211 lines
6.1 KiB
Groff
211 lines
6.1 KiB
Groff
.\"
|
|
.\" Copyright (C) 2001 Chad David <davidc@acns.ab.ca>. All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice(s), this list of conditions and the following disclaimer as
|
|
.\" the first lines of this file unmodified other than the possible
|
|
.\" addition of one or more copyright notices.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice(s), this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
|
|
.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
|
|
.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
|
|
.\" DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
|
|
.\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
|
|
.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
|
|
.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
|
|
.\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
|
|
.\" DAMAGE.
|
|
.\"
|
|
.\" $FreeBSD$
|
|
.\"
|
|
.Dd March 3, 2002
|
|
.Dt UCRED 9
|
|
.Os
|
|
.Sh NAME
|
|
.Nm ucred ,
|
|
.Nm crget ,
|
|
.Nm crhold ,
|
|
.Nm crfree ,
|
|
.Nm crshared ,
|
|
.Nm crcopy ,
|
|
.Nm crdup ,
|
|
.Nm cru2x ,
|
|
.Nm cred_update_thread
|
|
.Nd "functions related to user credentials"
|
|
.Sh SYNOPSIS
|
|
.In sys/param.h
|
|
.In sys/ucred.h
|
|
.Ft "struct ucred *"
|
|
.Fn crget void
|
|
.Ft "struct ucred *"
|
|
.Fn crhold "struct ucred *cr"
|
|
.Ft void
|
|
.Fn crfree "struct ucred *cr"
|
|
.Ft int
|
|
.Fn crshared "struct ucred *cr"
|
|
.Ft void
|
|
.Fn crcopy "struct ucred *dest" "struct ucred *src"
|
|
.Ft "struct ucred *"
|
|
.Fn crdup "struct ucred *cr"
|
|
.Ft void
|
|
.Fn cru2x "struct ucred *cr" "struct xucred *xcr"
|
|
.Ft void
|
|
.Fn cred_update_thread "struct thread *td"
|
|
.Sh DESCRIPTION
|
|
The
|
|
.Nm
|
|
family of functions is used to manage user credential structures
|
|
.Pq Vt "struct ucred"
|
|
within the kernel.
|
|
.Pp
|
|
The
|
|
.Fn crget
|
|
function allocates memory
|
|
for a new structure, sets its reference count to 1, and
|
|
initializes its lock.
|
|
.Pp
|
|
The
|
|
.Fn crhold
|
|
function increases the reference count on the credential.
|
|
.Pp
|
|
The
|
|
.Fn crfree
|
|
function decreases the reference count on the credential.
|
|
If the count drops to 0, the storage for the structure is freed.
|
|
.Pp
|
|
The
|
|
.Fn crshared
|
|
function returns true if the credential is shared.
|
|
A credential is considered to be shared if its reference
|
|
count is greater than one.
|
|
.Pp
|
|
The
|
|
.Fn crcopy
|
|
function copies the contents of the source (template)
|
|
credential into the destination template.
|
|
The
|
|
.Vt uidinfo
|
|
structure within the destination is referenced
|
|
by calling
|
|
.Xr uihold 9 .
|
|
.Pp
|
|
The
|
|
.Fn crdup
|
|
function allocates memory for a new structure and copies the
|
|
contents of
|
|
.Fa cr
|
|
into it.
|
|
The actual copying is performed by
|
|
.Fn crcopy .
|
|
.Pp
|
|
The
|
|
.Fn cru2x
|
|
function converts a
|
|
.Vt ucred
|
|
structure to an
|
|
.Vt xucred
|
|
structure.
|
|
That is,
|
|
it copies data from
|
|
.Fa cr
|
|
to
|
|
.Fa xcr ;
|
|
it ignores fields in the former that are not present in the latter
|
|
(e.g.,
|
|
.Va cr_uidinfo ) ,
|
|
and appropriately sets fields in the latter that are not present in
|
|
the former
|
|
(e.g.,
|
|
.Va cr_version ) .
|
|
.Pp
|
|
The
|
|
.Fn cred_update_thread
|
|
function sets the credentials of
|
|
.Fa td
|
|
to that of its process, freeing its old credential if required.
|
|
.Sh RETURN VALUES
|
|
.Fn crget ,
|
|
.Fn crhold
|
|
and
|
|
.Fn crdup
|
|
all return a pointer to a
|
|
.Vt ucred
|
|
structure.
|
|
.Pp
|
|
.Fn crshared
|
|
returns 0 if the credential has a reference count greater than 1;
|
|
otherwise, 1 is returned.
|
|
.Sh USAGE NOTES
|
|
As of
|
|
.Fx 5.0 ,
|
|
the
|
|
.Vt ucred
|
|
structure contains extensible fields.
|
|
This means that the correct protocol must always be followed to create
|
|
a fresh and writable credential structure: new credentials must always
|
|
be derived from existing credentials using
|
|
.Fn crget
|
|
and
|
|
.Fn crcopy .
|
|
.Pp
|
|
In the common case, credentials required for access control decisions are
|
|
used in a read-only manner.
|
|
In these circumstances, the thread credential
|
|
.Va td_ucred
|
|
should be used, as it requires no locking to access safely, and remains stable
|
|
for the duration of the call even in the face of a multi-threaded
|
|
application changing the process credentials from another thread.
|
|
Primitives such as
|
|
.Xr suser 9
|
|
will assume the use of
|
|
.Va td_ucred
|
|
unless explicitly specified using
|
|
.Xr suser_cred 9 .
|
|
.Pp
|
|
During a process credential update, the process lock must be held across
|
|
check and update, to prevent race conditions.
|
|
The process credential,
|
|
.Va td->td_proc->p_ucred ,
|
|
must be used both for check and update.
|
|
If a process credential is updated during a system call and checks against
|
|
the thread credential are to be made later during the same system call,
|
|
the thread credential must also be refreshed from the process credential
|
|
so as to prevent use of a stale value.
|
|
To avoid this scenario, it is recommended that system calls updating the
|
|
process credential be designed to avoid other authorization functions.
|
|
.Pp
|
|
If temporarily elevated privileges are required for a thread, the thread
|
|
credential can by replaced for the duration of an activity, or for
|
|
the remainder of the system call.
|
|
However, as a thread credential is often shared, appropriate care should be
|
|
taken to make sure modifications are made to a writable credential
|
|
through the use of
|
|
.Fn crget
|
|
and
|
|
.Fn crcopy .
|
|
.Pp
|
|
Caution should be exercised when checking authorization for a thread or
|
|
process perform an operation on another thread or process.
|
|
As a result of temporary elevation, the target thread credential should
|
|
.Em never
|
|
be used as the target credential in an access control decision: the process
|
|
credential associated with the thread,
|
|
.Va td->td_proc->p_ucred ,
|
|
should be used instead.
|
|
For example,
|
|
.Xr p_candebug 9
|
|
accepts a target process, not a target thread, for access control purposes.
|
|
.Sh SEE ALSO
|
|
.Xr uihold 9
|
|
.Sh AUTHORS
|
|
This manual page was written by
|
|
.An Chad David Aq davidc@acns.ab.ca .
|