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

[Mimi Zohar <zohar@linux.ibm.com>]

On Wed, 2025-09-24 at 16:03 +0800, Coiby Xu wrote:
> 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.

Calling evm_inode_updatexattr() is limited to EVM.  Only after verifying the
existing EVM value is evm_inode_updatexattr() called. For example, in
evm_inode_setxattr() the existing EVM value is verified and then updated in
evm_inode_post_setxattr(), by calling evm_inode_updatexattr().

In this case, the new function evm_fix_hmac() would call evm_update_evmxattr()
only after verifying the EVM mode is set to fix.