LSM and setxattr helpers

[Amir Goldstein]

security_inode_post_setxattr

On Mon, Apr 5, 2021 at 5:47 PM Mimi Zohar <zohar at linux.ibm.com> wrote:
>
> Hi Amir,
>
> On Sun, 2021-04-04 at 13:27 +0300, Amir Goldstein wrote:
> > [forking question about security modules]
> >
> > >
> > > Nice thing about vfs_{set,remove}xattr() is that they already have
> > > several levels of __vfs_ helpers and nfsd already calls those, so
> > > we can hoist fsnotify_xattr() hooks hooks up from the __vfs_xxx
> > > helpers to the common vfs_xxx helpers and add fsnotify hooks to
> > > the very few callers of __vfs_ helpers.
> > >
> > > nfsd is consistently calling __vfs_{set,remove}xattr_locked() which
> > > do generate events, but ecryptfs mixes __vfs_setxattr_locked() with
> > > __vfs_removexattr(), which does not generate event and does not
> > > check permissions - it looks like an oversight.
> > >
> > > The thing is, right now __vfs_setxattr_noperm() generates events,
> > > but looking at all the security/* callers, it feels to me like those are
> > > very internal operations and that "noperm" should also imply "nonotify".
> > >
> > > To prove my point, all those callers call __vfs_removexattr() which
> > > does NOT generate an event.
> > >
> > > Also, I *think* the EVM setxattr is something that usually follows
> > > another file data/metadata change, so some event would have been
> > > generated by the original change anyway.
> > >
> > > Mimi,
> > >
> > > Do you have an opinion on that?
>
> Right, EVM is re-calculating the EVM HMAC, which is based on other LSM
> xattrs and includes some misc file metadata (e.g. ino, generation, uid,
> gid, mode).
>

That explains why EVM registers to security_inode_post_setxattr() hook in
__vfs_setxattr_noperm() and which is the helper that selinux and smack call.

> > >
> > > The question is if you think it is important for an inotify/fanotify watcher
> > > that subscribed to IN_ATTRIB/FAN_ATTRIB events on a file to get an
> > > event when the IMA security blob changes.
>
> Probably not.  Programs could open files R/W, but never modify the
> file.  Perhaps to detect mutable file changes, but I'm not aware of
> anyone doing so.
>
> >
> > Guys,
> >
> > I was doing some re-factoring of the __vfs_setxattr helpers
> > and noticed some strange things.
> >
> > The wider context is fsnotify_xattr() hooks inside internal
> > setxattr,removexattr calls. I would like to move those hooks
> > to the common vfs_{set,remove}xattr() helpers.
> >
> > SMACK & SELINUX:
> > For the callers of __vfs_setxattr_noperm(),
> > smack_inode_setsecctx() and selinux_inode_setsecctx()
> > It seems that the only user is nfsd4_set_nfs4_label(), so it
> > makes sense for me to add the fsnotify_xattr() in nfsd context,
> > same as I did with other fsnotify_ hooks.
> >
> > Are there any other expected callers of security_inode_setsecctx()
> > except nfsd in the future? If so they would need to also add the
> > fsnotify_xattr() hook, if at all the user visible FS_ATTRIB event is
> > considered desirable.
> >
> > SMACK:
> > Just to be sure, is the call to __vfs_setxattr() from smack_d_instantiate()
> > guaranteed to be called for an inode whose S_NOSEC flag is already
> > cleared? Because the flag is being cleared by __vfs_setxattr_noperm().
> >
> > EVM:
> > I couldn't find what's stopping this recursion:
> > evm_update_evmxattr() => __vfs_setxattr_noperm() =>
> > security_inode_post_setxattr() => evm_inode_post_removexattr() =>
> > evm_update_evmxattr()
>
> EVM is triggered when file metadata changes, causing the EVM HMAC to be
> re-calculated. Before updating security.evm, EVM first verifies, on the
> evm_inode_setattr/setxattr/removexattr() hooks, that the existing
> security.evm value is correct.
>
> On the _post hooks, security.evm is updated or removed, if no LSM xattr
> exists.
>

I'm not sure I understand why evm_update_evmxattr() calls
__vfs_setxattr_noperm() and not __vfs_setxattr(), but it's not really important
for my needs to understand this. Neither helper will generate an fsnotify event.

> > It looks like the S_NOSEC should already be clear when
> > evm_update_evmxattr() is called(?), so it seems more logical to me to
> > call __vfs_setxattr() as there is no ->inode_setsecurity() hook for EVM.
> > Am I missing something?
>
> EVM is triggered when an LSM updates/removes its xattr.   The LSM is
> responsible for taking the inode lock.   Thus it is calling
> __vfs_setxattr_noperm.
>

Surely you need to call a variant that is __vfs_setxattr_locked() or
below it. I just did not understand why that variant is not  __vfs_setxattr().

> >
> > It seems to me that updating the EVM hmac should not generate
> > a visible FS_ATTRIB event to listeners, because it is an internal
> > implementation detail and because update EVM hmac happens
> > following another change to the inode which anyway reports a
> > visible event to listeners.
>
> Ok
>


OK. It looks like there is a consensus about losing those events.
That's what I thought, but wanted to check with you security guys.

Thanks,
Amir.