RE: [PATCH 07/11] evm: Set IMA_CHANGE_XATTR/ATTR bit if EVM_ALLOW_METADATA_WRITES is set

[Roberto Sassu <roberto.sassu@huawei.com>]

> From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> Sent: Monday, August 24, 2020 2:18 PM
> On Thu, 2020-06-18 at 18:04 +0200, Roberto Sassu wrote:
> > When EVM_ALLOW_METADATA_WRITES is set, EVM allows any operation
> on
> > metadata. Its main purpose is to allow users to freely set metadata when
> > they are protected by a portable signature, until the HMAC key is loaded.
> >
> > However, IMA is not notified about metadata changes and, after the first
> > appraisal, always allows access to the files without checking metadata
> > again.
> 
> ^after the first successful appraisal
> >
> > This patch checks in evm_reset_status() if EVM_ALLOW_METADATA
> WRITES is
> > enabled and if it is, sets the IMA_CHANGE_XATTR/ATTR bits depending on
> the
> > operation performed. At the next appraisal, metadata are revalidated.
> 
> EVM modifying IMA bits crosses the boundary between EVM and IMA.
> There
> is already an IMA post_setattr hook.  IMA could reset its own bit
> there.  If necessary EVM could export as a function it's status info.

I wouldn't try to guess in IMA when EVM resets its status. We would have
to duplicate the logic to check if an EVM key is loaded, if the passed xattr
is a POSIX ACL, ...

I think it is better to set a flag, maybe a new one, directly in EVM, to notify
the integrity subsystem that iint->evm_status is no longer valid.

If the EVM flag is set, IMA would reset the appraisal flags, as it uses
iint->evm_status for appraisal. We can consider to reset also the measure
flags when we have a template that includes file metadata.

Roberto

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

> Mimi
> 
> >
> > This patch also adds a call to evm_reset_status() in
> > evm_inode_post_setattr() so that EVM won't return the cached status
> the
> > next time appraisal is performed.
> >
> > Cc: stable@vger.kernel.org # 4.16.x
> > Fixes: ae1ba1676b88e ("EVM: Allow userland to permit modification of
> EVM-protected metadata")
> > Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> > ---
> >  security/integrity/evm/evm_main.c | 14 ++++++++++----
> >  1 file changed, 10 insertions(+), 4 deletions(-)
> >
> > diff --git a/security/integrity/evm/evm_main.c
> b/security/integrity/evm/evm_main.c
> > index 41cc6a4aaaab..d4d918183094 100644
> > --- a/security/integrity/evm/evm_main.c
> > +++ b/security/integrity/evm/evm_main.c
> > @@ -478,13 +478,17 @@ int evm_inode_removexattr(struct dentry
> *dentry, const char *xattr_name)
> >  	return evm_protect_xattr(dentry, xattr_name, NULL, 0);
> >  }
> >
> > -static void evm_reset_status(struct inode *inode)
> > +static void evm_reset_status(struct inode *inode, int bit)
> >  {
> >  	struct integrity_iint_cache *iint;
> >
> >  	iint = integrity_iint_find(inode);
> > -	if (iint)
> > +	if (iint) {
> > +		if (evm_initialized & EVM_ALLOW_METADATA_WRITES)
> > +			set_bit(bit, &iint->atomic_flags);
> > +
> >  		iint->evm_status = INTEGRITY_UNKNOWN;
> > +	}
> >  }
> >
> >  /**:q
> > @@ -507,7 +511,7 @@ void evm_inode_post_setxattr(struct dentry
> *dentry, const char *xattr_name,
> >  				  && !posix_xattr_acl(xattr_name)))
> >  		return;
> >
> > -	evm_reset_status(dentry->d_inode);
> > +	evm_reset_status(dentry->d_inode, IMA_CHANGE_XATTR);
> >
> >  	evm_update_evmxattr(dentry, xattr_name, xattr_value,
> xattr_value_len);
> >  }
> > @@ -527,7 +531,7 @@ void evm_inode_post_removexattr(struct dentry
> *dentry, const char *xattr_name)
> >  	if (!evm_key_loaded() || !evm_protected_xattr(xattr_name))
> >  		return;
> >
> > -	evm_reset_status(dentry->d_inode);
> > +	evm_reset_status(dentry->d_inode, IMA_CHANGE_XATTR);
> >
> >  	evm_update_evmxattr(dentry, xattr_name, NULL, 0);
> >  }
> > @@ -600,6 +604,8 @@ void evm_inode_post_setattr(struct dentry
> *dentry, int ia_valid)
> >  	if (!evm_key_loaded())
> >  		return;
> >
> > +	evm_reset_status(dentry->d_inode, IMA_CHANGE_ATTR);
> > +
> >  	if (ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID))
> >  		evm_update_evmxattr(dentry, NULL, NULL, 0);
> >  }
>