Re: [PATCH v6 08/20] evm: evm_inode_post_init

Dave Chinner
2011-06-04T23:51:10Z

On Fri, Jun 03, 2011 at 01:06:32AM -0400, Mimi Zohar wrote:
> On Fri, 2011-06-03 at 12:21 +1000, Dave Chinner wrote:
> > On Thu, Jun 02, 2011 at 08:23:31AM -0400, Mimi Zohar wrote:
> > > Initialize 'security.evm' for new files. Reduce number of arguments
> > > by defining 'struct xattr'.
> >
> > why does this need a new security callout from every filesystem?
> > Once the security xattr is initialised, the name, len and value is
> > not going to change so surely the evm xattr can be initialised at
> > the same time the lsm xattr is initialised.
>
> Steve Whitehouse asked a similar question, suggesting that
> security_inode_init_security() return a vector of xattrs to minimize the
> number of xattr writes. Casey pointed out the "stacking" of LSMs will
> result in multiple calls to security_inode_init_security(), once for
> each LSM. The conclusion (http://lkml.org/lkml/2011/5/19/125) was:
>
> Moving evm_inode_init_security() into security_inode_init_security()
> only works for the single LSM and EVM case, but not for the multiple
> LSMs and EVM case, as the 'stacker' would call each LSM's
> security_inode_iint_security(). Having the 'stacker' return an array of
> xattrs would make sense and, at the same time, resolve the EVM issue. In
> evm_inode_post_init_security(), EVM could then walk the list of xattrs.

But that does not change the fact that ther eis a _single external
call_ from the filesystem to security_inode_init_security(), and the
attribute (array) that it returns is only read by
evm_inode_post_init_security() to calculate a new attribute.

If evm_inode_post_init_security() only needs to read the security
attributes, then why does it need to be calculated _after_ the
security attributes are written to the filesystem inode?

i.e, your current code is:

security_inode_init_security(&lsm_xattr)
set_xattr(&lsm_xattr)
evm_inode_post_init_security(&lsm_xattr, &evm_xattr)
set_xattr(&evm_xattr)

and I'm asking why you can't do it like this:

security_inode_init_security(&lsm_xattr, &evm_xattr)
set_xattr(&lsm_xattr)
set_xattr(&evm_xattr)

where security_inode_init_security() calls:

evm_inode_post_init_security(&lsm_xattr, &evm_xattr)

before returning to calculate the evm xattr?

Indeed, if we are stacking LSMs, the iteration must occur internally
to security_inode_init_security(), and so that would mean the entire
stacking/multiple attr thing could be handled simply by passing an
array and having the EVM xattr always be the last in the array.
i.e.:

XXXfs_init_security()
{
xattr_count = security_inode_init_security(&xattr_array)

for (i = 0; i < xattr_count; i++)
set_xattr(&xattr_array[i])

security_free_xattr(&xattr_array);
}

And then the filesystems need to know _nothing at all_ about the
internals of the security subsystem or how it uses xattrs or even
whether EVM is enabled or active or neither. This is far cleaner
than spewing security-flavour-of-the-month junk widely across the
tree...

This also makes it possible for the filesystems to atomically set or
fail to set all the security attributes in one
operation/transaction, which will help guarantee the integrity of
the system in the face of externally induced failures.

> > Then all you need to do in each filesystem is add the evm_xattr
> > structure to the existing security init call and a:
> >
> > #ifdef CONFIG_EVM
> > /* set evm.xattr */
> > #endif
> >
> > to avoid adding code that is never executed when EVM is not
> > configured into the kernel.
> >
> > That way you don't create the lsm_xattr at all if the evm_xattr is
> > not created, and then the file creation should fail in an atomic
> > manner, right? i.e. you don't leave files with unverified security
> > attributes around when interesting failure corner cases occur (e.g.
> > ENOSPC).
>
> That would imply EVM must be enabled for all LSMs that define a security
> xattr. That's definitely a good goal, but probably not a good idea for
> right now.
>
> > And while you are there, it's probably also be a good idea to add
> > support for all filesystems that support xattrs, not just a random
> > subset of them...
> >
> > Cheers,
> >
> > Dave.
>
> The EVM xattr is initialized based on the LSM xattr. At this point, as
> far as I'm aware, the only remaining filesystems that call
> security_inode_init_security() to initialize the LSM xattr, are ocfs2
> and reiserfs. Both of which might have memory leaks. Tiger Yang is
> addressing the memory leak for ocfs2.

I don't follow you - I didn't see any patches that remove
security_inode_init_security() from any of the filesystems, so they
all still call that function....

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com