cred: kern_setgroups(): Internally use int as number of groups' type

sys_setgroups() (and sys_getgroups()) was changed in commit "kern: fail
getgroup and setgroup with negative int" (4bc2174a1b) to take the
number of groups as an 'int' (for sys_getgroups(), POSIX mandates this
change; for sys_setgroups(), which it does not standardize, it's
arguably for consistency).

All our internal APIs related to groups on 'struct ucred', as well as
related members on the latter, treat that number as an 'int' as well
(and not a 'u_int').

Consequently, to avoid surprises, change kern_setgroups() to behave the
same, and fix audit_arg_groupset() accordingly.  With that change,
everything is handled with signed integers internally.

Update sanity checks accordingly.

Reviewed by:    mhorne
Approved by:    markj (mentor)
MFC after:      3 days
Differential Revision:  https://reviews.freebsd.org/D46912

sys/kern/kern_prot.c

@@ -815,6 +815,15 @@ sys_setgroups(struct thread *td, struct setgroups_args *uap)
 	gid_t *groups;
 	int gidsetsize, error;
 
+	/*
+	 * Sanity check size now to avoid passing too big a value to copyin(),
+	 * even if kern_setgroups() will do it again.
+	 *
+	 * Ideally, the 'gidsetsize' argument should have been a 'u_int' (and it
+	 * was, in this implementation, for a long time), but POSIX standardized
+	 * getgroups() to take an 'int' and it would be quite entrapping to have
+	 * setgroups() differ.
+	 */
 	gidsetsize = uap->gidsetsize;
 	if (gidsetsize > ngroups_max + 1 || gidsetsize < 0)
 		return (EINVAL);
@@ -843,13 +852,16 @@ gidp_cmp(const void *p1, const void *p2)
 }
 
 int
-kern_setgroups(struct thread *td, u_int ngrp, gid_t *groups)
+kern_setgroups(struct thread *td, int ngrp, gid_t *groups)
 {
 	struct proc *p = td->td_proc;
 	struct ucred *newcred, *oldcred;
 	int error;
 
-	MPASS(ngrp <= ngroups_max + 1);
+	/* Sanity check size. */
+	if (ngrp < 0 || ngrp > ngroups_max + 1)
+		return (EINVAL);
+
 	AUDIT_ARG_GROUPSET(groups, ngrp);
 	newcred = crget();
 	crextend(newcred, ngrp);

sys/security/audit/audit.h

@@ -98,7 +98,7 @@ void	 audit_arg_rgid(gid_t rgid);
 void	 audit_arg_ruid(uid_t ruid);
 void	 audit_arg_sgid(gid_t sgid);
 void	 audit_arg_suid(uid_t suid);
-void	 audit_arg_groupset(gid_t *gidset, u_int gidset_size);
+void	 audit_arg_groupset(gid_t *gidset, int gidset_size);
 void	 audit_arg_login(char *login);
 void	 audit_arg_ctlname(int *name, int namelen);
 void	 audit_arg_mask(int mask);

sys/security/audit/audit_arg.c

@@ -263,13 +263,13 @@ audit_arg_suid(uid_t suid)
 }
 
 void
-audit_arg_groupset(gid_t *gidset, u_int gidset_size)
+audit_arg_groupset(gid_t *gidset, int gidset_size)
 {
-	u_int i;
+	int i;
 	struct kaudit_record *ar;
 
-	KASSERT(gidset_size <= ngroups_max + 1,
+	KASSERT(gidset_size >= 0 && gidset_size <= ngroups_max + 1,
-	    ("audit_arg_groupset: gidset_size > (kern.ngroups + 1)"));
+	    ("audit_arg_groupset: gidset_size < 0 or > (kern.ngroups + 1)"));
 
 	ar = currecord();
 	if (ar == NULL)

sys/sys/syscallsubr.h

@@ -320,7 +320,7 @@ int	kern_select(struct thread *td, int nd, fd_set *fd_in, fd_set *fd_ou,
 	    fd_set *fd_ex, struct timeval *tvp, int abi_nfdbits);
 int	kern_sendit(struct thread *td, int s, struct msghdr *mp, int flags,
 	    struct mbuf *control, enum uio_seg segflg);
-int	kern_setgroups(struct thread *td, u_int ngrp, gid_t *groups);
+int	kern_setgroups(struct thread *td, int ngrp, gid_t *groups);
 int	kern_setitimer(struct thread *, u_int, struct itimerval *,
 	    struct itimerval *);
 int	kern_setpriority(struct thread *td, int which, int who, int prio);