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Message-ID: <40e9c7bd15d4ab8b71ac335b5d896ed39c61980c.camel@linux.ibm.com>
Date: Mon, 15 Sep 2025 17:05:42 -0400
From: Mimi Zohar <zohar@...ux.ibm.com>
To: Coiby Xu <coxu@...hat.com>
Cc: linux-integrity@...r.kernel.org, Roberto Sassu
<roberto.sassu@...wei.com>,
Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
Eric Snowberg <eric.snowberg@...cle.com>,
Paul Moore
<paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
"Serge E. Hallyn"
<serge@...lyn.com>,
"open list:SECURITY SUBSYSTEM"
<linux-security-module@...r.kernel.org>,
open list
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ima: setting security.ima to fix security.evm for a
file with IMA signature
On Wed, 2025-09-10 at 09:20 +0800, Coiby Xu wrote:
> On Tue, Sep 09, 2025 at 11:31:20AM -0400, Mimi Zohar wrote:
> > On Tue, 2025-09-09 at 12:19 +0800, Coiby Xu wrote:
> > > When both IMA and EVM fix modes are enabled, accessing a file with IMA
> > > signature won't cause security.evm to be fixed. But this doesn't happen
> > > to a file with correct IMA hash already set because accessing it will
> > > cause setting security.ima again which triggers fixing security.evm
> > > thanks to security_inode_post_setxattr->evm_update_evmxattr.
> > >
> > > Let's use the same mechanism to fix security.evm for a file with IMA
> > > signature.
> > >
> > > Signed-off-by: Coiby Xu <coxu@...hat.com>
> >
> > Agreed, re-writing the file signature stored as security.ima would force
> > security.evm to be updated.
> >
> > Unfortunately, I'm missing something. ima_appraise_measurement() first verifies
> > the existing security.evm xattr, before verifying the security.ima xattr. If
> > the EVM HMAC fails to verify, it immediately exits ima_appraise_measurement().
> > security.ima in this case is never verified.
> >
> > This patch seems to address the case where the existing security.evm is valid,
> > but the file signature stored in security.ima is invalid. (To get to the new
> > code, the "status" flag is not INTEGRITY_PASS.) Re-writing the same invalid
> > file signature would solve an invalid security.evm, but not an invalid IMA file
> > signature. What am I missing?
>
> Hi, Mimi,
>
> Thanks for raising the question! This patch is to address the case where
> IMA signature is already added but security.evm doesn't yet exist. So
> EVM HMAC fails to verify but there is no exiting
> ima_appraise_measurement immediately.
>
> And you are right that re-writing an invalid IMA file won't fix an
> invalid IMA file signature. And even when IMA signature is valid, the
> verification may fail because the key is missing from .ima keyring. This
> happens because we need to turn off secure boot to enable fix mode. As a
> result, CA keys won't be loaded into .machine keyring. Btw, if I'm not
> mistaken, current IMA code assumes we are not supposed to fix IMA file
> signature.
> >
> > > ---
> > > security/integrity/ima/ima_appraise.c | 27 +++++++++++++++++++++------
> > > 1 file changed, 21 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> > > index f435eff4667f..18c3907c5e44 100644
> > > --- a/security/integrity/ima/ima_appraise.c
> > > +++ b/security/integrity/ima/ima_appraise.c
> > > @@ -595,12 +595,27 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
> > > integrity_audit_msg(audit_msgno, inode, filename,
> > > op, cause, rc, 0);
> > > } else if (status != INTEGRITY_PASS) {
> > > - /* Fix mode, but don't replace file signatures. */
> > > - if ((ima_appraise & IMA_APPRAISE_FIX) && !try_modsig &&
> > > - (!xattr_value ||
> > > - xattr_value->type != EVM_IMA_XATTR_DIGSIG)) {
> > > - if (!ima_fix_xattr(dentry, iint))
> > > - status = INTEGRITY_PASS;
> > > + /*
> > > + * Fix mode, but don't replace file signatures.
> > > + *
> > > + * When EVM fix mode is also enabled, security.evm will be
> > > + * fixed automatically when security.ima is set because of
> > > + * security_inode_post_setxattr->evm_update_evmxattr.
> > > + */
> > > + if ((ima_appraise & IMA_APPRAISE_FIX) && !try_modsig) {
> > > + if (!xattr_value ||
> > > + xattr_value->type != EVM_IMA_XATTR_DIGSIG) {
> > > + if (ima_fix_xattr(dentry, iint))
> > > + status = INTEGRITY_PASS;
> > > + } else if (xattr_value->type == EVM_IMA_XATTR_DIGSIG &&
> > > + evm_revalidate_status(XATTR_NAME_IMA)) {
> > > + if (!__vfs_setxattr_noperm(&nop_mnt_idmap,
> > > + dentry,
> > > + XATTR_NAME_IMA,
> > > + xattr_value,
> > > + xattr_len, 0))
> > > + status = INTEGRITY_PASS;
> > > + }
> > > }
Instead of re-writing the IMA signature without a clear explanation, define a
new EVM function named evm_fix_hmac() and add a call here in IMA. Only in EVM
fix mode would evm_fix_hmac() update the EVM hmac.
} else if (status != INTEGRITY_PASS) {
/*
* IMA fix mode updates the IMA file hash, which triggers EVM
* to update security.evm. ....
*
* Similarly, trigger fixing EVM HMAC for IMA file signatures.
*/
if ((ima_appraise & IMA_APPRAISE_FIX) && !try_modsig) {
if (!xattr_value ||
xattr_value->type != EVM_IMA_XATTR_DIGSIG) {
if (ima_fix_xattr(dentry, iint))
status = INTEGRITY_PASS;
} else if (status == INTEGRITY_NOLABEL) {
evm_fix_hmac(dentry, XATTR_NAME_IMA, ....);
}
}
> > >
> > > /*
> > >
> > > base-commit: b320789d6883cc00ac78ce83bccbfe7ed58afcf0
> >
>
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