Re: [PATCH 7/8] SELinux: Create a common helper to determine an inode label

From: Stephen Smalley
Date: Thu Jun 18 2015 - 11:22:00 EST

On 06/18/2015 11:13 AM, David Howells wrote:
> Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:
>>> + if ((sbsec->flags & SE_SBINITIALIZED) &&
>>> + (sbsec->behavior == SECURITY_FS_USE_MNTPOINT)) {
>>> + *_new_isid = sbsec->mntpoint_sid;
>>> + } else if (tsec->create_sid) {
>> This doesn't quite match the logic in inode_init_security today, see its
>> checking of SBLABEL_MNT.
> Fair point. What does SBLABEL_MNT mean precisely? It seems to indicate one
> of an odd mix of behaviours. I presume it means that we *have* to calculate a
> label and can't get one from the underlying fs if it is not set.

It means the filesystem supports per-file labeling and you can use
setxattr(..."security.selinux") and setfscreatecon() for files on it.
You can see whether it is set on a filesystem by looking for the
seclabel option in cat /proc/mounts. If it is not set, then we ignore
tsec->create_sid. It is arguable as to whether it is correct to always
call security_transition_sid() there either, but that's another topic.

> Also, in:
> sbsec->flags |= SE_SBINITIALIZED;
> if (selinux_is_sblabel_mnt(sb))
> sbsec->flags |= SBLABEL_MNT;
> should SE_SBINITIALIZED be set after SBLABEL_MNT? And should there be a
> memory barrier in here somewhere before the setting of SE_SBINITIALIZED?

I believe that's all under sbsec->lock held by the caller, but that code
has changed a lot and been refactored significantly by others.

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