is the difference between
execveat() calls and execveat() + AT_EXECVE_CHECK calls?  Both are from
user space, the only difference is that the first can lead to a full
execution, but the intent is the same.

> 
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
> ---
>  security/integrity/ima/ima_appraise.c | 84 ++++++++++++++++++++-------
>  security/integrity/ima/ima_main.c     | 22 +++++++
>  2 files changed, 86 insertions(+), 20 deletions(-)
> 
> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> index 656c709b974f..b5f8e49cde9d 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>
> @@ -16,6 +17,7 @@
>  #include <linux/fsverity.h>
>  #include <keys/system_keyring.h>
>  #include <uapi/linux/fsverity.h>
> +#include <linux/securebits.h>
>  
>  #include "ima.h"
>  
> @@ -276,7 +278,8 @@ static int calc_file_id_hash(enum evm_ima_xattr_type type,
>   */
>  static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  			struct evm_ima_xattr_data *xattr_value, int xattr_len,
> -			enum integrity_status *status, const char **cause)
> +			enum integrity_status *status, const char **cause,
> +			bool is_check)
>  {
>  	struct ima_max_digest_data hash;
>  	struct signature_v2_hdr *sig;
> @@ -292,9 +295,11 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  		if (*status != INTEGRITY_PASS_IMMUTABLE) {
>  			if (iint->flags & IMA_DIGSIG_REQUIRED) {
>  				if (iint->flags & IMA_VERITY_REQUIRED)
> -					*cause = "verity-signature-required";
> +					*cause = !is_check ? "verity-signature-required" :
> +						"verity-signature-required(userspace)";

This looks simpler (same for all following checks):
is_check ? "verity-signature-required(userspace)" : "verity-signature-required";

>  				else
> -					*cause = "IMA-signature-required";
> +					*cause = !is_check ? "IMA-signature-required" :
> +						"IMA-signature-required(userspace)";
>  				*status = INTEGRITY_FAIL;
>  				break;
>  			}
> @@ -314,7 +319,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  		else
>  			rc = -EINVAL;
>  		if (rc) {
> -			*cause = "invalid-hash";
> +			*cause = !is_check ? "invalid-hash" :
> +				"invalid-hash(userspace)";
>  			*status = INTEGRITY_FAIL;
>  			break;
>  		}
> @@ -325,14 +331,16 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  
>  		mask = IMA_DIGSIG_REQUIRED | IMA_VERITY_REQUIRED;
>  		if ((iint->flags & mask) == mask) {
> -			*cause = "verity-signature-required";
> +			*cause = !is_check ? "verity-signature-required" :
> +				"verity-signature-required(userspace)";
>  			*status = INTEGRITY_FAIL;
>  			break;
>  		}
>  
>  		sig = (typeof(sig))xattr_value;
>  		if (sig->version >= 3) {
> -			*cause = "invalid-signature-version";
> +			*cause = !is_check ? "invalid-signature-version" :
> +				"invalid-signature-version(userspace)";
>  			*status = INTEGRITY_FAIL;
>  			break;
>  		}
> @@ -353,7 +361,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  						     iint->ima_hash->digest,
>  						     iint->ima_hash->length);
>  		if (rc) {
> -			*cause = "invalid-signature";
> +			*cause = !is_check ? "invalid-signature" :
> +				"invalid-signature(userspace)";
>  			*status = INTEGRITY_FAIL;
>  		} else {
>  			*status = INTEGRITY_PASS;
> @@ -364,7 +373,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  
>  		if (iint->flags & IMA_DIGSIG_REQUIRED) {
>  			if (!(iint->flags & IMA_VERITY_REQUIRED)) {
> -				*cause = "IMA-signature-required";
> +				*cause = !is_check ? "IMA-signature-required" :
> +					"IMA-signature-required(userspace)";
>  				*status = INTEGRITY_FAIL;
>  				break;
>  			}
> @@ -372,7 +382,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  
>  		sig = (typeof(sig))xattr_value;
>  		if (sig->version != 3) {
> -			*cause = "invalid-signature-version";
> +			*cause = !is_check ? "invalid-signature-version" :
> +				"invalid-signature-version(userspace)";
>  			*status = INTEGRITY_FAIL;
>  			break;
>  		}
> @@ -382,7 +393,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  				       container_of(&hash.hdr,
>  					       struct ima_digest_data, hdr));
>  		if (rc) {
> -			*cause = "sigv3-hashing-error";
> +			*cause = !is_check ? "sigv3-hashing-error" :
> +				"sigv3-hashing-error(userspace)";
>  			*status = INTEGRITY_FAIL;
>  			break;
>  		}
> @@ -392,7 +404,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  					     xattr_len, hash.digest,
>  					     hash.hdr.length);
>  		if (rc) {
> -			*cause = "invalid-verity-signature";
> +			*cause = !is_check ? "invalid-verity-signature" :
> +				"invalid-verify-signature(userspace)";
>  			*status = INTEGRITY_FAIL;
>  		} else {
>  			*status = INTEGRITY_PASS;
> @@ -401,7 +414,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  		break;
>  	default:
>  		*status = INTEGRITY_UNKNOWN;
> -		*cause = "unknown-ima-data";
> +		*cause = !is_check ? "unknown-ima-data" :
> +			"unknown-ima-data(userspace)";
>  		break;
>  	}
>  
> @@ -469,6 +483,18 @@ int ima_check_blacklist(struct ima_iint_cache *iint,
>  	return rc;
>  }
>  
> +static int is_bprm_creds_for_exec(enum ima_hooks func, struct file *file)
> +{
> +	struct linux_binprm *bprm = NULL;
> +
> +	if (func == BPRM_CHECK) {
> +		bprm = container_of(&file, struct linux_binprm, file);
> +		if (bprm->is_check)
> +			return 1;
> +	}
> +	return 0;
> +}
> +
>  /*
>   * ima_appraise_measurement - appraise file measurement
>   *
> @@ -489,11 +515,24 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
>  	enum integrity_status status = INTEGRITY_UNKNOWN;
>  	int rc = xattr_len;
>  	bool try_modsig = iint->flags & IMA_MODSIG_ALLOWED && modsig;
> +	bool is_check = false;
>  
>  	/* If not appraising a modsig, we need an xattr. */
>  	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 is left up to the discretion of the script interpreter
> +	 * (userspace).
> +	 *
> +	 * Since the SECBIT_EXEC_RESTRICT_FILE flag is just a hint as to
> +	 * userspace intentions, simply annotate the audit messages indicating
> +	 * a userspace based query.
> +	 */
> +	is_check = is_bprm_creds_for_exec(func, file);
> +
>  	/* If reading the xattr failed and there's no modsig, error out. */
>  	if (rc <= 0 && !try_modsig) {
>  		if (rc && rc != -ENODATA)
> @@ -501,11 +540,14 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
>  
>  		if (iint->flags & IMA_DIGSIG_REQUIRED) {
>  			if (iint->flags & IMA_VERITY_REQUIRED)
> -				cause = "verity-signature-required";
> +				cause = !is_check ? "verity-signature-required" :
> +					"verity-signature-required(userspace)";
>  			else
> -				cause = "IMA-signature-required";
> +				cause = !is_check ? "IMA-signature-required" :
> +					"IMA-signature-required(userspace)";
>  		} else {
> -			cause = "missing-hash";
> +			cause = !is_check ? "missing-hash" :
> +				"missing-hash(userspace)";
>  		}
>  
>  		status = INTEGRITY_NOLABEL;
> @@ -531,14 +573,15 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
>  			break;
>  		fallthrough;
>  	case INTEGRITY_NOLABEL:		/* No security.evm xattr. */
> -		cause = "missing-HMAC";
> +		cause = !is_check ? "missing-HMAC" : "missing-HMAC(userspace)";
>  		goto out;
>  	case INTEGRITY_FAIL_IMMUTABLE:
>  		set_bit(IMA_DIGSIG, &iint->atomic_flags);
> -		cause = "invalid-fail-immutable";
> +		cause = !is_check ? "invalid-fail-immutable" :
> +		       "invalid-fail-immutable(userspace)";
>  		goto out;
>  	case INTEGRITY_FAIL:		/* Invalid HMAC/signature. */
> -		cause = "invalid-HMAC";
> +		cause = !is_check ? "invalid-HMAC" : "invalid-HMAC(userspace)";
>  		goto out;
>  	default:
>  		WARN_ONCE(true, "Unexpected integrity status %d\n", status);
> @@ -546,7 +589,7 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
>  
>  	if (xattr_value)
>  		rc = xattr_verify(func, iint, xattr_value, xattr_len, &status,
> -				  &cause);
> +				  &cause, is_check);
>  
>  	/*
>  	 * If we have a modsig and either no imasig or the imasig's key isn't
> @@ -568,7 +611,8 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
>  	    ((inode->i_sb->s_iflags & SB_I_UNTRUSTED_MOUNTER) ||
>  	     (iint->flags & IMA_FAIL_UNVERIFIABLE_SIGS))) {
>  		status = INTEGRITY_FAIL;
> -		cause = "unverifiable-signature";
> +		cause = !is_check ? "unverifiable-signature" :
> +			"unverifiable-signature(userspace)";
>  		integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, filename,
>  				    op, cause, rc, 0);

