[PATCH 5.15.y] fs/proc: do_task_stat: use sig->stats_lock to gather the threads/children stats

[PATCH 5.15.y] fs/proc: do_task_stat: use sig->stats_lock to gather the threads/children stats

[David Sauerwein]

From: Oleg Nesterov <oleg@redhat.com>

[ Upstream commit 7601df8031fd67310af891897ef6cc0df4209305 ]

lock_task_sighand() can trigger a hard lockup.  If NR_CPUS threads call
do_task_stat() at the same time and the process has NR_THREADS, it will
spin with irqs disabled O(NR_CPUS * NR_THREADS) time.

Change do_task_stat() to use sig->stats_lock to gather the statistics
outside of ->siglock protected section, in the likely case this code will
run lockless.

Link: https://lkml.kernel.org/r/20240123153357.GA21857@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Dylan Hatch <dylanbhatch@google.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: David Sauerwein <dssauerw@amazon.de>
---
 fs/proc/array.c | 53 +++++++++++++++++++++++++++----------------------
 1 file changed, 29 insertions(+), 24 deletions(-)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index c925287a4dc4..2cb01aaa6718 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -462,12 +462,12 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 	int permitted;
 	struct mm_struct *mm;
 	unsigned long long start_time;
-	unsigned long cmin_flt = 0, cmaj_flt = 0;
-	unsigned long  min_flt = 0,  maj_flt = 0;
-	u64 cutime, cstime, utime, stime;
-	u64 cgtime, gtime;
+	unsigned long cmin_flt, cmaj_flt, min_flt, maj_flt;
+	u64 cutime, cstime, cgtime, utime, stime, gtime;
 	unsigned long rsslim = 0;
 	unsigned long flags;
+	struct signal_struct *sig = task->signal;
+	unsigned int seq = 1;
 
 	state = *get_task_state(task);
 	vsize = eip = esp = 0;
@@ -495,12 +495,8 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 
 	sigemptyset(&sigign);
 	sigemptyset(&sigcatch);
-	cutime = cstime = 0;
-	cgtime = gtime = 0;
 
 	if (lock_task_sighand(task, &flags)) {
-		struct signal_struct *sig = task->signal;
-
 		if (sig->tty) {
 			struct pid *pgrp = tty_get_pgrp(sig->tty);
 			tty_pgrp = pid_nr_ns(pgrp, ns);
@@ -511,36 +507,45 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 		num_threads = get_nr_threads(task);
 		collect_sigign_sigcatch(task, &sigign, &sigcatch);
 
+		rsslim = READ_ONCE(sig->rlim[RLIMIT_RSS].rlim_cur);
+
+		sid = task_session_nr_ns(task, ns);
+		ppid = task_tgid_nr_ns(task->real_parent, ns);
+		pgid = task_pgrp_nr_ns(task, ns);
+
+		unlock_task_sighand(task, &flags);
+	}
+
+	if (permitted && (!whole || num_threads < 2))
+		wchan = !task_is_running(task);
+
+	do {
+		seq++; /* 2 on the 1st/lockless path, otherwise odd */
+		flags = read_seqbegin_or_lock_irqsave(&sig->stats_lock, &seq);
+
 		cmin_flt = sig->cmin_flt;
 		cmaj_flt = sig->cmaj_flt;
 		cutime = sig->cutime;
 		cstime = sig->cstime;
 		cgtime = sig->cgtime;
-		rsslim = READ_ONCE(sig->rlim[RLIMIT_RSS].rlim_cur);
 
-		/* add up live thread stats at the group level */
 		if (whole) {
 			struct task_struct *t = task;
+
+			min_flt = sig->min_flt;
+			maj_flt = sig->maj_flt;
+			gtime = sig->gtime;
+
+			rcu_read_lock();
 			do {
 				min_flt += t->min_flt;
 				maj_flt += t->maj_flt;
 				gtime += task_gtime(t);
 			} while_each_thread(task, t);
-
-			min_flt += sig->min_flt;
-			maj_flt += sig->maj_flt;
-			gtime += sig->gtime;
+			rcu_read_unlock();
 		}
-
-		sid = task_session_nr_ns(task, ns);
-		ppid = task_tgid_nr_ns(task->real_parent, ns);
-		pgid = task_pgrp_nr_ns(task, ns);
-
-		unlock_task_sighand(task, &flags);
-	}
-
-	if (permitted && (!whole || num_threads < 2))
-		wchan = !task_is_running(task);
+	} while (need_seqretry(&sig->stats_lock, seq));
+	done_seqretry_irqrestore(&sig->stats_lock, seq, flags);
 
 	if (whole) {
 		thread_group_cputime_adjusted(task, &utime, &stime);
-- 
2.47.1

Re: [PATCH 5.15.y] fs/proc: do_task_stat: use sig->stats_lock to gather the threads/children stats

[Sasha Levin]

[ Sasha's backport helper bot ]

Hi,

✅ All tests passed successfully. No issues detected.
No action required from the submitter.

The upstream commit SHA1 provided is correct: 7601df8031fd67310af891897ef6cc0df4209305

WARNING: Author mismatch between patch and upstream commit:
Backport author: David Sauerwein<dssauerw@amazon.de>
Commit author: Oleg Nesterov<oleg@redhat.com>

Status in newer kernel trees:
6.14.y | Present (exact SHA1)
6.13.y | Present (exact SHA1)
6.12.y | Present (exact SHA1)
6.6.y | Present (different SHA1: 3820b0fac773)
6.1.y | Present (different SHA1: cf4b8c39b9a0)

Note: The patch differs from the upstream commit:
---
1:  7601df8031fd6 < -:  ------------- fs/proc: do_task_stat: use sig->stats_lock to gather the threads/children stats
-:  ------------- > 1:  49c9b6bb0f69c fs/proc: do_task_stat: use sig->stats_lock to gather the threads/children stats
---

Results of testing on various branches:

| Branch                    | Patch Apply | Build Test |
|---------------------------|-------------|------------|
| stable/linux-5.15.y       |  Success    |  Success   |