sys_capability.c

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/*-
 * Copyright (c) 2008-2011 Robert N. M. Watson
 * Copyright (c) 2010-2011 Jonathan Anderson
 * All rights reserved.
 *
 * This software was developed at the University of Cambridge Computer
 * Laboratory with support from a grant from Google, Inc.
 *
 * 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, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, 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 AUTHOR AND CONTRIBUTORS ``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 AUTHOR OR CONTRIBUTORS 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 kernel capability facility.
 *
 * Two kernel features are implemented here: capability mode, a sandboxed mode
 * of execution for processes, and capabilities, a refinement on file
 * descriptors that allows fine-grained control over operations on the file
 * descriptor.  Collectively, these allow processes to run in the style of a
 * historic "capability system" in which they can use only resources
 * explicitly delegated to them.  This model is enforced by restricting access
 * to global namespaces in capability mode.
 *
 * Capabilities wrap other file descriptor types, binding them to a constant
 * rights mask set when the capability is created.  New capabilities may be
 * derived from existing capabilities, but only if they have the same or a
 * strict subset of the rights on the original capability.
 *
 * System calls permitted in capability mode are defined in capabilities.conf;
 * calls must be carefully audited for safety to ensure that they don't allow
 * escape from a sandbox.  Some calls permit only a subset of operations in
 * capability mode -- for example, shm_open(2) is limited to creating
 * anonymous, rather than named, POSIX shared memory objects.
 */

#include "opt_capsicum.h"

#include <sys/cdefs.h>
__FBSDID("$BSDSUniX$");

#include <sys/param.h>
#include <sys/capability.h>
#include <sys/file.h>
#include <sys/filedesc.h>
#include <sys/kernel.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/proc.h>
#include <sys/sysproto.h>
#include <sys/sysctl.h>
#include <sys/systm.h>
#include <sys/ucred.h>

#include <security/audit/audit.h>

#include <vm/uma.h>
#include <vm/vm.h>

#ifdef CAPABILITY_MODE

FEATURE(security_capability_mode, "Capsicum Capability Mode");

/*
 * System call to enter capability mode for the process.
 */
int
sys_cap_enter(struct thread *td, struct cap_enter_args *uap)
{
        struct ucred *newcred, *oldcred;
        struct proc *p;

        if (IN_CAPABILITY_MODE(td))
                return (0);

        newcred = crget();
        p = td->td_proc;
        PROC_LOCK(p);
        oldcred = p->p_ucred;
        crcopy(newcred, oldcred);
        newcred->cr_flags |= CRED_FLAG_CAPMODE;
        p->p_ucred = newcred;
        PROC_UNLOCK(p);
        crfree(oldcred);
        return (0);
}

/*
 * System call to query whether the process is in capability mode.
 */
int
sys_cap_getmode(struct thread *td, struct cap_getmode_args *uap)
{
        u_int i;

        i = (IN_CAPABILITY_MODE(td)) ? 1 : 0;
        return (copyout(&i, uap->modep, sizeof(i)));
}

#else /* !CAPABILITY_MODE */

int
sys_cap_enter(struct thread *td, struct cap_enter_args *uap)
{

        return (ENOSYS);
}

int
sys_cap_getmode(struct thread *td, struct cap_getmode_args *uap)
{

        return (ENOSYS);
}

#endif /* CAPABILITY_MODE */

#ifdef CAPABILITIES

FEATURE(security_capabilities, "Capsicum Capabilities");

/*
 * struct capability describes a capability, and is hung off of its struct
 * file f_data field.  cap_file and cap_rightss are static once hooked up, as
 * neither the object it references nor the rights it encapsulates are
 * permitted to change.
 */
struct capability {
        struct file     *cap_object;    /* Underlying object's file. */
        struct file     *cap_file;      /* Back-pointer to cap's file. */
        cap_rights_t     cap_rights;    /* Mask of rights on object. */
};

/*
 * Capabilities have a fileops vector, but in practice none should ever be
 * called except for fo_close, as the capability will normally not be
 * returned during a file descriptor lookup in the system call code.
 */
static fo_rdwr_t capability_read;
static fo_rdwr_t capability_write;
static fo_truncate_t capability_truncate;
static fo_ioctl_t capability_ioctl;
static fo_poll_t capability_poll;
static fo_kqfilter_t capability_kqfilter;
static fo_stat_t capability_stat;
static fo_close_t capability_close;
static fo_chmod_t capability_chmod;
static fo_chown_t capability_chown;

static struct fileops capability_ops = {
        .fo_read = capability_read,
        .fo_write = capability_write,
        .fo_truncate = capability_truncate,
        .fo_ioctl = capability_ioctl,
        .fo_poll = capability_poll,
        .fo_kqfilter = capability_kqfilter,
        .fo_stat = capability_stat,
        .fo_close = capability_close,
        .fo_chmod = capability_chmod,
        .fo_chown = capability_chown,
        .fo_flags = DFLAG_PASSABLE,
};

