Hi Al, Christoph, Trond, Stephen, Casey,

Here's a set of patches that implement a very basic set of COW credentials. It compiles, links and runs for x86_64 with EXT3, (V)FAT, NFS, AFS, SELinux and keyrings all enabled. Most other filesystems are disabled, apart from things like proc. It is not intended to completely cover the kernel at this point.

The cred struct contains the credentials that the kernel needs to act upon something or to create something. Credentials that govern how a task may be acted upon remain in the task struct.

In essence, the introduction of the cred struct separates a task's subjective context (the authority with which it acts) from its objective context (the authorisation required by others that want to act upon it), and permits overriding of the subjective context by a kernel service so that the service can act on the task's behalf to do something the task couldn't do on its own authority.

Because keyrings and effective capabilities can be installed or changed in one process by another process, they are shadowed by the cred structure rather than residing there. Additionally, the session and process keyrings are shared between all the threads of a process. The shadowing is performed by update_current_cred() which is invoked on entry to any system call that might need it.

A thread's cred struct may be read by that thread without any RCU precautions as only that thread may replace the its own cred struct. To change a thread's credentials, dup_cred() should be called to create a new copy, the copy should be changed, and then set_current_cred() should be called to make it live. Once live, it may not be changed as it may then be shared with file descriptors, RPC calls and other threads. RCU will be used to dispose of the old structure.

The four patches are:

(1) Introduce struct cred and migrate fsuid, fsgid, the groups list and the

     keyrings pointer to it.

(2) Introduce a security pointer into the cred struct and add LSM hooks to

     duplicate the information pointed to thereby and to free it.

     Make SELinux implement the hooks, splitting out some the task security
     data to be associated with struct cred instead.

(3) Migrate the effective capabilities mask into the cred struct.

(4) Provide a pair of LSM hooks so that a kernel service can (a) get a

     credential record representing the authority with which it is permitted to
     act, and (b) alter the file creation context in a credential record.

In addition, as this works with cachefiles, I've included all the FS-Cache, CacheFiles, NFS and AFS patches.

To substitute a temporary set of credentials, the cred struct attached to the task should be altered, like so:

	int get_privileged_creds(...)
	{
		/* get special privileged creds */
		my_special_cred = get_kernel_cred("cachefiles", current);
		change_create_files_as(my_special_cred, my_cache_dir);
	}

	int do_stuff(...)
	{
		struct cred *cred;

		/* rotate in the new creds, saving the old */
		cred = __set_current_cred(get_cred(my_special_cred));

		do_privileged_stuff();

		/* restore the old creds */
		set_current_cred(cred);
	}

One thing I'm not certain about is how this should interact with /proc, which can display some of the stuff in the cred struct. I think it may be necessary to have a real cred pointer and an effective cred pointer, with the contents of /proc coming from the real, but the effective governing what actually goes on.

David

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