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Subject[PATCH 00/22] Introduce credential record
Date


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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