RE: [PATCH v5 06/12] evm: Ignore INTEGRITY_NOLABEL/INTEGRITY_NOXATTRS if conditions are safe

[Roberto Sassu]

> From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx]
> Sent: Monday, May 3, 2021 2:13 AM
> Hi Roberto,
> 
> On Wed, 2021-04-07 at 12:52 +0200, Roberto Sassu wrote:
> > When a file is being created, LSMs can set the initial label with the
> > inode_init_security hook. If no HMAC key is loaded, the new file will have
> > LSM xattrs but not the HMAC. It is also possible that the file remains
> > without protected xattrs after creation if no active LSM provided it.
> >
> > Unfortunately, EVM will deny any further metadata operation on new files,
> > as evm_protect_xattr() will always return the INTEGRITY_NOLABEL error, or
> > INTEGRITY_NOXATTRS if no protected xattrs exist. This would limit the
> > usability of EVM when only a public key is loaded, as commands such as cp
> > or tar with the option to preserve xattrs won't work.
> >
> > This patch ignores these errors when they won't be an issue, if no HMAC
> key
> > is loaded and cannot be loaded in the future (which can be enforced by
> > setting the EVM_SETUP_COMPLETE initialization flag).
> >
> > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > ---
> >  security/integrity/evm/evm_main.c | 23 ++++++++++++++++++++++-
> >  1 file changed, 22 insertions(+), 1 deletion(-)
> >
> > diff --git a/security/integrity/evm/evm_main.c
> b/security/integrity/evm/evm_main.c
> > index 998818283fda..6556e8c22da9 100644
> > --- a/security/integrity/evm/evm_main.c
> > +++ b/security/integrity/evm/evm_main.c
> > @@ -90,6 +90,24 @@ static bool evm_key_loaded(void)
> >  	return (bool)(evm_initialized & EVM_KEY_MASK);
> >  }
> >
> > +/*
> > + * Ignoring INTEGRITY_NOLABEL/INTEGRITY_NOXATTRS is safe if no HMAC
> key
> > + * is loaded and the EVM_SETUP_COMPLETE initialization flag is set.
> > + */
> > +static bool evm_ignore_error_safe(enum integrity_status evm_status)
> > +{
> > +	if (evm_initialized & EVM_INIT_HMAC)
> > +		return false;
> > +
> > +	if (!(evm_initialized & EVM_SETUP_COMPLETE))
> > +		return false;
> > +
> > +	if (evm_status != INTEGRITY_NOLABEL && evm_status !=
> INTEGRITY_NOXATTRS)
> > +		return false;
> > +
> > +	return true;
> > +}
> > +
> >  static int evm_find_protected_xattrs(struct dentry *dentry)
> >  {
> >  	struct inode *inode = d_backing_inode(dentry);
> > @@ -354,6 +372,8 @@ static int evm_protect_xattr(struct dentry *dentry,
> const char *xattr_name,
> >  				    -EPERM, 0);
> >  	}
> >  out:
> > +	if (evm_ignore_error_safe(evm_status))
> > +		return 0;
> 
> I agree with the concept, but the function name doesn't provide enough
> context.  Perhaps defining a function more along the lines of
> "evm_hmac_disabled()" would be more appropriate and at the same time
> self documenting.

Since the function checks if the passed error can be ignored,
would evm_ignore_error_hmac_disabled() also be ok?

> >  	if (evm_status != INTEGRITY_PASS)
> >  		integrity_audit_msg(AUDIT_INTEGRITY_METADATA,
> d_backing_inode(dentry),
> >  				    dentry->d_name.name,
> "appraise_metadata",
> > @@ -515,7 +535,8 @@ int evm_inode_setattr(struct dentry *dentry, struct
> iattr *attr)
> >  		return 0;
> >  	evm_status = evm_verify_current_integrity(dentry);
> >  	if ((evm_status == INTEGRITY_PASS) ||
> > -	    (evm_status == INTEGRITY_NOXATTRS))
> > +	    (evm_status == INTEGRITY_NOXATTRS) ||
> > +	    (evm_ignore_error_safe(evm_status)))
> 
> It would also remove the INTEGRITY_NOXATTRS test duplication here.

Ok.

Thanks

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

> thanks,
> 
> Mimi
> 
> >  		return 0;
> >  	integrity_audit_msg(AUDIT_INTEGRITY_METADATA,
> d_backing_inode(dentry),
> >  			    dentry->d_name.name, "appraise_metadata",