[PATCH] ima: instantiate the bprm_creds_for_exec() hook
Mickaël Salaün
mic at digikod.net
Tue Dec 3 11:53:25 UTC 2024
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 at huawei.com>
> > > Signed-off-by: Mimi Zohar <zohar at 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
> > >
> > >
> >
>
>
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