linux-kernel - [PATCH V2 3/4] posix-timers: Initialise timer->it_signal in posix_timer

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Message-ID: <20250219125522.2535263-4-edumazet@google.com>
Date: Wed, 19 Feb 2025 12:55:21 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: Anna-Maria Behnsen <anna-maria@...utronix.de>, Frederic Weisbecker <frederic@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel <linux-kernel@...r.kernel.org>, Benjamin Segall <bsegall@...gle.com>, 
	Eric Dumazet <eric.dumazet@...il.com>, Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH V2 3/4] posix-timers: Initialise timer->it_signal in posix_timer_add()

Instead of leaving a NULL value in timer->it_signal,
set it to the current sig pointer, but with the low order bit set.

This fixes a potential race, in the unlikely case a thread
was preempted long enough that other threads created more than
2^31 itimers.

Rename __posix_timers_find() to posix_timers_find()

Mask the low order bit in posix_timers_find().

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 kernel/time/posix-timers.c | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 1f73ea955756..ed27c7eab456 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -72,15 +72,22 @@ static int hash(struct signal_struct *sig, unsigned int nr)
 	return hash_32(hash32_ptr(sig) ^ nr, HASH_BITS(posix_timers_hashtable));
 }
 
-static struct k_itimer *__posix_timers_find(struct hlist_head *head,
+static struct signal_struct *posix_sig_owner(const struct k_itimer *timer)
+{
+	/* timer->it_signal can be set concurrently */
+	unsigned long val = (unsigned long)READ_ONCE(timer->it_signal);
+
+	return (struct signal_struct *)(val & ~1UL);
+}
+
+static struct k_itimer *posix_timers_find(struct hlist_head *head,
 					    struct signal_struct *sig,
 					    timer_t id)
 {
 	struct k_itimer *timer;
 
 	hlist_for_each_entry_rcu(timer, head, t_hash, lockdep_is_held(&hash_lock)) {
-		/* timer->it_signal can be set concurrently */
-		if ((READ_ONCE(timer->it_signal) == sig) && (timer->it_id == id))
+		if ((posix_sig_owner(timer) == sig) && (timer->it_id == id))
 			return timer;
 	}
 	return NULL;
@@ -90,8 +97,14 @@ static struct k_itimer *posix_timer_by_id(timer_t id)
 {
 	struct signal_struct *sig = current->signal;
 	struct hlist_head *head = &posix_timers_hashtable[hash(sig, id)];
+	struct k_itimer *timer;
 
-	return __posix_timers_find(head, sig, id);
+	hlist_for_each_entry_rcu(timer, head, t_hash) {
+		/* timer->it_signal can be set concurrently */
+		if ((READ_ONCE(timer->it_signal) == sig) && (timer->it_id == id))
+			return timer;
+	}
+	return NULL;
 }
 
 static int posix_timer_add(struct k_itimer *timer)
@@ -113,8 +126,9 @@ static int posix_timer_add(struct k_itimer *timer)
 		head = &posix_timers_hashtable[hash(sig, id)];
 
 		spin_lock(&hash_lock);
-		if (!__posix_timers_find(head, sig, id)) {
+		if (!posix_timers_find(head, sig, id)) {
 			timer->it_id = (timer_t)id;
+			timer->it_signal = (struct signal_struct *)((unsigned long)sig | 1UL);
 			hlist_add_head_rcu(&timer->t_hash, head);
 			spin_unlock(&hash_lock);
 			return id;
@@ -453,7 +467,7 @@ static int do_timer_create(clockid_t which_clock, struct sigevent *event,
 	}
 	/*
 	 * After succesful copy out, the timer ID is visible to user space
-	 * now but not yet valid because new_timer::signal is still NULL.
+	 * now but not yet valid because new_timer::signal low order bit is 1.
 	 *
 	 * Complete the initialization with the clock specific create
 	 * callback.
@@ -463,7 +477,7 @@ static int do_timer_create(clockid_t which_clock, struct sigevent *event,
 		goto out;
 
 	spin_lock_irq(&current->sighand->siglock);
-	/* This makes the timer valid in the hash table */
+	/* This makes the timer valid in the hash table, clearing low order bit. */
 	WRITE_ONCE(new_timer->it_signal, current->signal);
 	hlist_add_head(&new_timer->list, &current->signal->posix_timers);
 	spin_unlock_irq(&current->sighand->siglock);
-- 
2.48.1.601.g30ceb7b040-goog