LSM and setxattr helpers

[Casey Schaufler]

On 4/4/2021 3:27 AM, 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?
>>
>> 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.
>>
> 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.

That seems reasonable to me, but the SELinux team would
have more experience with NFS deployemnts than Smack does.

> 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.

Not that I know of.

> 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().

My understanding is that the inode is always in the process of being
initialized, and that S_NOSEC should never have been set.

>
> 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()
>
> 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?
>
> 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.
> Also, please note that evm_update_evmxattr() may also call
> __vfs_removexattr() which does not call the fsnotify_xattr() hook.
>
> IMA:
> Similarly, ima_fix_xattr() should be called on an inode without
> S_NOSEC flag and no other LSM should be interested in the
> IMA hash update, right? So wouldn't it be better to use
> __vfs_setxattr() in this case as well?
>
> ima_fix_xattr() can be called after file data update, which again
> will have other visible events, but it can also be called in "fix mode"
> I suppose also when reading files? Still, it seems to me like an
> internal implementation detail that should not generate a user
> visible event.
>
> If you agree with my proposed changes, please ACK the
> respective bits of your subsystem from the attached patch.
> Note that my patch does not contain the proposed change to
> use __vfs_setxattr() in IMA/EVM.
>
> Thanks,
> Amir.