sys.c

在linux下使用c语言编写的一个文件拷贝的源码

 * Unprivileged users may change the real gid to the effective gid
 * or vice versa.  (BSD-style)
 *
 * If you set the real gid at all, or set the effective gid to a value not
 * equal to the real gid, then the saved gid is set to the new effective gid.
 *
 * This makes it possible for a setgid program to completely drop its
 * privileges, which is often a useful assertion to make when you are doing
 * a security audit over a program.
 *
 * The general idea is that a program which uses just setregid() will be
 * 100% compatible with BSD.  A program which uses just setgid() will be
 * 100% compatible with POSIX with saved IDs. 
 *
 * SMP: There are not races, the GIDs are checked only by filesystem
 *      operations (as far as semantic preservation is concerned).
 */
asmlinkage long sys_setregid(gid_t rgid, gid_t egid)
{
    int old_rgid = current->gid;
    int old_egid = current->egid;
    int new_rgid = old_rgid;
    int new_egid = old_egid;

    if (rgid != (gid_t) - 1) {
	if ((old_rgid == rgid) ||
	    (current->egid == rgid) || capable(CAP_SETGID))
	    new_rgid = rgid;
	else
	    return -EPERM;
    }
    if (egid != (gid_t) - 1) {
	if ((old_rgid == egid) ||
	    (current->egid == egid) ||
	    (current->sgid == egid) || capable(CAP_SETGID))
	    new_egid = egid;
	else {
	    return -EPERM;
	}
    }
    if (new_egid != old_egid) {
	current->mm->dumpable = 0;
	wmb();
    }
    if (rgid != (gid_t) - 1 || (egid != (gid_t) - 1 && egid != old_rgid))
	current->sgid = new_egid;
    current->fsgid = new_egid;
    current->egid = new_egid;
    current->gid = new_rgid;
    return 0;
}

/*
 * setgid() is implemented like SysV w/ SAVED_IDS 
 *
 * SMP: Same implicit races as above.
 */
asmlinkage long sys_setgid(gid_t gid)
{
    int old_egid = current->egid;

    if (capable(CAP_SETGID)) {
	if (old_egid != gid) {
	    current->mm->dumpable = 0;
	    wmb();
	}
	current->gid = current->egid = current->sgid = current->fsgid =
	    gid;
    } else if ((gid == current->gid) || (gid == current->sgid)) {
	if (old_egid != gid) {
	    current->mm->dumpable = 0;
	    wmb();
	}
	current->egid = current->fsgid = gid;
    } else
	return -EPERM;
    return 0;
}

/* 
 * cap_emulate_setxuid() fixes the effective / permitted capabilities of
 * a process after a call to setuid, setreuid, or setresuid.
 *
 *  1) When set*uiding _from_ one of {r,e,s}uid == 0 _to_ all of
 *  {r,e,s}uid != 0, the permitted and effective capabilities are
 *  cleared.
 *
 *  2) When set*uiding _from_ euid == 0 _to_ euid != 0, the effective
 *  capabilities of the process are cleared.
 *
 *  3) When set*uiding _from_ euid != 0 _to_ euid == 0, the effective
 *  capabilities are set to the permitted capabilities.
 *
 *  fsuid is handled elsewhere. fsuid == 0 and {r,e,s}uid!= 0 should 
 *  never happen.
 *
 *  -astor 
 *
 * cevans - New behaviour, Oct '99
 * A process may, via prctl(), elect to keep its capabilities when it
 * calls setuid() and switches away from uid==0. Both permitted and
 * effective sets will be retained.
 * Without this change, it was impossible for a daemon to drop only some
 * of its privilege. The call to setuid(!=0) would drop all privileges!
 * Keeping uid 0 is not an option because uid 0 owns too many vital
 * files..
 * Thanks to Olaf Kirch and Peter Benie for spotting this.
 */
extern inline void cap_emulate_setxuid(int old_ruid, int old_euid,
				       int old_suid)
{
    if ((old_ruid == 0 || old_euid == 0 || old_suid == 0) &&
	(current->uid != 0 && current->euid != 0 && current->suid != 0) &&
	!current->keep_capabilities) {
	cap_clear(current->cap_permitted);
	cap_clear(current->cap_effective);
    }
    if (old_euid == 0 && current->euid != 0) {
	cap_clear(current->cap_effective);
    }
    if (old_euid != 0 && current->euid == 0) {
	current->cap_effective = current->cap_permitted;
    }
}

static int set_user(uid_t new_ruid, int dumpclear)
{
    struct user_struct *new_user, *old_user;

