RE: [PATCH 03/11] evm: Refuse EVM_ALLOW_METADATA_WRITES only if the HMAC key is loaded

[Roberto Sassu]

> From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx]
> Sent: Friday, August 21, 2020 10:15 PM
> Hi Roberto,
> 
> On Thu, 2020-06-18 at 18:01 +0200, Roberto Sassu wrote:
> > Granting metadata write is safe if the HMAC key is not loaded, as it won't
> > let an attacker obtain a valid HMAC from corrupted xattrs.
> evm_write_key()
> > however does not allow it if any key is loaded, including a public key,
> > which should not be a problem.
> >
> 
> Why is the existing hebavior a problem?  What is the problem being
> solved?

Hi Mimi

currently it is not possible to set EVM_ALLOW_METADATA_WRITES when
only a public key is loaded and the HMAC key is not. The patch removes
this limitation.

> > This patch allows setting EVM_ALLOW_METADATA_WRITES if the
> EVM_INIT_HMAC
> > flag is not set.
> >
> > Cc: stable@xxxxxxxxxxxxxxx # 4.16.x
> > Fixes: ae1ba1676b88e ("EVM: Allow userland to permit modification of
> EVM-protected metadata")
> > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > ---
> >  security/integrity/evm/evm_secfs.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/security/integrity/evm/evm_secfs.c
> b/security/integrity/evm/evm_secfs.c
> > index cfc3075769bb..92fe26ace797 100644
> > --- a/security/integrity/evm/evm_secfs.c
> > +++ b/security/integrity/evm/evm_secfs.c
> > @@ -84,7 +84,7 @@ static ssize_t evm_write_key(struct file *file, const
> char __user *buf,
> >  	 * keys are loaded.
> >  	 */
> >  	if ((i & EVM_ALLOW_METADATA_WRITES) &&
> > -	    ((evm_initialized & EVM_KEY_MASK) != 0) &&
> > +	    ((evm_initialized & EVM_INIT_HMAC) != 0) &&
> >  	    !(evm_initialized & EVM_ALLOW_METADATA_WRITES))
> >  		return -EPERM;
> 
> >
> 
> Documentation/ABI/testing/evm needs to be updated as well.

Ok.

Thanks

Roberto

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

> thanks,
> 
> Mimi
> 
>