proc: fix a race which could result in dereference of bad p_pgrp pointer on fork

During fork p_starcopy - p_endcopy area of a process is populated with bcopy
with only proc lock held. Another forking thread can find such a process and
proceed to access p_pgrp included in said area.

Fix the problem by moving the field outside. It is being properly assigned
later.

Reviewed by:	kib
Diagnosed by:	kib
Tested by:	Fabian Keil <freebsd-listen fabiankeil.de>
MFC after:	10 days

2 files changed, 2 insertions, 1 deletions

diff --git a/sys/kern/kern_proc.c b/sys/kern/kern_proc.c
index 8a3b6ca..bbedd9b 100644
--- a/sys/kern/kern_proc.c
+++ b/sys/kern/kern_proc.c
@@ -248,6 +248,7 @@ proc_init(void *mem, int size, int flags)
 	TAILQ_INIT(&p->p_threads);	     /* all threads in proc */
 	EVENTHANDLER_INVOKE(process_init, p);
 	p->p_stats = pstats_alloc();
+	p->p_pgrp = NULL;
 	SDT_PROBE3(proc, , init, return, p, size, flags);
 	return (0);
 }
diff --git a/sys/sys/proc.h b/sys/sys/proc.h
index 90effa6..cb94318 100644
--- a/sys/sys/proc.h
+++ b/sys/sys/proc.h
@@ -586,7 +586,6 @@ struct proc {
 	int		p_osrel;	/* (x) osreldate for the
 					       binary (from ELF note, if any) */
 	char		p_comm[MAXCOMLEN + 1];	/* (b) Process name. */
-	struct pgrp	*p_pgrp;	/* (c + e) Pointer to process group. */
 	struct sysentvec *p_sysent;	/* (b) Syscall dispatch info. */
 	struct pargs	*p_args;	/* (c) Process arguments. */
 	rlim_t		p_cpulimit;	/* (c) Current CPU limit in seconds. */
@@ -599,6 +598,7 @@ struct proc {
 	u_int		p_xsig;		/* (c) Stop/kill sig. */
 /* End area that is copied on creation. */
 #define	p_endcopy	p_xsig
+	struct pgrp	*p_pgrp;	/* (c + e) Pointer to process group. */
 	struct knlist	p_klist;	/* (c) Knotes attached to this proc. */
 	int		p_numthreads;	/* (c) Number of threads. */
 	struct mdproc	p_md;		/* Any machine-dependent fields. */