I have been working on SELinux support for Puppet. One issue that has cropped up is the behavior on filesystems which don't support SELinux.
They all appear to get a default label, some seem to allow changing the label (VFAT) in a non-persistent manner, some seem to throw "not supported" errors (NFS).
How can I detect if a file is on a filesystem which supports SELinux without trying to update the label?
The best idea so far as been to parse /proc/mounts and use that to determine what type of filesystem a file lives on, then check it against a whitelist (which would include ext3, xfs, ?) but it seems like there has to be a cleaner/simpler way.
What I would like would be a "getfilecon" call that returns the real label, ignoring any mount-time defaults.
Any ideas?
Thanks, Sean
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Sean E. Millichamp wrote:
I have been working on SELinux support for Puppet. One issue that has cropped up is the behavior on filesystems which don't support SELinux.
They all appear to get a default label, some seem to allow changing the label (VFAT) in a non-persistent manner, some seem to throw "not supported" errors (NFS).
How can I detect if a file is on a filesystem which supports SELinux without trying to update the label?
The best idea so far as been to parse /proc/mounts and use that to determine what type of filesystem a file lives on, then check it against a whitelist (which would include ext3, xfs, ?) but it seems like there has to be a cleaner/simpler way.
What I would like would be a "getfilecon" call that returns the real label, ignoring any mount-time defaults.
Any ideas?
Thanks, Sean
-- fedora-selinux-list mailing list fedora-selinux-list@redhat.com https://www.redhat.com/mailman/listinfo/fedora-selinux-list
I have been waiting for some one else to respond to this. I think this would be better sent to the nsa selinux list for better discussion.
The problem with your parsing of the /proc/mounts is that it would not give you an accurate idea of what supports and what does not support SELinux labeling. Also this can change over time.
If I mount an ext3 file system with a context mount, then it will no longer allow you to set the file context. I think the best idea is just attempt to assign the context and if it fails, ignore the error. I guess you can report it, if in verbose mode as a warning.
Others may have different ideas.
On Mon, 2008-11-17 at 09:45 -0500, Daniel J Walsh wrote:
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Sean E. Millichamp wrote:
I have been working on SELinux support for Puppet. One issue that has cropped up is the behavior on filesystems which don't support SELinux.
They all appear to get a default label, some seem to allow changing the label (VFAT) in a non-persistent manner, some seem to throw "not supported" errors (NFS).
How can I detect if a file is on a filesystem which supports SELinux without trying to update the label?
The best idea so far as been to parse /proc/mounts and use that to determine what type of filesystem a file lives on, then check it against a whitelist (which would include ext3, xfs, ?) but it seems like there has to be a cleaner/simpler way.
What I would like would be a "getfilecon" call that returns the real label, ignoring any mount-time defaults.
Any ideas?
Thanks, Sean
I have been waiting for some one else to respond to this. I think this would be better sent to the nsa selinux list for better discussion.
The problem with your parsing of the /proc/mounts is that it would not give you an accurate idea of what supports and what does not support SELinux labeling. Also this can change over time.
If I mount an ext3 file system with a context mount, then it will no longer allow you to set the file context. I think the best idea is just attempt to assign the context and if it fails, ignore the error. I guess you can report it, if in verbose mode as a warning.
Others may have different ideas.
You'd want to distinguish EOPNOTSUPP from other errors in that case. But note that this won't catch certain filesystems (like the vfat example he gave), as changing the in-core context of a file labeled via genfscon rules is supported presently. We could possibly change that to also return EOPNOTSUPP.
The problem with using getfilecon() to probe for support is that SELinux always assigns some security context to each file for access control purposes, even if the underlying filesystem doesn't support storage. If we had separate getfilecon() vs. getxattr() kernel interface ala FreeBSD, applications could test for support for storage separately, but that isn't the case and is unlikely to change.
On Mon, 2008-11-17 at 10:26 -0500, Stephen Smalley wrote:
On Mon, 2008-11-17 at 09:45 -0500, Daniel J Walsh wrote:
I have been waiting for some one else to respond to this. I think this would be better sent to the nsa selinux list for better discussion.