Instead of adding new causes, another option would be to add a new audit
record type (e.g. AUDIT_INTEGRITY_DATA_CHECK).  This would help filter
these new kind of messages and I guess scale better.

Another alternative would be to extend the audit message with a new
field (e.g. "check=1"), but that would not help for efficient filtering.

>  	} else if (status != INTEGRITY_PASS) {
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 06132cf47016..2b5d6bae77a4 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 - based on policy, collect/store/appraise measurement.
> + * @bprm: contains the linux_binprm structure
> + *
> + * Based on the IMA policy and the execvat(2) AT_CHECK flag, measure and
> + * 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)
> +{
> +	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),

Why not replace bprm_check_security with bprm_creds_for_exec
implementation altogether?

>  	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
> 
> 

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

[Mimi Zohar]

On Fri, 2024-11-29 at 12:06 +0100, Mickaël Salaün wrote:
> For reference, here is the base patch series:
> https://lore.kernel.org/all/20241112191858.162021-1-mic@digikod.net/
> 
> CCing audit@
> 
> On Wed, Nov 27, 2024 at 04:02:34PM -0500, Mimi Zohar wrote:
> > Like direct file execution (e.g. ./script.sh), indirect file execution
> > (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 integrity is optionally enforced by the interpreter.
> > 
> > Update the audit messages to differentiate between kernel and userspace
> > enforced integrity.
> 
> I'm not sure to see the full picture.  What is the difference between
> execveat() calls and execveat() + AT_EXECVE_CHECK calls?  Both are from
> user space, the only difference is that the first can lead to a full
> execution, but the intent is the same.

We do want the full execution in order to measure/appraise/audit both the direct
file execution (e.g. ./script.sh) and the interpreter (e.g. #!/usr/bin/bash)
specified.

