Re: [PATCH V3 2/2] EVM: Allow runtime modification of the set of verified xattrs

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

On Tue, 2018-05-08 at 21:30 +0000, Matthew Garrett wrote:
> On Wed, May 2, 2018 at 8:17 PM Mimi Zohar <zohar@linux.vnet.ibm.com> wrote:
> > On Tue, 2018-05-01 at 10:51 -0700, Matthew Garrett wrote:
> > > +             Shows the set of extended attributes used to calculate or
> > > +             validate the EVM signature, and allows additional
> attributes
> > > +             to be added at runtime. Adding additional extended
> attributes
> > > +             will result in any existing signatures generated without
> the
> > > +             additional attributes becoming invalid, and any signatures
> > > +             generated after additional attributes are added will only
> be
> > > +             valid if the same additional attributes are configured on
> > > +             system boot.
> 
> > This needs to be updated ...
> 
> Yup.
> 
> > > +     if (*ppos != 0)
> > > +             return -EINVAL;
> 
> > Is there a maximum xattr name size?  Should there be?
> 
> There is - I'll add a check.
> 
> > > +     /* Remove any trailing newline */
> > > +     len = strlen(xattr->name);
> > > +     if (xattr->name[len-1] == '\n')
> > > +             xattr->name[len-1] = '\0';
> 
> > Shouldn't we somehow sanity check userspace provided strings, before
> > adding them to the list of xattrs?  Perhaps limit the string to the
> > upper and lower case letters?
> 
> As far as I can tell the VFS doesn't do that, and I wouldn't put it past
> someone to use UTF-8 at some point...

The audit subsystem uses audit_log_untrustedstrings() to check for
control characters.  I'm not sure what should be included, but not
checking userspace strings doesn't sound right.

> 
>   > +     evm_xattrs = securityfs_create_file("evm_xattrs", 0440, NULL, NULL,
> > > +                                         &evm_xattr_ops);
> > > +     if (!evm_xattrs || IS_ERR(evm_xattrs)) {
> > > +             securityfs_remove(evm_init_tpm);
> > > +             return -EFAULT;
> > > +     }
> > > +
> 
> > Do we really want this feature unconditionally enabled?  How often do
> > you expect to add xattrs?  Is there ever a point where you would want
> > to lock the list of xattrs from changing?
> 
> I think a config option would make sense here. Locking doesn't seem
> unreasonable, but I'm not sure what the threat model would really be -
> adding new xattrs would only result in additional signatures validating if
> they were signed with a trusted key in the first place.

Remember adding additional EVM xattrs isn't limited to just EVM
signatures, but for the original EVM HMAC as well.  Did you want to
limit it to the EVM portable & immutable signatures?

Mimi