The problem with your parsing of the /proc/mounts is that it would not give you an accurate idea of what supports and what does not support SELinux labeling. Also this can change over time.
If I mount an ext3 file system with a context mount, then it will no longer allow you to set the file context. I think the best idea is just attempt to assign the context and if it fails, ignore the error. I guess you can report it, if in verbose mode as a warning.
Others may have different ideas.
You'd want to distinguish EOPNOTSUPP from other errors in that case. But note that this won't catch certain filesystems (like the vfat example he gave), as changing the in-core context of a file labeled via genfscon rules is supported presently. We could possibly change that to also return EOPNOTSUPP.
The problem with using getfilecon() to probe for support is that SELinux always assigns some security context to each file for access control purposes, even if the underlying filesystem doesn't support storage. If we had separate getfilecon() vs. getxattr() kernel interface ala FreeBSD, applications could test for support for storage separately, but that isn't the case and is unlikely to change.
Hmm... I believe that checking for an error at assignment time is not going to be a workable solution for Puppet.
The problem is that Puppet prepares what it needs to do in a transaction before doing it. Take the situation where /usr/local is NFS mounted:
# ls -Z /usr/local/bin/foo -rwxr-xr-x root root system_u:object_r:nfs_t:s0 /usr/local/bin/foo # matchpathcon /usr/local/bin/foo /usr/local/bin/foo system_u:object_r:bin_t:s0
Then you run puppet with a manifest that includes management of /usr/local/bin/foo. The first thing Puppet does is determine default values. For SELinux this means a call to matchpathcon. Then Puppet determines the current values with lgetfilecon. It notices that the default value for the type should be bin_t, but the current is nfs_t so it adds changing the type to its list of things to do. As it is building this list it reports on the things it intends to do.
Once it determines all of the actions that it needs to take only then does it perform the setfilecon call to update the context. Even if we catch and silently ignore the error here the logging for the steps it intends to take will occur on every single Puppet run, filling the logs with what amounts to garbage and making email reports of changes essentially useless (as you would get an email on every run telling you of the changes it was going to make).
Performing a setfilecon call to the same context that exists during the first phase to determine if a value can be set would be the only way I could see to handle this, but it violates Puppet's promise of not touching anything during a noop run and will update the ctime of the file.
In the case of filesystems which behave like vfat Puppet would set the label the first time following the mount and until it is remounted wouldn't generate any additional messages. Filesystems which behave like NFS are the real problem case though and NFS is far more likely to be mounted at a spot where matchpathcon returns a default then (for example) vfat is.
I'm not a fan of hardcoding a whitelist of supported filesystems for the very reasons Dan mentioned but it sounds like there isn't a good option for Puppet at the moment (and since I couldn't find any better options, this is what is going into the next Puppet release). No chance of seeing a "supports_setfilecon" type call?
Sean
On Mon, 2008-11-17 at 11:16 -0500, Sean E. Millichamp wrote:
Hmm... I believe that checking for an error at assignment time is not going to be a workable solution for Puppet.
The problem is that Puppet prepares what it needs to do in a transaction before doing it. Take the situation where /usr/local is NFS mounted:
# ls -Z /usr/local/bin/foo -rwxr-xr-x root root system_u:object_r:nfs_t:s0 /usr/local/bin/foo # matchpathcon /usr/local/bin/foo /usr/local/bin/foo system_u:object_r:bin_t:s0
Then you run puppet with a manifest that includes management of /usr/local/bin/foo.
Can you explain what it means for puppet to manage a NFS-mounted filesystem? I'd tend to think that file management would happen on the server, not on a client. And puppet could easily run into problems with e.g. setting ownership/mode information on a NFS-mounted filesystem due to squashroot, uid/gid remapping, etc.
