linux-kernel - [05/45] futex: Handle user space corruption gracefully

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Message-Id: <20100330224942.518182836@linux.site>
Date:	Tue, 30 Mar 2010 15:48:25 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Thomas Gleixner <tglx@...utronix.de>,
	Darren Hart <dvhltc@...ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: [05/45] futex: Handle user space corruption gracefully

2.6.27-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Thomas Gleixner <tglx@...utronix.de>

commit 51246bfd189064079c54421507236fd2723b18f3 upstream.

If the owner of a PI futex dies we fix up the pi_state and set
pi_state->owner to NULL. When a malicious or just sloppy programmed
user space application sets the futex value to 0 e.g. by calling
pthread_mutex_init(), then the futex can be acquired again. A new
waiter manages to enqueue itself on the pi_state w/o damage, but on
unlock the kernel dereferences pi_state->owner and oopses.

Prevent this by checking pi_state->owner in the unlock path. If
pi_state->owner is not current we know that user space manipulated the
futex value. Ignore the mess and return -EINVAL.

This catches the above case and also the case where a task hijacks the
futex by setting the tid value and then tries to unlock it.

Reported-by: Jermome Marchand <jmarchan@...hat.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Acked-by: Darren Hart <dvhltc@...ibm.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 kernel/futex.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -647,6 +647,13 @@ static int wake_futex_pi(u32 __user *uad
 	if (!pi_state)
 		return -EINVAL;

+	/*
+	 * If current does not own the pi_state then the futex is
+	 * inconsistent and user space fiddled with the futex value.
+	 */
+	if (pi_state->owner != current)
+		return -EINVAL;
+
 	spin_lock(&pi_state->pi_mutex.wait_lock);
 	new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);

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