> 
> > 
> > Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> > Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
> > ---
> >  security/integrity/ima/ima_appraise.c | 84 ++++++++++++++++++++-------
> >  security/integrity/ima/ima_main.c     | 22 +++++++
> >  2 files changed, 86 insertions(+), 20 deletions(-)
> > 
> > diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> > index 656c709b974f..b5f8e49cde9d 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>
> > @@ -16,6 +17,7 @@
> >  #include <linux/fsverity.h>
> >  #include <keys/system_keyring.h>
> >  #include <uapi/linux/fsverity.h>
> > +#include <linux/securebits.h>
> >  
> >  #include "ima.h"
> >  
> > @@ -276,7 +278,8 @@ static int calc_file_id_hash(enum evm_ima_xattr_type type,
> >   */
> >  static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> >  			struct evm_ima_xattr_data *xattr_value, int xattr_len,
> > -			enum integrity_status *status, const char **cause)
> > +			enum integrity_status *status, const char **cause,
> > +			bool is_check)
> >  {
> >  	struct ima_max_digest_data hash;
> >  	struct signature_v2_hdr *sig;
> > @@ -292,9 +295,11 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> >  		if (*status != INTEGRITY_PASS_IMMUTABLE) {
> >  			if (iint->flags & IMA_DIGSIG_REQUIRED) {
> >  				if (iint->flags & IMA_VERITY_REQUIRED)
> > -					*cause = "verity-signature-required";
> > +					*cause = !is_check ? "verity-signature-required" :
> > +						"verity-signature-required(userspace)";
> 
> This looks simpler (same for all following checks):
> is_check ? "verity-signature-required(userspace)" : "verity-signature-required";
> 
> >  				else
> > -					*cause = "IMA-signature-required";
> > +					*cause = !is_check ? "IMA-signature-required" :
> > +						"IMA-signature-required(userspace)";
> >  				*status = INTEGRITY_FAIL;
> >  				break;
> >  			}
> > @@ -314,7 +319,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> >  		else
> >  			rc = -EINVAL;
> >  		if (rc) {
> > -			*cause = "invalid-hash";
> > +			*cause = !is_check ? "invalid-hash" :
> > +				"invalid-hash(userspace)";
> >  			*status = INTEGRITY_FAIL;
> >  			break;
> >  		}
> > @@ -325,14 +331,16 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> >  
> >  		mask = IMA_DIGSIG_REQUIRED | IMA_VERITY_REQUIRED;
> >  		if ((iint->flags & mask) == mask) {
> > -			*cause = "verity-signature-required";
> > +			*cause = !is_check ? "verity-signature-required" :
> > +				"verity-signature-required(userspace)";
> >  			*status = INTEGRITY_FAIL;
> >  			break;
> >  		}
> >  
> >  		sig = (typeof(sig))xattr_value;
> >  		if (sig->version >= 3) {
> > -			*cause = "invalid-signature-version";
> > +			*cause = !is_check ? "invalid-signature-version" :
> > +				"invalid-signature-version(userspace)";
> >  			*status = INTEGRITY_FAIL;
> >  			break;
> >  		}
> > @@ -353,7 +361,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> >  						     iint->ima_hash->digest,
> >  						     iint->ima_hash->length);
> >  		if (rc) {
> > -			*cause = "invalid-signature";
> > +			*cause = !is_check ? "invalid-signature" :
> > +				"invalid-signature(userspace)";
> >  			*status = INTEGRITY_FAIL;
> >  		} else {
> >  			*status = INTEGRITY_PASS;
> > @@ -364,7 +373,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> >  
> >  		if (iint->flags & IMA_DIGSIG_REQUIRED) {
> >  			if (!(iint->flags & IMA_VERITY_REQUIRED)) {
> > -				*cause = "IMA-signature-required";
> > +				*cause = !is_check ? "IMA-signature-required" :
> > +					"IMA-signature-required(userspace)";
> >  				*status = INTEGRITY_FAIL;
> >  				break;
> >  			}
> > @@ -372,7 +382,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> >  
> >  		sig = (typeof(sig))xattr_value;
> >  		if (sig->version != 3) {
> > -			*cause = "invalid-signature-version";
> > +			*cause = !is_check ? "invalid-signature-version" :
> > +				"invalid-signature-version(userspace)";
> >  			*status = INTEGRITY_FAIL;
> >  			break;
> >  		}
> > @@ -382,7 +393,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> >  				       container_of(&hash.hdr,
> >  					       struct ima_digest_data, hdr));
> >  		if (rc) {
> > -			*cause = "sigv3-hashing-error";
> > +			*cause = !is_check ? "sigv3-hashing-error" :
> > +				"sigv3-hashing-error(userspace)";
> >  			*status = INTEGRITY_FAIL;
> >  			break;
> >  		}
> > @@ -392,7 +404,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> >  					     xattr_len, hash.digest,
> >  					     hash.hdr.length);
> >  		if (rc) {
> > -			*cause = "invalid-verity-signature";
> > +			*cause = !is_check ? "invalid-verity-signature" :
> > +				"invalid-verify-signature(userspace)";
> >  			*status = INTEGRITY_FAIL;
> >  		} else {
> >  			*status = INTEGRITY_PASS;
> > @@ -401,7 +414,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> >  		break;
> >  	default:
> >  		*status = INTEGRITY_UNKNOWN;
> > -		*cause = "unknown-ima-data";
> > +		*cause = !is_check ? "unknown-ima-data" :
> > +			"unknown-ima-data(userspace)";
> >  		break;
> >  	}
> >  
> > @@ -469,6 +483,18 @@ int ima_check_blacklist(struct ima_iint_cache *iint,
> >  	return rc;
> >  }
> >  
> > +static int is_bprm_creds_for_exec(enum ima_hooks func, struct file *file)
> > +{
> > +	struct linux_binprm *bprm = NULL;
> > +
> > +	if (func == BPRM_CHECK) {
> > +		bprm = container_of(&file, struct linux_binprm, file);
> > +		if (bprm->is_check)
> > +			return 1;
> > +	}
> > +	return 0;
> > +}
> > +
> >  /*
> >   * ima_appraise_measurement - appraise file measurement
> >   *
> > @@ -489,11 +515,24 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> >  	enum integrity_status status = INTEGRITY_UNKNOWN;
> >  	int rc = xattr_len;
> >  	bool try_modsig = iint->flags & IMA_MODSIG_ALLOWED && modsig;
> > +	bool is_check = false;
> >  
> >  	/* If not appraising a modsig, we need an xattr. */
> >  	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 is left up to the discretion of the script interpreter
> > +	 * (userspace).
> > +	 *
> > +	 * Since the SECBIT_EXEC_RESTRICT_FILE flag is just a hint as to
> > +	 * userspace intentions, simply annotate the audit messages indicating
> > +	 * a userspace based query.
> > +	 */
> > +	is_check = is_bprm_creds_for_exec(func, file);
> > +
> >  	/* If reading the xattr failed and there's no modsig, error out. */
> >  	if (rc <= 0 && !try_modsig) {
> >  		if (rc && rc != -ENODATA)
> > @@ -501,11 +540,14 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> >  
> >  		if (iint->flags & IMA_DIGSIG_REQUIRED) {
> >  			if (iint->flags & IMA_VERITY_REQUIRED)
> > -				cause = "verity-signature-required";
> > +				cause = !is_check ? "verity-signature-required" :
> > +					"verity-signature-required(userspace)";
> >  			else
> > -				cause = "IMA-signature-required";
> > +				cause = !is_check ? "IMA-signature-required" :
> > +					"IMA-signature-required(userspace)";
> >  		} else {
> > -			cause = "missing-hash";
> > +			cause = !is_check ? "missing-hash" :
> > +				"missing-hash(userspace)";
> >  		}
> >  
> >  		status = INTEGRITY_NOLABEL;
> > @@ -531,14 +573,15 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> >  			break;
> >  		fallthrough;
> >  	case INTEGRITY_NOLABEL:		/* No security.evm xattr. */
> > -		cause = "missing-HMAC";
> > +		cause = !is_check ? "missing-HMAC" : "missing-HMAC(userspace)";
> >  		goto out;
> >  	case INTEGRITY_FAIL_IMMUTABLE:
> >  		set_bit(IMA_DIGSIG, &iint->atomic_flags);
> > -		cause = "invalid-fail-immutable";
> > +		cause = !is_check ? "invalid-fail-immutable" :
> > +		       "invalid-fail-immutable(userspace)";
> >  		goto out;
> >  	case INTEGRITY_FAIL:		/* Invalid HMAC/signature. */
> > -		cause = "invalid-HMAC";
> > +		cause = !is_check ? "invalid-HMAC" : "invalid-HMAC(userspace)";
> >  		goto out;
> >  	default:
> >  		WARN_ONCE(true, "Unexpected integrity status %d\n", status);
> > @@ -546,7 +589,7 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> >  
> >  	if (xattr_value)
> >  		rc = xattr_verify(func, iint, xattr_value, xattr_len, &status,
> > -				  &cause);
> > +				  &cause, is_check);
> >  
> >  	/*
> >  	 * If we have a modsig and either no imasig or the imasig's key isn't
> > @@ -568,7 +611,8 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> >  	    ((inode->i_sb->s_iflags & SB_I_UNTRUSTED_MOUNTER) ||
> >  	     (iint->flags & IMA_FAIL_UNVERIFIABLE_SIGS))) {
> >  		status = INTEGRITY_FAIL;
> > -		cause = "unverifiable-signature";
> > +		cause = !is_check ? "unverifiable-signature" :
> > +			"unverifiable-signature(userspace)";
> >  		integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, filename,
> >  				    op, cause, rc, 0);
> 
> Instead of adding new causes, another option would be to add a new audit
> record type (e.g. AUDIT_INTEGRITY_DATA_CHECK).  This would help filter
> these new kind of messages and I guess scale better.