The first thing Puppet does is determine default values. For SELinux this means a call to matchpathcon. Then Puppet determines the current values with lgetfilecon. It notices that the default value for the type should be bin_t, but the current is nfs_t so it adds changing the type to its list of things to do. As it is building this list it reports on the things it intends to do.
Once it determines all of the actions that it needs to take only then does it perform the setfilecon call to update the context. Even if we catch and silently ignore the error here the logging for the steps it intends to take will occur on every single Puppet run, filling the logs with what amounts to garbage and making email reports of changes essentially useless (as you would get an email on every run telling you of the changes it was going to make).
Performing a setfilecon call to the same context that exists during the first phase to determine if a value can be set would be the only way I could see to handle this, but it violates Puppet's promise of not touching anything during a noop run and will update the ctime of the file.
In the case of filesystems which behave like vfat Puppet would set the label the first time following the mount and until it is remounted wouldn't generate any additional messages. Filesystems which behave like NFS are the real problem case though and NFS is far more likely to be mounted at a spot where matchpathcon returns a default then (for example) vfat is.
I'm not a fan of hardcoding a whitelist of supported filesystems for the very reasons Dan mentioned but it sounds like there isn't a good option for Puppet at the moment (and since I couldn't find any better options, this is what is going into the next Puppet release).
Ok - that's essentially what Dan does in his /sbin/fixfiles script as well.
No chance of seeing a "supports_setfilecon" type call?
Possibly an interface could be added to selinuxfs and wrapped with a libselinux function of that nature.
Or possibly we could export that via a new mount option that shows up in /proc/mounts since we now support exporting information about context mounts there? There are already mount options for user_xattr and acl for example, but not explicitly for security contexts.
On Mon, 2008-11-17 at 11:34 -0500, Stephen Smalley wrote:
Can you explain what it means for puppet to manage a NFS-mounted filesystem? I'd tend to think that file management would happen on the server, not on a client. And puppet could easily run into problems with e.g. setting ownership/mode information on a NFS-mounted filesystem due to squashroot, uid/gid remapping, etc.
Managing a file essentially means "ensure the existence/absence of a file/directory, its ownership/permissions, and contents". The first use case that pops to mind is for NFS servers where you can't run Puppet on the server itself (an appliance device). Then you would need to manage the files from one or more clients.
The big difference between SELinux contexts and the other attributes is that in the absence of uid/gid/mode being explicitly specified Puppet sees the default as "nil". This causes Puppet to ignore those attributes and, if creating a file, will just default to the umask/user that Puppet is running with. With SELinux the proper way to determine the default is to ask matchpathcon - so for any file where the user doesn't explicitly specify a context but matchpathcon returns one there is a non-nil default, causing Puppet to try to make the file in-sync if the actual doesn't match it. If an NFS filesystem was mounted at /mnt/nfs on my Fedora 9 system matchpathcon won't return a default for /mnt/nfs/foo so Puppet won't trigger any SELinux actions (unless the user explicitly lists a context).
I'm not a fan of hardcoding a whitelist of supported filesystems for the very reasons Dan mentioned but it sounds like there isn't a good option for Puppet at the moment (and since I couldn't find any better options, this is what is going into the next Puppet release).
Ok - that's essentially what Dan does in his /sbin/fixfiles script as well.
Ah, I just skimmed the script. I feel much better about this approach now and it will hopefully suffice. Also, since the whitelist exclusion is only for determining defaults, a user that wanted to actively manage an SELinux context could still explicitly list it in the file attributes and Puppet will try to set it regardless.
No chance of seeing a "supports_setfilecon" type call?
Possibly an interface could be added to selinuxfs and wrapped with a libselinux function of that nature.
Or possibly we could export that via a new mount option that shows up in /proc/mounts since we now support exporting information about context mounts there? There are already mount options for user_xattr and acl for example, but not explicitly for security contexts.
We'll see how well the filesystem whitelist works. I think it will probably be good enough - at least for the foreseeable future.
Thanks for the feedback.
Sean
selinux@lists.fedoraproject.org
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Daniel J Walsh -
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Stephen Smalley