SETCRED(2)

NAME
setcred - set current process credentials atomically

LIBRARY
libc

SYNOPSIS
#include <sys/ucred.h>
int
setcred(u_int flags, const struct setcred *wcred, size_t size);

DESCRIPTION
The setcred() system call can set any combination of user-accessible credentials of the current process in an atomic manner.

This system call is normally permitted only for processes having the ID of the super-user (0) as their effective user ID, or not at all if the sysctl(8) variable security.bsd.suser_enabled is zero or some active MAC policy specifically denies these processes.

Some MAC policies, such as mac_do(4), may also allow unprivileged users to call it successfully, possibly depending on the exact credentials transition requested, once again unless any active MAC policy specifically denies that.

The flags argument serves to indicate which process credentials should be changed by the call. Allowed flags are:

SETCREDF_UID
    Set the effective user ID.
SETCREDF_RUID
    Set the real user ID.
SETCREDF_SVUID
    Set the saved user ID.
SETCREDF_GID
    Set the effective group ID.
SETCREDF_RGID
    Set the real group ID.
SETCREDF_SVGID
    Set the saved group ID.
SETCREDF_SUPP_GROUPS
    Set the supplementary group list.
SETCREDF_MAC_LABEL
    Set the MAC label.

The struct setcred structure is currently defined as:

    struct setcred {
    	uid_t	 sc_uid;		/* effective user id */
    	uid_t	 sc_ruid;		/* real user id */
    	uid_t	 sc_svuid;		/* saved user id */
    	gid_t	 sc_gid;		/* effective group id */
    	gid_t	 sc_rgid;		/* real group id */
    	gid_t	 sc_svgid;		/* saved group id */
    	u_int	 sc_pad;		/* padding, unused */
    	u_int	 sc_supp_groups_nb;	/* supplementary groups number */
    	gid_t	*sc_supp_groups;	/* supplementary groups */
    	struct mac *sc_label;		/* MAC label */
    };

Its fields are:

sc_uid
    The ID to set the effective user to, if flag SETCREDF_UID is specified.
sc_ruid
    The ID to set the real user to, if flag SETCREDF_RUID is specified.
sc_svuid
    The ID to set the saved user to, if flag SETCREDF_SVUID is specified.
sc_gid
    The ID to set the effective group to, if flag SETCREDF_GID is specified.
sc_rgid
    The ID to set the real group to, if flag SETCREDF_RGID is specified.
sc_svgid
    The ID to set the saved group to, if flag SETCREDF_SVGID is specified.
sc_supp_groups_nb
    The size of array sc_supp_groups, if flag SETCREDF_SUPP_GROUPS is specified. It must be less than or equal to {NGROUPS_MAX}.
sc_supp_groups
    An array of IDs to set the supplementary groups to, if flag SETCREDF_SUPP_GROUPS is specified. Note that all groups in this array will be set as supplementary groups only, in contrast to setgroups(2) which treats the first element specially as the new effective group, not adding it to supplementary groups.
sc_label
    A pointer to a valid MAC label structure, e.g., built with the mac_from_text(3) function, if flag SETCREDF_MAC_LABEL is specified.

For forward compatibility and security reasons, it is recommended that users always initialize objects of type struct setcred with the provided initializer: SETCRED_INITIALIZER.

The size argument must be the size of the passed wcred structure.

RETURN VALUES

ERRORS
The setcred() system call will fail if:

[EINVAL]
    Unrecognized flags were passed in flags, or the size parameter does not match the size of struct setcred, or the field sc_supp_group_nb has a value strictly greater than {NGROUPS_MAX} (if flag SETCREDF_SUPP_GROUPS was supplied), or the MAC label pointed to by field sc_label is invalid (if flag SETCREDF_MAC_LABEL was supplied).

[EFAULT]
    The wcred pointer, or pointers in fields sc_supp_groups (if flag SETCREDF_SUPP_GROUPS was supplied) or sc_label (if flag SETCREDF_MAC_LABEL was supplied) point to invalid locations.

[EPERM]
    The user is not the super-user and/or the requested credentials transition is not allowed by the system or MAC modules.

[EOPNOTSUPP]
    Some of the requested credentials have a type that the system does not support. This currently can occur only if the kernel has been compiled without MAC and SETCREDF_MAC_LABEL has been passed.

SEE ALSO
issetugid(2), setregid(2), setreuid(2), setuid(2), mac_text(3), mac(4), mac_do(4), maclabel(7)

STANDARDS
The setcred() system call is specific to FreeBSD.

A call to setcred() usually changes process credentials that are listed by POSIX/SUS standards. The changed values then produce the effects with respect to the rest of the system that are described in these standards, as if these changes had resulted from calling standard or traditional credentials-setting functions. Currently, all flags but SETCREDF_MAC_LABEL lead to modifying standard credentials.

The only differences in using setcred() to change standard credentials instead of standard or traditional functions are:

•   All requested changes are performed atomically.
•   Only the super-user or an unprivileged user authorized by some MAC module can successfully call setcred(), even if the standard system calls would have authorized any unprivileged user to effect the same changes. For example, seteuid() allows any unprivileged user to change the effective user ID to either the real or saved ones, while setcred() called with flag SETCREDF_UID does not.

HISTORY
The setcred() system call appeared in FreeBSD 14.3.

Traditionally in UNIX, all credential changes beyond shuffles of effective, real and saved IDs have been done by setuid binaries that successively call multiple credentials-setting system calls and in a specific order. For example, to change all user IDs to that of some unprivileged user, setuid() must be called last so that all other credentials-changing calls can be performed successfully beforehand, as they require super-user privileges.

This piecewise approach causes such a process to transiently hold high privilege credentials that are neither the original nor necessarily the desired final ones. Besides opening a transition window where possible vulnerabilities could have catastrophic consequences, it makes it impossible for the kernel to enforce that only certain transitions of credentials are allowed.

The necessity of an atomic, global approach to changing credentials clearly appeared while working on extending mac_do(4) to allow rules to authorize only specific changes of primary or supplementary groups, which prompted the addition of setcred().

AUTHORS
The setcred() system call and this manual page were written by Olivier Certner <olce.freebsd@certner.fr>.

SECURITY CONSIDERATIONS
The same considerations as those of standard or traditional credentials-setting system calls apply to setcred(), except for the lack of atomicity of successive such calls.

In particular, please consult section SECURITY CONSIDERATIONS of the setuid(2) manual page about the absence of effect of changing standard credentials on already open files.