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

Mimi Zohar
2011-06-03T05:06:41Z

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.

> 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.

thanks,

Mimi