static struct fileops capability_ops_unpassable = {
        .fo_read = capability_read,
        .fo_write = capability_write,
        .fo_truncate = capability_truncate,
        .fo_ioctl = capability_ioctl,
        .fo_poll = capability_poll,
        .fo_kqfilter = capability_kqfilter,
        .fo_stat = capability_stat,
        .fo_close = capability_close,
        .fo_chmod = capability_chmod,
        .fo_chown = capability_chown,
        .fo_flags = 0,
};

static uma_zone_t capability_zone;

static void
capability_init(void *dummy __unused)
{

        capability_zone = uma_zcreate("capability", sizeof(struct capability),
            NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
        if (capability_zone == NULL)
                panic("capability_init: capability_zone not initialized");
}
SYSINIT(vfs, SI_SUB_VFS, SI_ORDER_ANY, capability_init, NULL);

/*
 * Test whether a capability grants the requested rights.
 */
static int
cap_check(struct capability *c, cap_rights_t rights)
{

        if ((c->cap_rights | rights) != c->cap_rights)
                return (ENOTCAPABLE);
        return (0);
}

/*
 * Extract rights from a capability for monitoring purposes -- not for use in
 * any other way, as we want to keep all capability permission evaluation in
 * this one file.
 */
cap_rights_t
cap_rights(struct file *fp_cap)
{
        struct capability *c;

        KASSERT(fp_cap->f_type == DTYPE_CAPABILITY,
            ("cap_rights: !capability"));

        c = fp_cap->f_data;
        return (c->cap_rights);
}

/*
 * System call to create a new capability reference to either an existing
 * file object or an an existing capability.
 */
int
sys_cap_new(struct thread *td, struct cap_new_args *uap)
{
        int error, capfd;
        int fd = uap->fd;
        struct file *fp;
        cap_rights_t rights = uap->rights;

        AUDIT_ARG_FD(fd);
        AUDIT_ARG_RIGHTS(rights);
        error = fget(td, fd, rights, &fp);
        if (error)
                return (error);
        AUDIT_ARG_FILE(td->td_proc, fp);
        error = kern_capwrap(td, fp, rights, &capfd);
        /*
         * Release our reference to the file (kern_capwrap has held a reference
         * for the filedesc array).
         */
        fdrop(fp, td);
        if (error == 0)
                td->td_retval[0] = capfd;
        return (error);
}

/*
 * System call to query the rights mask associated with a capability.
 */
int
sys_cap_getrights(struct thread *td, struct cap_getrights_args *uap)
{
        struct capability *cp;
        struct file *fp;
        int error;

        AUDIT_ARG_FD(uap->fd);
        error = fgetcap(td, uap->fd, &fp);
        if (error)
                return (error);
        cp = fp->f_data;
        error = copyout(&cp->cap_rights, uap->rightsp, sizeof(*uap->rightsp));
        fdrop(fp, td);
        return (error);
}

/*
 * Create a capability to wrap around an existing file.
 */
int
kern_capwrap(struct thread *td, struct file *fp, cap_rights_t rights,
    int *capfdp)
{
        struct capability *cp, *cp_old;
        struct file *fp_object, *fcapp;
        int error;

        if ((rights | CAP_MASK_VALID) != CAP_MASK_VALID)
                return (EINVAL);

        /*
         * If a new capability is being derived from an existing capability,
         * then the new capability rights must be a subset of the existing
         * rights.
         */
        if (fp->f_type == DTYPE_CAPABILITY) {
                cp_old = fp->f_data;
                if ((cp_old->cap_rights | rights) != cp_old->cap_rights)
                        return (ENOTCAPABLE);
        }

        /*
         * Allocate a new file descriptor to hang the capability off of.
         */
        error = falloc(td, &fcapp, capfdp, fp->f_flag);
        if (error)
                return (error);

        /*
         * Rather than nesting capabilities, directly reference the object an
         * existing capability references.  There's nothing else interesting
         * to preserve for future use, as we've incorporated the previous
         * rights mask into the new one.  This prevents us from having to
         * deal with capability chains.
         */
        if (fp->f_type == DTYPE_CAPABILITY)
                fp_object = ((struct capability *)fp->f_data)->cap_object;
        else
                fp_object = fp;
        fhold(fp_object);
        cp = uma_zalloc(capability_zone, M_WAITOK | M_ZERO);
        cp->cap_rights = rights;
        cp->cap_object = fp_object;
        cp->cap_file = fcapp;
        if (fp->f_flag & DFLAG_PASSABLE)
                finit(fcapp, fp->f_flag, DTYPE_CAPABILITY, cp,
                    &capability_ops);
        else
                finit(fcapp, fp->f_flag, DTYPE_CAPABILITY, cp,
                    &capability_ops_unpassable);

        /*
         * Release our private reference (the proc filedesc still has one).
         */
        fdrop(fcapp, td);
        return (0);
}

