linux-kernel - Re: [PATCH] ima: setting security.ima to fix security.evm for a file with IMA signature

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Message-ID: <4l7l4duxfximhzixruim3lnij5mhnlqs5srzycc6j6c2bu5zda@ogoj3ade37fd>
Date: Wed, 24 Sep 2025 16:03:34 +0800
From: Coiby Xu <coxu@...hat.com>
To: Mimi Zohar <zohar@...ux.ibm.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 Mon, Sep 15, 2025 at 05:05:42PM -0400, Mimi Zohar wrote:
>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.
>> > >
[...]
>> > > ---
>> > >  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, ....);
>                        }
>                }

Thanks for the advice! I wonder if we should use existing
evm_update_evmxattr instead of defining a new EVM function. 

     /*
      * Calculate the hmac and update security.evm xattr
      *
      * Expects to be called with i_mutex locked.
      */
     int evm_update_evmxattr(struct dentry *dentry, const char *xattr_name,
     			const char *xattr_value, size_t xattr_value_len)
     {
     }


I already tried evm_update_evmxattr and can confirm it works.  But later
I switched to __vfs_setxattr_noperm because I thought it's consistent
with current logic of adding security.evm when there is already correct
security.ima and it's a slightly smaller change.

-- 
Best regards,
Coiby