Thanks.  This sounds like a better alternative.

> 
> Another alternative would be to extend the audit message with a new
> field (e.g. "check=1"), but that would not help for efficient filtering.
> 
> >  	} else if (status != INTEGRITY_PASS) {
> > diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> > index 06132cf47016..2b5d6bae77a4 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 - based on policy, collect/store/appraise measurement.
> > + * @bprm: contains the linux_binprm structure
> > + *
> > + * Based on the IMA policy and the execvat(2) AT_CHECK flag, measure and
> > + * 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)
> > +{
> > +	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),
> 
> Why not replace bprm_check_security with bprm_creds_for_exec
> implementation altogether?

To measure/appraise/audit the interpreter specified in the direct file (e.g.
./script.sh).

> 
> >  	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
> > 
> > 
> 

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

[Mickaël Salaün]

On Mon, Dec 02, 2024 at 02:40:35PM -0500, Mimi Zohar wrote:
> On Fri, 2024-11-29 at 12:06 +0100, Mickaël Salaün wrote:
> > For reference, here is the base patch series:
> > https://lore.kernel.org/all/20241112191858.162021-1-mic@digikod.net/
> > 
> > CCing audit@
> > 
> > On Wed, Nov 27, 2024 at 04:02:34PM -0500, Mimi Zohar wrote:
> > > Like direct file execution (e.g. ./script.sh), indirect file execution
> > > (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 integrity is optionally enforced by the interpreter.
> > > 
> > > Update the audit messages to differentiate between kernel and userspace
> > > enforced integrity.
> > 
> > I'm not sure to see the full picture.  What is the difference between
> > execveat() calls and execveat() + AT_EXECVE_CHECK calls?  Both are from
> > user space, the only difference is that the first can lead to a full
> > execution, but the intent is the same.
> 
> We do want the full execution in order to measure/appraise/audit both the direct
> file execution (e.g. ./script.sh) and the interpreter (e.g. #!/usr/bin/bash)
> specified.

Yes, but I was wondering about the difference in the log messages.  In
both cases the script is checked, but only without AT_EXECVE_CHECK its
"dependencies" (e.g. script interpreter) are checked.  I guess it could
be useful to differenciate those but I wanted to make sure we were on
the same page.

