fix races in the uidinfo subsystem, several problems existed:

1) while allocating a uidinfo struct malloc is called with M_WAITOK,
   it's possible that while asleep another process by the same user
   could have woken up earlier and inserted an entry into the uid
   hash table.  Having redundant entries causes inconsistancies that
   we can't handle.

   fix: do a non-waiting malloc, and if that fails then do a blocking
   malloc, after waking up check that no one else has inserted an entry
   for us already.

2) Because many checks for sbsize were done as "test then set" in a non
   atomic manner it was possible to exceed the limits put up via races.

   fix: instead of querying the count then setting, we just attempt to
   set the count and leave it up to the function to return success or
   failure.

3) The uidinfo code was inlining and repeating, lookups and insertions
   and deletions needed to be in their own functions for clarity.

Reviewed by: green

1 files changed, 92 insertions, 54 deletions

diff --git a/sys/kern/kern_proc.c b/sys/kern/kern_proc.c
index df9f1c5..72069d0 100644
--- a/sys/kern/kern_proc.c
+++ b/sys/kern/kern_proc.c
@@ -72,6 +72,10 @@ static u_long uihash;		/* size of hash table - 1 */
 
 static void	orphanpg __P((struct pgrp *pg));
 
+static struct uidinfo	*uifind(uid_t uid);
+static struct uidinfo	*uicreate(uid_t uid);
+static int 	uifree(struct uidinfo *uip);
+
 /*
  * Other process lists
  */
@@ -99,85 +103,119 @@ procinit()
 }
 
 /*
- * Change the count associated with number of processes
- * a given user is using.
+ * find/create a uidinfo struct for the uid passed in
  */
-int
-chgproccnt(uid, diff)
-	uid_t	uid;
-	int	diff;
+static struct uidinfo *
+uifind(uid)
+	uid_t uid;
 {
-	register struct uidinfo *uip;
-	register struct uihashhead *uipp;
+	struct uihashhead *uipp;
+	struct uidinfo *uip;
 
 	uipp = UIHASH(uid);
 	LIST_FOREACH(uip, uipp, ui_hash)
 		if (uip->ui_uid == uid)
 			break;
-	if (uip) {
-		uip->ui_proccnt += diff;
-		if (uip->ui_proccnt < 0)
-			panic("chgproccnt: procs < 0");
-		if (uip->ui_proccnt > 0 || uip->ui_sbsize > 0)
-			return (uip->ui_proccnt);
+
+	return (uip);
+}
+
+static struct uidinfo *
+uicreate(uid)
+	uid_t uid;
+{
+	struct uidinfo *uip, *norace;
+
+	MALLOC(uip, struct uidinfo *, sizeof(*uip), M_PROC, M_NOWAIT);
+	if (uip == NULL) {
+		MALLOC(uip, struct uidinfo *, sizeof(*uip), M_PROC, M_WAITOK);
+		/*
+		 * if we M_WAITOK we must look afterwards or risk
+		 * redundant entries
+		 */
+		norace = uifind(uid);
+		if (norace != NULL) {
+			FREE(uip, M_PROC);
+			return (norace);
+		}
+	}
+	LIST_INSERT_HEAD(UIHASH(uid), uip, ui_hash);
+	uip->ui_uid = uid;
+	uip->ui_proccnt = 0;
+	uip->ui_sbsize = 0;
+	return (uip);
+}
+
+static int
+uifree(uip)
+	struct uidinfo *uip;
+{
+
+	if (uip->ui_sbsize == 0 && uip->ui_proccnt == 0) {
 		LIST_REMOVE(uip, ui_hash);
 		FREE(uip, M_PROC);
-		return (0);
+		return (1);
 	}
-	if (diff <= 0) {
-		if (diff == 0)
-			return(0);
-		panic("chgproccnt: lost user");
+	return (0);
+}
+
+/*
+ * Change the count associated with number of processes
+ * a given user is using.  When 'max' is 0, don't enforce a limit
+ */
+int
+chgproccnt(uid, diff, max)
+	uid_t	uid;
+	int	diff;
+	int	max;
+{
+	struct uidinfo *uip;
+
+	uip = uifind(uid);
+	if (diff < 0)
+		KASSERT(uip != NULL, ("reducing proccnt: lost count, uid = %d", uid));
+	if (uip == NULL)
+		uip = uicreate(uid);
+	/* don't allow them to exceed max, but allow subtraction */
+	if (diff > 0 && uip->ui_proccnt + diff > max && max != 0) {
+		(void)uifree(uip);
+		return (0);
 	}
-	MALLOC(uip, struct uidinfo *, sizeof(*uip), M_PROC, M_WAITOK);
-	LIST_INSERT_HEAD(uipp, uip, ui_hash);
-	uip->ui_uid = uid;
-	uip->ui_proccnt = diff;
-	uip->ui_sbsize = 0;
-	return (diff);
+	uip->ui_proccnt += diff;
+	(void)uifree(uip);
+	return (1);
 }
 
 /*
  * Change the total socket buffer size a user has used.
  */
-rlim_t
-chgsbsize(uid, diff)
+int
+chgsbsize(uid, diff, max)
 	uid_t	uid;
 	rlim_t	diff;
+	rlim_t	max;
 {
-	register struct uidinfo *uip;
-	register struct uihashhead *uipp;
-
-	uipp = UIHASH(uid);
-	LIST_FOREACH(uip, uipp, ui_hash)
-		if (uip->ui_uid == uid)
-			break;
-	if (diff <= 0) {
-		if (diff == 0)
-			return (uip ? uip->ui_sbsize : 0);
-		KASSERT(uip != NULL, ("uidinfo (%d) gone", uid));
-	}
-	if (uip) {
-		uip->ui_sbsize += diff;
-		if (uip->ui_sbsize == 0 && uip->ui_proccnt == 0) {
-			LIST_REMOVE(uip, ui_hash);
-			FREE(uip, M_PROC);
-			return (0);
-		}
-		return (uip->ui_sbsize);
+	struct uidinfo *uip;
+
+	uip = uifind(uid);
+	if (diff < 0)
+		KASSERT(uip != NULL, ("reducing sbsize: lost count, uid = %d", uid));
+	if (uip == NULL)
+		uip = uicreate(uid);
+	/* don't allow them to exceed max, but allow subtraction */
+	if (diff > 0 && uip->ui_sbsize + diff > max) {
+		(void)uifree(uip);
+		return (0);
 	}
-	MALLOC(uip, struct uidinfo *, sizeof(*uip), M_PROC, M_WAITOK);
-	LIST_INSERT_HEAD(uipp, uip, ui_hash);
-	uip->ui_uid = uid;
-	uip->ui_proccnt = 0;
-	uip->ui_sbsize = diff;
-	return (diff);
+	uip->ui_sbsize += diff;
+	(void)uifree(uip);
+	return (1);
 }
 
 /*
  * Is p an inferior of the current process?
  */
-int
+inT
 inferior(p)
 	register struct proc *p;
 {