[patch 3/3] futex: Prevent attaching to kernel threads

[Thomas Gleixner]

We happily allow userspace to declare a random kernel thread to be the
owner of a user space PI futex.

Found while analysing the fallout of Dave Jones syscall fuzzer.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 kernel/futex.c |    7 +++++++
 1 file changed, 7 insertions(+)

Index: linux-2.6/kernel/futex.c
===================================================================
--- linux-2.6.orig/kernel/futex.c
+++ linux-2.6/kernel/futex.c
@@ -682,6 +682,8 @@ static struct task_struct * futex_find_g
 	p = find_task_by_vpid(pid);
 	if (p)
 		get_task_struct(p);
+	else
+		p = NULL;
 
 	rcu_read_unlock();
 
@@ -814,6 +816,11 @@ lookup_pi_state(u32 uval, struct futex_h
 	if (!p)
 		return -ESRCH;
 
+	if (!p->mm) {
+		put_task_struct(p);
+		return -EPERM;
+	}
+
 	/*
 	 * We need to look at the task state flags to figure out,
 	 * whether the task is exiting. To protect against the do_exit


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