    /* What if a process setreuid()'s and this brings the
     * new uid over his NPROC rlimit?  We can check this now
     * cheaply with the new uid cache, so if it matters
     * we should be checking for it.  -DaveM
     */
    new_user = alloc_uid(new_ruid);
    if (!new_user)
	return -EAGAIN;
    old_user = current->user;
    atomic_dec(&old_user->processes);
    atomic_inc(&new_user->processes);

    if (dumpclear) {
	current->mm->dumpable = 0;
	wmb();
    }
    current->uid = new_ruid;
    current->user = new_user;
    free_uid(old_user);
    return 0;
}

/*
 * Unprivileged users may change the real uid to the effective uid
 * or vice versa.  (BSD-style)
 *
 * If you set the real uid at all, or set the effective uid to a value not
 * equal to the real uid, then the saved uid is set to the new effective uid.
 *
 * This makes it possible for a setuid program to completely drop its
 * privileges, which is often a useful assertion to make when you are doing
 * a security audit over a program.
 *
 * The general idea is that a program which uses just setreuid() will be
 * 100% compatible with BSD.  A program which uses just setuid() will be
 * 100% compatible with POSIX with saved IDs. 
 */
asmlinkage long sys_setreuid(uid_t ruid, uid_t euid)
{
    int old_ruid, old_euid, old_suid, new_ruid, new_euid;

    new_ruid = old_ruid = current->uid;
    new_euid = old_euid = current->euid;
    old_suid = current->suid;

    if (ruid != (uid_t) - 1) {
	new_ruid = ruid;
	if ((old_ruid != ruid) &&
	    (current->euid != ruid) && !capable(CAP_SETUID))
	    return -EPERM;
    }

    if (euid != (uid_t) - 1) {
	new_euid = euid;
	if ((old_ruid != euid) &&
	    (current->euid != euid) &&
	    (current->suid != euid) && !capable(CAP_SETUID))
	    return -EPERM;
    }

    if (new_ruid != old_ruid
	&& set_user(new_ruid, new_euid != old_euid) < 0)
	return -EAGAIN;

    if (new_euid != old_euid) {
	current->mm->dumpable = 0;
	wmb();
    }
    current->fsuid = current->euid = new_euid;
    if (ruid != (uid_t) - 1 || (euid != (uid_t) - 1 && euid != old_ruid))
	current->suid = current->euid;
    current->fsuid = current->euid;

    if (!issecure(SECURE_NO_SETUID_FIXUP)) {
	cap_emulate_setxuid(old_ruid, old_euid, old_suid);
    }

    return 0;
}



/*
 * setuid() is implemented like SysV with SAVED_IDS 
 * 
 * Note that SAVED_ID's is deficient in that a setuid root program
 * like sendmail, for example, cannot set its uid to be a normal 
 * user and then switch back, because if you're root, setuid() sets
 * the saved uid too.  If you don't like this, blame the bright people
 * in the POSIX committee and/or USG.  Note that the BSD-style setreuid()
 * will allow a root program to temporarily drop privileges and be able to
 * regain them by swapping the real and effective uid.  
 */
asmlinkage long sys_setuid(uid_t uid)
{
    int old_euid = current->euid;
    int old_ruid, old_suid, new_ruid, new_suid;

    old_ruid = new_ruid = current->uid;
    old_suid = current->suid;
    new_suid = old_suid;

    if (capable(CAP_SETUID)) {
	if (uid != old_ruid && set_user(uid, old_euid != uid) < 0)
	    return -EAGAIN;
	new_suid = uid;
    } else if ((uid != current->uid) && (uid != new_suid))
	return -EPERM;

    if (old_euid != uid) {
	current->mm->dumpable = 0;
	wmb();
    }
    current->fsuid = current->euid = uid;
    current->suid = new_suid;

    if (!issecure(SECURE_NO_SETUID_FIXUP)) {
	cap_emulate_setxuid(old_ruid, old_euid, old_suid);
    }

    return 0;
}


/*
 * This function implements a generic ability to update ruid, euid,
 * and suid.  This allows you to implement the 4.4 compatible seteuid().
 */
asmlinkage long sys_setresuid(uid_t ruid, uid_t euid, uid_t suid)
{
    int old_ruid = current->uid;
    int old_euid = current->euid;
    int old_suid = current->suid;