> 
> > 
> > > 
> > > Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> > > Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
> > > ---
> > >  security/integrity/ima/ima_appraise.c | 84 ++++++++++++++++++++-------
> > >  security/integrity/ima/ima_main.c     | 22 +++++++
> > >  2 files changed, 86 insertions(+), 20 deletions(-)
> > > 
> > > diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> > > index 656c709b974f..b5f8e49cde9d 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>
> > > @@ -16,6 +17,7 @@
> > >  #include <linux/fsverity.h>
> > >  #include <keys/system_keyring.h>
> > >  #include <uapi/linux/fsverity.h>
> > > +#include <linux/securebits.h>
> > >  
> > >  #include "ima.h"
> > >  
> > > @@ -276,7 +278,8 @@ static int calc_file_id_hash(enum evm_ima_xattr_type type,
> > >   */
> > >  static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  			struct evm_ima_xattr_data *xattr_value, int xattr_len,
> > > -			enum integrity_status *status, const char **cause)
> > > +			enum integrity_status *status, const char **cause,
> > > +			bool is_check)
> > >  {
> > >  	struct ima_max_digest_data hash;
> > >  	struct signature_v2_hdr *sig;
> > > @@ -292,9 +295,11 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  		if (*status != INTEGRITY_PASS_IMMUTABLE) {
> > >  			if (iint->flags & IMA_DIGSIG_REQUIRED) {
> > >  				if (iint->flags & IMA_VERITY_REQUIRED)
> > > -					*cause = "verity-signature-required";
> > > +					*cause = !is_check ? "verity-signature-required" :
> > > +						"verity-signature-required(userspace)";
> > 
> > This looks simpler (same for all following checks):
> > is_check ? "verity-signature-required(userspace)" : "verity-signature-required";
> > 
> > >  				else
> > > -					*cause = "IMA-signature-required";
> > > +					*cause = !is_check ? "IMA-signature-required" :
> > > +						"IMA-signature-required(userspace)";
> > >  				*status = INTEGRITY_FAIL;
> > >  				break;
> > >  			}
> > > @@ -314,7 +319,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  		else
> > >  			rc = -EINVAL;
> > >  		if (rc) {
> > > -			*cause = "invalid-hash";
> > > +			*cause = !is_check ? "invalid-hash" :
> > > +				"invalid-hash(userspace)";
> > >  			*status = INTEGRITY_FAIL;
> > >  			break;
> > >  		}
> > > @@ -325,14 +331,16 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  
> > >  		mask = IMA_DIGSIG_REQUIRED | IMA_VERITY_REQUIRED;
> > >  		if ((iint->flags & mask) == mask) {
> > > -			*cause = "verity-signature-required";
> > > +			*cause = !is_check ? "verity-signature-required" :
> > > +				"verity-signature-required(userspace)";
> > >  			*status = INTEGRITY_FAIL;
> > >  			break;
> > >  		}
> > >  
> > >  		sig = (typeof(sig))xattr_value;
> > >  		if (sig->version >= 3) {
> > > -			*cause = "invalid-signature-version";
> > > +			*cause = !is_check ? "invalid-signature-version" :
> > > +				"invalid-signature-version(userspace)";
> > >  			*status = INTEGRITY_FAIL;
> > >  			break;
> > >  		}
> > > @@ -353,7 +361,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  						     iint->ima_hash->digest,
> > >  						     iint->ima_hash->length);
> > >  		if (rc) {
> > > -			*cause = "invalid-signature";
> > > +			*cause = !is_check ? "invalid-signature" :
> > > +				"invalid-signature(userspace)";
> > >  			*status = INTEGRITY_FAIL;
> > >  		} else {
> > >  			*status = INTEGRITY_PASS;
> > > @@ -364,7 +373,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  
> > >  		if (iint->flags & IMA_DIGSIG_REQUIRED) {
> > >  			if (!(iint->flags & IMA_VERITY_REQUIRED)) {
> > > -				*cause = "IMA-signature-required";
> > > +				*cause = !is_check ? "IMA-signature-required" :
> > > +					"IMA-signature-required(userspace)";
> > >  				*status = INTEGRITY_FAIL;
> > >  				break;
> > >  			}
> > > @@ -372,7 +382,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  
> > >  		sig = (typeof(sig))xattr_value;
> > >  		if (sig->version != 3) {
> > > -			*cause = "invalid-signature-version";
> > > +			*cause = !is_check ? "invalid-signature-version" :
> > > +				"invalid-signature-version(userspace)";
> > >  			*status = INTEGRITY_FAIL;
> > >  			break;
> > >  		}
> > > @@ -382,7 +393,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  				       container_of(&hash.hdr,
> > >  					       struct ima_digest_data, hdr));
> > >  		if (rc) {
> > > -			*cause = "sigv3-hashing-error";
> > > +			*cause = !is_check ? "sigv3-hashing-error" :
> > > +				"sigv3-hashing-error(userspace)";
> > >  			*status = INTEGRITY_FAIL;
> > >  			break;
> > >  		}
> > > @@ -392,7 +404,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  					     xattr_len, hash.digest,
> > >  					     hash.hdr.length);
> > >  		if (rc) {
> > > -			*cause = "invalid-verity-signature";
> > > +			*cause = !is_check ? "invalid-verity-signature" :
> > > +				"invalid-verify-signature(userspace)";
> > >  			*status = INTEGRITY_FAIL;
> > >  		} else {
> > >  			*status = INTEGRITY_PASS;
> > > @@ -401,7 +414,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  		break;
> > >  	default:
> > >  		*status = INTEGRITY_UNKNOWN;
