LKML Archive on lore.kernel.org
 help / color / Atom feed
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
	linux-kernel@vger.kernel.org,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: [PATCH] kernel/futex: handle the case where we got a "late" waiter
Date: Fri, 15 Apr 2016 14:35:39 +0200
Message-ID: <1460723739-5195-1-git-send-email-bigeasy@linutronix.de> (raw)

futex_unlock_pi() gets uval before taking the hb lock. Now imagine
someone in futex_lock_pi() took the lock. While futex_unlock_pi() waits
for the hb lock, the LOCK_PI sets FUTEX_WAITERS and drops the lock.
Now, futex_unlock_pi() figures out that there is waiter and invokes
wake_futex_pi() with the old uval which does not yet have FUTEX_WAITERS
set. This flaw lets cmpxchg_futex_value_locked() fail and return -EINVAL.

To address this race we could either use get_futex_value_locked() after
we obtained the lock but then we would need to handle the EFAULT case
(which I think means get_user() without the lock).
To avoid this I let wake_futex_pi() detect this and return -EAGAIN so it
can read the new value and try again.

As bad as this may sound: FUTEX_OWNER_DIED is not set while the owner is
unlocking the lock. FUTEX_WAITERS gets set by another task *but* this
means that the owner invoked the futex() syscall rather than doing a
cmpxchg() in userland (and libc does not do that).

Fixes: ccf9e6a80d9e ("futex: Make unlock_pi more robust")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/futex.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 0d557e13823a..2911f54a6dde 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1496,8 +1496,12 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this,
 
 	if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval))
 		ret = -EFAULT;
-	else if (curval != uval)
-		ret = -EINVAL;
+	else if (curval != uval) {
+		if ((FUTEX_TID_MASK & curval) == uval)
+			ret = -EAGAIN;
+		else
+			ret = -EINVAL;
+	}
 	if (ret) {
 		raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
 		return ret;
@@ -2852,6 +2856,12 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
 		if (ret == -EFAULT)
 			goto pi_faulted;
 		/*
+		 * Between get_user() and obtaining the hb->lock the uval
+		 * gained a flag. Retry with the new value.
+		 */
+		if (ret == -EAGAIN)
+			goto pi_faulted;
+		/*
 		 * wake_futex_pi has detected invalid state. Tell user
 		 * space.
 		 */
@@ -2883,6 +2893,8 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
 	spin_unlock(&hb->lock);
 	put_futex_key(&key);
 
+	if (ret == -EAGAIN)
+		goto retry;
 	ret = fault_in_user_writeable(uaddr);
 	if (!ret)
 		goto retry;
-- 
2.8.0.rc3

             reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-04-15 12:35 Sebastian Andrzej Siewior [this message]
2016-04-19 22:27 ` Davidlohr Bueso
2016-04-20  7:09   ` Sebastian Andrzej Siewior
2016-04-20  7:36   ` Thomas Gleixner
2016-04-20 11:51 ` [tip:locking/urgent] futex: Handle unlock_pi race gracefully tip-bot for Sebastian Andrzej Siewior

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=1460723739-5195-1-git-send-email-bigeasy@linutronix.de \
    --to=bigeasy@linutronix.de \
    --cc=dave@stgolabs.net \
    --cc=linux-kernel@vger.kernel.org \
    --cc=tglx@linutronix.de \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lore.kernel.org/lkml/9 lkml/git/9.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git