fs/proc/array.c: make safe access to group_leader

As mentioned in commit 52ee2df
("pids: refactor vnr/nr_ns helpers to make them safe"). *_nr_ns
helpers used to be buggy. The commit addresses most of the helpers but
is missing task_tgid_xxx()

Without this protection there is a possible use after free reported by
kasan instrumented kernel:

==================================================================
BUG: KASAN: use-after-free in task_tgid_nr_ns+0x2c/0x44 at addr ***
Read of size 8 by task cat/2472
CPU: 1 PID: 2472 Comm: cat Tainted: ****
Hardware name: Google Tegra210 Smaug Rev 1,3+ (DT)
Call trace:
[<ffffffc00020ad2c>] dump_backtrace+0x0/0x17c
[<ffffffc00020aec0>] show_stack+0x18/0x24
[<ffffffc0011573d0>] dump_stack+0x94/0x100
[<ffffffc0003c7dc0>] kasan_report+0x308/0x554
[<ffffffc0003c7518>] __asan_load8+0x20/0x7c
[<ffffffc00025a54c>] task_tgid_nr_ns+0x28/0x44
[<ffffffc00046951c>] proc_pid_status+0x444/0x1080
[<ffffffc000460f60>] proc_single_show+0x8c/0xdc
[<ffffffc0004081b0>] seq_read+0x2e8/0x6f0
[<ffffffc0003d1420>] vfs_read+0xd8/0x1e0
[<ffffffc0003d1b98>] SyS_read+0x68/0xd4

Accessing group_leader while holding rcu_lock and using the now safe
helpers introduced in the commit mentioned, this race condition is
addressed.

Signed-off-by: Adrian Salido <salidoa@google.com>
Change-Id: I4315217922dda375a30a3581c0c1740dda7b531b
Bug: 31495866

fs/proc/array.c

@@ -165,16 +165,16 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 	int g;
 	struct fdtable *fdt = NULL;
 	const struct cred *cred;
-	pid_t ppid, tpid;
+	pid_t ppid = 0, tpid = 0;
+	struct task_struct *leader = NULL;
 
 	rcu_read_lock();
-	ppid = pid_alive(p) ?
-		task_tgid_nr_ns(rcu_dereference(p->real_parent), ns) : 0;
-	tpid = 0;
 	if (pid_alive(p)) {
 		struct task_struct *tracer = tracehook_tracer_task(p);
 		if (tracer)
 			tpid = task_pid_nr_ns(tracer, ns);
+		ppid = task_tgid_nr_ns(rcu_dereference(p->real_parent), ns);
+		leader = p->group_leader;
 	}
 	cred = get_task_cred(p);
 	seq_printf(m,
@@ -186,7 +186,7 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 		"Uid:\t%d\t%d\t%d\t%d\n"
 		"Gid:\t%d\t%d\t%d\t%d\n",
 		get_task_state(p),
-		task_tgid_nr_ns(p, ns),
+		leader ? task_pid_nr_ns(leader, ns) : 0,
 		pid_nr_ns(pid, ns),
 		ppid, tpid,
 		cred->uid, cred->euid, cred->suid, cred->fsuid,