CREDENTIALS(9F) Kernel Functions for Drivers CREDENTIALS(9F)

credentials, CRED, crdup, crfree, crget, crhold, kcred, zone_kcred
Functions for obtaining credentials in the kernel

#include <sys/cred.h>

cred_t *
CRED();

cred_t *
crdup(cred_t *cr);

void
crfree(cred_t *cr);

cred_t *
crget();

void
crhold(cred_t *cr);

cred_t *
zone_kcred();

cred_t *kcred

Volatile - This interface is still evolving in illumos. API and ABI stability is not guaranteed.

cr
pointer to the user credential structure.

Some kernel interfaces require a credential as an argument. This page documents the credentials available in the system, as well as interfaces for creating new credentials.

Most users do not need to create new credentials. Instead, users should generally use the credentials of the executing context.

This interface is primarily intended for services that must perform operations on behalf of multiple remotely authenticated users, whose authentication context is unrelated to the context of the executing thread or process. Drivers MUST NOT create new credentials. Drivers should use the provided credential.

For functions that do not return new credentials, if the credentials will be used outside of their context (i.e. if the output of zone_kcred() is referenced outside of the zone), then one should use crdup() or crhold() to ensure that the credentials remain valid.

CRED()

The CRED() function returns the credential of the calling thread. Its contents depend on the calling context (user, kernel, interrupt).

crdup()

crdup() returns a newly-allocated copy of cr with reference count of 1. It sleeps until the allocation succeeds.

crfree()

crfree() releases a reference to cr. If this is the last reference, the credential is destroyed.

crhold()

crhold() takes a reference to cr.

kcred

kcred is the root credential of the global zone. Its UIDs and GIDs are all 0. It has the following privilege sets by default:
E: basic,proc_secflags
I: basic,proc_secflags
P: basic,proc_secflags
L: all

NET_MAC_AWARE is set in the credential's flags. It is not marked privilege-aware.

kcred will never be freed by the system.

zone_kcred()

The zone_kcred() function returns the root credential of the zone to which the calling thread belongs. This cred is derived from the global kcred, minus any privileges denied to the zone.

crget()

The crget() function returns a copy of zone_kcred() suitable for modification by the caller. This is useful for obtaining a default, well-initialized credential in the appropriate zone context, that is free of privileges or limitations of the originating thread. It must be freed with crfree(). It sleeps until the allocation succeeds.

kcred and zone_kcred() are not privilege-aware, and have all IDs set to 0. This causes their Limit set to be used in place of the Effective and Permitted sets, which significantly expands their privileges.

If the output of crget() is not later marked as privilege aware, and its UID is not set to a non-zero value, then its Limit set will be used in place of its Effective and Permitted sets, significantly expanding its privileges. Callers should either mark the credential as privilege-aware, reduce the Limit set appropriately, or ensure that they intend for zero-uid users to have expanded privileges.

kcred, zone_kcred(), and CRED() are not suitable for modfication by the caller. Callers must use crdup() to create a copy of these credentials that are suitable for modification.

Callers of zone_kcred() and crget() must take care to ensure that the calling thread is executing in the context of the appropriate zone. If the thread is performing work on behalf of a different zone, or if one is uncertain of the zone context of the calling thread, then one should find the appropriate zone by other means, and reference zone->zone_kcred explicitly.

These functions can be called from user and kernel contexts.

zone_kcred() and CRED() return a pointer to a cred_t that should not be modified.

crget() and crdup() return a pointer to a newly allocated cred_t.

zone_kcred(), CRED(), crdup(), and crget() can never fail, and always return a valid credential.

ddi_cred(9f)
August 21, 2020 OmniOS