> > > -		*cause = "unknown-ima-data";
> > > +		*cause = !is_check ? "unknown-ima-data" :
> > > +			"unknown-ima-data(userspace)";
> > >  		break;
> > >  	}
> > >  
> > > @@ -469,6 +483,18 @@ int ima_check_blacklist(struct ima_iint_cache *iint,
> > >  	return rc;
> > >  }
> > >  
> > > +static int is_bprm_creds_for_exec(enum ima_hooks func, struct file *file)
> > > +{
> > > +	struct linux_binprm *bprm = NULL;
> > > +
> > > +	if (func == BPRM_CHECK) {
> > > +		bprm = container_of(&file, struct linux_binprm, file);
> > > +		if (bprm->is_check)
> > > +			return 1;
> > > +	}
> > > +	return 0;
> > > +}
> > > +
> > >  /*
> > >   * ima_appraise_measurement - appraise file measurement
> > >   *
> > > @@ -489,11 +515,24 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  	enum integrity_status status = INTEGRITY_UNKNOWN;
> > >  	int rc = xattr_len;
> > >  	bool try_modsig = iint->flags & IMA_MODSIG_ALLOWED && modsig;
> > > +	bool is_check = false;
> > >  
> > >  	/* If not appraising a modsig, we need an xattr. */
> > >  	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 is left up to the discretion of the script interpreter
> > > +	 * (userspace).
> > > +	 *
> > > +	 * Since the SECBIT_EXEC_RESTRICT_FILE flag is just a hint as to
> > > +	 * userspace intentions, simply annotate the audit messages indicating
> > > +	 * a userspace based query.
> > > +	 */
> > > +	is_check = is_bprm_creds_for_exec(func, file);
> > > +
> > >  	/* If reading the xattr failed and there's no modsig, error out. */
> > >  	if (rc <= 0 && !try_modsig) {
> > >  		if (rc && rc != -ENODATA)
> > > @@ -501,11 +540,14 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  
> > >  		if (iint->flags & IMA_DIGSIG_REQUIRED) {
> > >  			if (iint->flags & IMA_VERITY_REQUIRED)
> > > -				cause = "verity-signature-required";
> > > +				cause = !is_check ? "verity-signature-required" :
> > > +					"verity-signature-required(userspace)";
> > >  			else
> > > -				cause = "IMA-signature-required";
> > > +				cause = !is_check ? "IMA-signature-required" :
> > > +					"IMA-signature-required(userspace)";
> > >  		} else {
> > > -			cause = "missing-hash";
> > > +			cause = !is_check ? "missing-hash" :
> > > +				"missing-hash(userspace)";
> > >  		}
> > >  
> > >  		status = INTEGRITY_NOLABEL;
> > > @@ -531,14 +573,15 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  			break;
> > >  		fallthrough;
> > >  	case INTEGRITY_NOLABEL:		/* No security.evm xattr. */
> > > -		cause = "missing-HMAC";
> > > +		cause = !is_check ? "missing-HMAC" : "missing-HMAC(userspace)";
> > >  		goto out;
> > >  	case INTEGRITY_FAIL_IMMUTABLE:
> > >  		set_bit(IMA_DIGSIG, &iint->atomic_flags);
> > > -		cause = "invalid-fail-immutable";
> > > +		cause = !is_check ? "invalid-fail-immutable" :
> > > +		       "invalid-fail-immutable(userspace)";
> > >  		goto out;
> > >  	case INTEGRITY_FAIL:		/* Invalid HMAC/signature. */
> > > -		cause = "invalid-HMAC";
> > > +		cause = !is_check ? "invalid-HMAC" : "invalid-HMAC(userspace)";
> > >  		goto out;
> > >  	default:
> > >  		WARN_ONCE(true, "Unexpected integrity status %d\n", status);
> > > @@ -546,7 +589,7 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  
> > >  	if (xattr_value)
> > >  		rc = xattr_verify(func, iint, xattr_value, xattr_len, &status,
> > > -				  &cause);
> > > +				  &cause, is_check);
> > >  
> > >  	/*
> > >  	 * If we have a modsig and either no imasig or the imasig's key isn't
> > > @@ -568,7 +611,8 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> > >  	    ((inode->i_sb->s_iflags & SB_I_UNTRUSTED_MOUNTER) ||
> > >  	     (iint->flags & IMA_FAIL_UNVERIFIABLE_SIGS))) {
> > >  		status = INTEGRITY_FAIL;
> > > -		cause = "unverifiable-signature";
> > > +		cause = !is_check ? "unverifiable-signature" :
> > > +			"unverifiable-signature(userspace)";
> > >  		integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, filename,
> > >  				    op, cause, rc, 0);
> > 
> > Instead of adding new causes, another option would be to add a new audit
> > record type (e.g. AUDIT_INTEGRITY_DATA_CHECK).  This would help filter
> > these new kind of messages and I guess scale better.
> 
> Thanks.  This sounds like a better alternative.
> 
> > 
> > Another alternative would be to extend the audit message with a new
> > field (e.g. "check=1"), but that would not help for efficient filtering.
> > 
> > >  	} else if (status != INTEGRITY_PASS) {
> > > diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> > > index 06132cf47016..2b5d6bae77a4 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 - based on policy, collect/store/appraise measurement.
> > > + * @bprm: contains the linux_binprm structure
> > > + *
> > > + * Based on the IMA policy and the execvat(2) AT_CHECK flag, measure and
> > > + * 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)
> > > +{
> > > +	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),
> > 
> > Why not replace bprm_check_security with bprm_creds_for_exec
> > implementation altogether?
> 
> To measure/appraise/audit the interpreter specified in the direct file (e.g.
> ./script.sh).