/*
 * Given a file descriptor, test it against a capability rights mask and then
 * return the file descriptor on which to actually perform the requested
 * operation.  As long as the reference to fp_cap remains valid, the returned
 * pointer in *fp will remain valid, so no extra reference management is
 * required, and the caller should fdrop() fp_cap as normal when done with
 * both.
 */
int
cap_funwrap(struct file *fp_cap, cap_rights_t rights, struct file **fpp)
{
        struct capability *c;
        int error;

        if (fp_cap->f_type != DTYPE_CAPABILITY) {
                *fpp = fp_cap;
                return (0);
        }
        c = fp_cap->f_data;
        error = cap_check(c, rights);
        if (error)
                return (error);
        *fpp = c->cap_object;
        return (0);
}

/*
 * Slightly different routine for memory mapping file descriptors: unwrap the
 * capability and check CAP_MMAP, but also return a bitmask representing the
 * maximum mapping rights the capability allows on the object.
 */
int
cap_funwrap_mmap(struct file *fp_cap, cap_rights_t rights, u_char *maxprotp,
    struct file **fpp)
{
        struct capability *c;
        u_char maxprot;
        int error;

        if (fp_cap->f_type != DTYPE_CAPABILITY) {
                *fpp = fp_cap;
                *maxprotp = VM_PROT_ALL;
                return (0);
        }
        c = fp_cap->f_data;
        error = cap_check(c, rights | CAP_MMAP);
        if (error)
                return (error);
        *fpp = c->cap_object;
        maxprot = 0;
        if (c->cap_rights & CAP_READ)
                maxprot |= VM_PROT_READ;
        if (c->cap_rights & CAP_WRITE)
                maxprot |= VM_PROT_WRITE;
        if (c->cap_rights & CAP_MAPEXEC)
                maxprot |= VM_PROT_EXECUTE;
        *maxprotp = maxprot;
        return (0);
}

/*
 * When a capability is closed, simply drop the reference on the underlying
 * object and free the capability.  fdrop() will handle the case where the
 * underlying object also needs to close, and the caller will have already
 * performed any object-specific lock or mqueue handling.
 */
static int
capability_close(struct file *fp, struct thread *td)
{
        struct capability *c;
        struct file *fp_object;

        KASSERT(fp->f_type == DTYPE_CAPABILITY,
            ("capability_close: !capability"));

        c = fp->f_data;
        fp->f_ops = &badfileops;
        fp->f_data = NULL;
        fp_object = c->cap_object;
        uma_zfree(capability_zone, c);
        return (fdrop(fp_object, td));
}

/*
 * In general, file descriptor operations should never make it to the
 * capability, only the underlying file descriptor operation vector, so panic
 * if we do turn up here.
 */
static int
capability_read(struct file *fp, struct uio *uio, struct ucred *active_cred,
    int flags, struct thread *td)
{

        panic("capability_read");
}

static int
capability_write(struct file *fp, struct uio *uio, struct ucred *active_cred,
    int flags, struct thread *td)
{

        panic("capability_write");
}

static int
capability_truncate(struct file *fp, off_t length, struct ucred *active_cred,
    struct thread *td)
{

        panic("capability_truncate");
}

static int
capability_ioctl(struct file *fp, u_long com, void *data,
    struct ucred *active_cred, struct thread *td)
{

        panic("capability_ioctl");
}

static int
capability_poll(struct file *fp, int events, struct ucred *active_cred,
    struct thread *td)
{

        panic("capability_poll");
}

static int
capability_kqfilter(struct file *fp, struct knote *kn)
{

        panic("capability_kqfilter");
}

static int
capability_stat(struct file *fp, struct stat *sb, struct ucred *active_cred,
    struct thread *td)
{

        panic("capability_stat");
}

int
capability_chmod(struct file *fp, mode_t mode, struct ucred *active_cred,
    struct thread *td)
{

        panic("capability_chmod");
}

int
capability_chown(struct file *fp, uid_t uid, gid_t gid,
    struct ucred *active_cred, struct thread *td)
{

        panic("capability_chown");
}

#else /* !CAPABILITIES */

/*
 * Stub Capability functions for when options CAPABILITIES isn't compiled
 * into the kernel.
 */
int
sys_cap_new(struct thread *td, struct cap_new_args *uap)
{

        return (ENOSYS);
}

int
sys_cap_getrights(struct thread *td, struct cap_getrights_args *uap)
{

        return (ENOSYS);
}

int
cap_funwrap(struct file *fp_cap, cap_rights_t rights, struct file **fpp)
{

        KASSERT(fp_cap->f_type != DTYPE_CAPABILITY,
            ("cap_funwrap: saw capability"));

        *fpp = fp_cap;
        return (0);
}

int
cap_funwrap_mmap(struct file *fp_cap, cap_rights_t rights, u_char *maxprotp,
    struct file **fpp)
{

        KASSERT(fp_cap->f_type != DTYPE_CAPABILITY,
            ("cap_funwrap_mmap: saw capability"));

        *fpp = fp_cap;
        *maxprotp = VM_PROT_ALL;
        return (0);
}

#endif /* CAPABILITIES */