    if (!capable(CAP_SETUID)) {
	if ((ruid != (uid_t) - 1) && (ruid != current->uid) &&
	    (ruid != current->euid) && (ruid != current->suid))
	    return -EPERM;
	if ((euid != (uid_t) - 1) && (euid != current->uid) &&
	    (euid != current->euid) && (euid != current->suid))
	    return -EPERM;
	if ((suid != (uid_t) - 1) && (suid != current->uid) &&
	    (suid != current->euid) && (suid != current->suid))
	    return -EPERM;
    }
    if (ruid != (uid_t) - 1) {
	if (ruid != current->uid
	    && set_user(ruid, euid != current->euid) < 0)
	    return -EAGAIN;
    }
    if (euid != (uid_t) - 1) {
	if (euid != current->euid) {
	    current->mm->dumpable = 0;
	    wmb();
	}
	current->euid = euid;
	current->fsuid = euid;
    }
    if (suid != (uid_t) - 1)
	current->suid = suid;

    if (!issecure(SECURE_NO_SETUID_FIXUP)) {
	cap_emulate_setxuid(old_ruid, old_euid, old_suid);
    }

    return 0;
}

asmlinkage long sys_getresuid(uid_t * ruid, uid_t * euid, uid_t * suid)
{
    int retval;

    if (!(retval = put_user(current->uid, ruid)) &&
	!(retval = put_user(current->euid, euid)))
	retval = put_user(current->suid, suid);

    return retval;
}

/*
 * Same as above, but for rgid, egid, sgid.
 */
asmlinkage long sys_setresgid(gid_t rgid, gid_t egid, gid_t sgid)
{
    if (!capable(CAP_SETGID)) {
	if ((rgid != (gid_t) - 1) && (rgid != current->gid) &&
	    (rgid != current->egid) && (rgid != current->sgid))
	    return -EPERM;
	if ((egid != (gid_t) - 1) && (egid != current->gid) &&
	    (egid != current->egid) && (egid != current->sgid))
	    return -EPERM;
	if ((sgid != (gid_t) - 1) && (sgid != current->gid) &&
	    (sgid != current->egid) && (sgid != current->sgid))
	    return -EPERM;
    }
    if (egid != (gid_t) - 1) {
	if (egid != current->egid) {
	    current->mm->dumpable = 0;
	    wmb();
	}
	current->egid = egid;
	current->fsgid = egid;
    }
    if (rgid != (gid_t) - 1)
	current->gid = rgid;
    if (sgid != (gid_t) - 1)
	current->sgid = sgid;
    return 0;
}

asmlinkage long sys_getresgid(gid_t * rgid, gid_t * egid, gid_t * sgid)
{
    int retval;

    if (!(retval = put_user(current->gid, rgid)) &&
	!(retval = put_user(current->egid, egid)))
	retval = put_user(current->sgid, sgid);

    return retval;
}


/*
 * "setfsuid()" sets the fsuid - the uid used for filesystem checks. This
 * is used for "access()" and for the NFS daemon (letting nfsd stay at
 * whatever uid it wants to). It normally shadows "euid", except when
 * explicitly set by setfsuid() or for access..
 */
asmlinkage long sys_setfsuid(uid_t uid)
{
    int old_fsuid;

    old_fsuid = current->fsuid;
    if (uid == current->uid || uid == current->euid ||
	uid == current->suid || uid == current->fsuid ||
	capable(CAP_SETUID)) {
	if (uid != old_fsuid) {
	    current->mm->dumpable = 0;
	    wmb();
	}
	current->fsuid = uid;
    }

    /* We emulate fsuid by essentially doing a scaled-down version
     * of what we did in setresuid and friends. However, we only
     * operate on the fs-specific bits of the process' effective
     * capabilities 
     *
     * FIXME - is fsuser used for all CAP_FS_MASK capabilities?
     *          if not, we might be a bit too harsh here.
     */

    if (!issecure(SECURE_NO_SETUID_FIXUP)) {
	if (old_fsuid == 0 && current->fsuid != 0) {
	    cap_t(current->cap_effective) &= ~CAP_FS_MASK;
	}
	if (old_fsuid != 0 && current->fsuid == 0) {
	    cap_t(current->cap_effective) |=
		(cap_t(current->cap_permitted) & CAP_FS_MASK);
	}
    }

    return old_fsuid;
}

/*
 * Samma p