It makes sense. :)

And there is no need to add another is_check check in the
bprm_check_security implementation because it will not be called when
is_check is set.

> 
> > 
> > >  	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
> > > 
> > > 
> > 
> 
> 

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

[Mimi Zohar]

On Tue, 2024-12-03 at 12:53 +0100, Mickaël Salaün wrote:
> On Mon, Dec 02, 2024 at 02:40:35PM -0500, Mimi Zohar wrote:
> > On Fri, 2024-11-29 at 12:06 +0100, Mickaël Salaün wrote:
> > > For reference, here is the base patch series:
> > > https://lore.kernel.org/all/20241112191858.162021-1-mic@digikod.net/
> > > 
> > > CCing audit@
> > > 
> > > On Wed, Nov 27, 2024 at 04:02:34PM -0500, Mimi Zohar wrote:
> > > > Like direct file execution (e.g. ./script.sh), indirect file execution
> > > > (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 integrity is optionally enforced by the interpreter.
> > > > 
> > > > Update the audit messages to differentiate between kernel and userspace
> > > > enforced integrity.
> > > 
> > > I'm not sure to see the full picture.  What is the difference between
> > > execveat() calls and execveat() + AT_EXECVE_CHECK calls?  Both are from
> > > user space, the only difference is that the first can lead to a full
> > > execution, but the intent is the same.
> > 
> > We do want the full execution in order to measure/appraise/audit both the direct
> > file execution (e.g. ./script.sh) and the interpreter (e.g. #!/usr/bin/bash)
> > specified.
> 
> Yes, but I was wondering about the difference in the log messages.  In
> both cases the script is checked, but only without AT_EXECVE_CHECK its
> "dependencies" (e.g. script interpreter) are checked.  I guess it could
> be useful to differenciate those but I wanted to make sure we were on
> the same page.

By "those" I assume you're referring to with/without AT_EXECVE_CHECK and not the
missing "dependencies".

In both cases the integrity of the script is being checked, but in one case the
integrity is being enforced by the kernel, while in the other case userspace may
enforce integrity.  The audit message should different between these two cases.

Mimi

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

[Mickaël Salaün]

Fan, would IPE need a similar update?

