elf_core_dump: use rcu_read_lock() to access ->real_parent

In theory it is not safe to dereference ->parent/real_parent without
tasklist or rcu lock, we can race with re-parenting.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/binfmt_elf.c

@@ -1340,8 +1340,10 @@ static void fill_prstatus(struct elf_prstatus *prstatus,
 	prstatus->pr_info.si_signo = prstatus->pr_cursig = signr;
 	prstatus->pr_sigpend = p->pending.signal.sig[0];
 	prstatus->pr_sighold = p->blocked.sig[0];
+	rcu_read_lock();
+	prstatus->pr_ppid = task_pid_vnr(rcu_dereference(p->real_parent));
+	rcu_read_unlock();
 	prstatus->pr_pid = task_pid_vnr(p);
-	prstatus->pr_ppid = task_pid_vnr(p->real_parent);
 	prstatus->pr_pgrp = task_pgrp_vnr(p);
 	prstatus->pr_sid = task_session_vnr(p);
 	if (thread_group_leader(p)) {
@@ -1382,8 +1384,10 @@ static int fill_psinfo(struct elf_prpsinfo *psinfo, struct task_struct *p,
 			psinfo->pr_psargs[i] = ' ';
 	psinfo->pr_psargs[len] = 0;
 
+	rcu_read_lock();
+	psinfo->pr_ppid = task_pid_vnr(rcu_dereference(p->real_parent));
+	rcu_read_unlock();
 	psinfo->pr_pid = task_pid_vnr(p);
-	psinfo->pr_ppid = task_pid_vnr(p->real_parent);
 	psinfo->pr_pgrp = task_pgrp_vnr(p);
 	psinfo->pr_sid = task_session_vnr(p);
 

fs/binfmt_elf_fdpic.c

@@ -1387,8 +1387,10 @@ static void fill_prstatus(struct elf_prstatus *prstatus,
 	prstatus->pr_info.si_signo = prstatus->pr_cursig = signr;
 	prstatus->pr_sigpend = p->pending.signal.sig[0];
 	prstatus->pr_sighold = p->blocked.sig[0];
+	rcu_read_lock();
+	prstatus->pr_ppid = task_pid_vnr(rcu_dereference(p->real_parent));
+	rcu_read_unlock();
 	prstatus->pr_pid = task_pid_vnr(p);
-	prstatus->pr_ppid = task_pid_vnr(p->real_parent);
 	prstatus->pr_pgrp = task_pgrp_vnr(p);
 	prstatus->pr_sid = task_session_vnr(p);
 	if (thread_group_leader(p)) {
@@ -1432,8 +1434,10 @@ static int fill_psinfo(struct elf_prpsinfo *psinfo, struct task_struct *p,
 			psinfo->pr_psargs[i] = ' ';
 	psinfo->pr_psargs[len] = 0;
 
+	rcu_read_lock();
+	psinfo->pr_ppid = task_pid_vnr(rcu_dereference(p->real_parent));
+	rcu_read_unlock();
 	psinfo->pr_pid = task_pid_vnr(p);
-	psinfo->pr_ppid = task_pid_vnr(p->real_parent);
 	psinfo->pr_pgrp = task_pgrp_vnr(p);
 	psinfo->pr_sid = task_session_vnr(p);