See https://lore.kernel.org/r/20241127210234.121546-1-zohar@linux.ibm.com

On Wed, Nov 27, 2024 at 04:02:34PM -0500, Mimi Zohar wrote:
> Like direct file execution (e.g. ./script.sh), indirect file execution
> (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 integrity is optionally enforced by the interpreter.
> 
> Update the audit messages to differentiate between kernel and userspace
> enforced integrity.
> 
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
> ---
>  security/integrity/ima/ima_appraise.c | 84 ++++++++++++++++++++-------
>  security/integrity/ima/ima_main.c     | 22 +++++++
>  2 files changed, 86 insertions(+), 20 deletions(-)
> 
> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> index 656c709b974f..b5f8e49cde9d 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>
> @@ -16,6 +17,7 @@
>  #include <linux/fsverity.h>
>  #include <keys/system_keyring.h>
>  #include <uapi/linux/fsverity.h>
> +#include <linux/securebits.h>
>  
>  #include "ima.h"
>  
> @@ -276,7 +278,8 @@ static int calc_file_id_hash(enum evm_ima_xattr_type type,
>   */
>  static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  			struct evm_ima_xattr_data *xattr_value, int xattr_len,
> -			enum integrity_status *status, const char **cause)
> +			enum integrity_status *status, const char **cause,
> +			bool is_check)
>  {
>  	struct ima_max_digest_data hash;
>  	struct signature_v2_hdr *sig;
> @@ -292,9 +295,11 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  		if (*status != INTEGRITY_PASS_IMMUTABLE) {
>  			if (iint->flags & IMA_DIGSIG_REQUIRED) {
>  				if (iint->flags & IMA_VERITY_REQUIRED)
> -					*cause = "verity-signature-required";
> +					*cause = !is_check ? "verity-signature-required" :
> +						"verity-signature-required(userspace)";
>  				else
> -					*cause = "IMA-signature-required";
> +					*cause = !is_check ? "IMA-signature-required" :
> +						"IMA-signature-required(userspace)";
>  				*status = INTEGRITY_FAIL;
>  				break;
>  			}
> @@ -314,7 +319,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  		else
>  			rc = -EINVAL;
>  		if (rc) {
> -			*cause = "invalid-hash";
> +			*cause = !is_check ? "invalid-hash" :
> +				"invalid-hash(userspace)";
>  			*status = INTEGRITY_FAIL;
>  			break;
>  		}
> @@ -325,14 +331,16 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  
>  		mask = IMA_DIGSIG_REQUIRED | IMA_VERITY_REQUIRED;
>  		if ((iint->flags & mask) == mask) {
> -			*cause = "verity-signature-required";
> +			*cause = !is_check ? "verity-signature-required" :
> +				"verity-signature-required(userspace)";
>  			*status = INTEGRITY_FAIL;
>  			break;
>  		}
>  
>  		sig = (typeof(sig))xattr_value;
>  		if (sig->version >= 3) {
> -			*cause = "invalid-signature-version";
> +			*cause = !is_check ? "invalid-signature-version" :
> +				"invalid-signature-version(userspace)";
>  			*status = INTEGRITY_FAIL;
>  			break;
>  		}
> @@ -353,7 +361,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  						     iint->ima_hash->digest,
>  						     iint->ima_hash->length);
>  		if (rc) {
> -			*cause = "invalid-signature";
> +			*cause = !is_check ? "invalid-signature" :
> +				"invalid-signature(userspace)";
>  			*status = INTEGRITY_FAIL;
>  		} else {
>  			*status = INTEGRITY_PASS;
> @@ -364,7 +373,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  
>  		if (iint->flags & IMA_DIGSIG_REQUIRED) {
>  			if (!(iint->flags & IMA_VERITY_REQUIRED)) {
> -				*cause = "IMA-signature-required";
> +				*cause = !is_check ? "IMA-signature-required" :
> +					"IMA-signature-required(userspace)";
>  				*status = INTEGRITY_FAIL;
>  				break;
>  			}
> @@ -372,7 +382,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  
>  		sig = (typeof(sig))xattr_value;
>  		if (sig->version != 3) {
> -			*cause = "invalid-signature-version";
> +			*cause = !is_check ? "invalid-signature-version" :
> +				"invalid-signature-version(userspace)";
>  			*status = INTEGRITY_FAIL;
>  			break;
>  		}
> @@ -382,7 +393,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  				       container_of(&hash.hdr,
>  					       struct ima_digest_data, hdr));
>  		if (rc) {
> -			*cause = "sigv3-hashing-error";
> +			*cause = !is_check ? "sigv3-hashing-error" :
> +				"sigv3-hashing-error(userspace)";
>  			*status = INTEGRITY_FAIL;
>  			break;
>  		}
> @@ -392,7 +404,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  					     xattr_len, hash.digest,
>  					     hash.hdr.length);
>  		if (rc) {
> -			*cause = "invalid-verity-signature";
> +			*cause = !is_check ? "invalid-verity-signature" :
> +				"invalid-verify-signature(userspace)";
>  			*status = INTEGRITY_FAIL;
>  		} else {
>  			*status = INTEGRITY_PASS;
> @@ -401,7 +414,8 @@ static int xattr_verify(enum ima_hooks func, struct ima_iint_cache *iint,
>  		break;
>  	default:
>  		*status = INTEGRITY_UNKNOWN;
> -		*cause = "unknown-ima-data";
> +		*cause = !is_check ? "unknown-ima-data" :
> +			"unknown-ima-data(userspace)";
>  		break;
>  	}
>  
> @@ -469,6 +483,18 @@ int ima_check_blacklist(struct ima_iint_cache *iint,
>  	return rc;
>  }
>  
> +static int is_bprm_creds_for_exec(enum ima_hooks func, struct file *file)
> +{
> +	struct linux_binprm *bprm = NULL;
> +
> +	if (func == BPRM_CHECK) {
> +		bprm = container_of(&file, struct linux_binprm, file);
> +		if (bprm->is_check)
> +			return 1;
> +	}
> +	return 0;
> +}
> +
>  /*
>   * ima_appraise_measurement - appraise file measurement
>   *
> @@ -489,11 +515,24 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
>  	enum integrity_status status = INTEGRITY_UNKNOWN;
>  	int rc = xattr_len;
>  	bool try_modsig = iint->flags & IMA_MODSIG_ALLOWED && modsig;
> +	bool is_check = false;
>  
>  	/* If not appraising a modsig, we need an xattr. */
>  	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 is left up to the discretion of the script interpreter
> +	 * (userspace).
> +	 *
> +	 * Since the SECBIT_EXEC_RESTRICT_FILE flag is just a hint as to
> +	 * userspace intentions, simply annotate the audit messages indicating
> +	 * a userspace based query.
> +	 */
> +	is_check = is_bprm_creds_for_exec(func, file);
> +
>  	/* If reading the xattr failed and there's no modsig, error out. */
>  	if (rc <= 0 && !try_modsig) {
>  		if (rc && rc != -ENODATA)
> @@ -501,11 +540,14 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
>  
>  		if (iint->flags & IMA_DIGSIG_REQUIRED) {
>  			if (iint->flags & IMA_VERITY_REQUIRED)
> -				cause = "verity-signature-required";
> +				cause = !is_check ? "verity-signature-required" :
> +					"verity-signature-required(userspace)";
>  			else
> -				cause = "IMA-signature-required";
> +				cause = !is_check ? "IMA-signature-required" :
> +					"IMA-signature-required(userspace)";
>  		} else {
> -			cause = "missing-hash";
> +			cause = !is_check ? "missing-hash" :
> +				"missing-hash(userspace)";
>  		}
>  
>  		status = INTEGRITY_NOLABEL;
> @@ -531,14 +573,15 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
>  			break;
>  		fallthrough;
>  	case INTEGRITY_NOLABEL:		/* No security.evm xattr. */
> -		cause = "missing-HMAC";
> +		cause = !is_check ? "missing-HMAC" : "missing-HMAC(userspace)";
>  		goto out;
>  	case INTEGRITY_FAIL_IMMUTABLE:
>  		set_bit(IMA_DIGSIG, &iint->atomic_flags);
> -		cause = "invalid-fail-immutable";
> +		cause = !is_check ? "invalid-fail-immutable" :
> +		       "invalid-fail-immutable(userspace)";
>  		goto out;
>  	case INTEGRITY_FAIL:		/* Invalid HMAC/signature. */
> -		cause = "invalid-HMAC";
> +		cause = !is_check ? "invalid-HMAC" : "invalid-HMAC(userspace)";
>  		goto out;
>  	default:
>  		WARN_ONCE(true, "Unexpected integrity status %d\n", status);
> @@ -546,7 +589,7 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
>  
>  	if (xattr_value)
>  		rc = xattr_verify(func, iint, xattr_value, xattr_len, &status,
> -				  &cause);
> +				  &cause, is_check);
>  
>  	/*
>  	 * If we have a modsig and either no imasig or the imasig's key isn't
> @@ -568,7 +611,8 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
>  	    ((inode->i_sb->s_iflags & SB_I_UNTRUSTED_MOUNTER) ||
>  	     (iint->flags & IMA_FAIL_UNVERIFIABLE_SIGS))) {
>  		status = INTEGRITY_FAIL;
> -		cause = "unverifiable-signature";
> +		cause = !is_check ? "unverifiable-signature" :
> +			"unverifiable-signature(userspace)";
>  		integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, filename,
>  				    op, cause, rc, 0);
>  	} else if (status != INTEGRITY_PASS) {
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 06132cf47016..2b5d6bae77a4 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 - based on policy, collect/store/appraise measurement.
> + * @bprm: contains the linux_binprm structure
> + *
> + * Based on the IMA policy and the execvat(2) AT_CHECK flag, measure and
> + * 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)
> +{
> +	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
> 
>