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SubjectRe: Bug: fio traps into kernel without exiting because futex has a deadloop
On Fri, 12 Jun 2009, Thomas Gleixner wrote:
> On Fri, 12 Jun 2009, Zhang, Yanmin wrote:
> > On Thu, 2009-06-11 at 13:36 +0200, Peter Zijlstra wrote:
> > > FWIW, using a private futex on a shm section is wrong in and of itself.
> >
> > What I mean is it could be used as a DOS attack.
>
> Right. Fix is on the way.
>
> > Did you try my test case? Could you kill it when it runs?
>
> No, you can not kill it. That's why it needs a proper fix. Will send
> out today.

Can you please verify the patch below ? It's against 2.6.30.

Thanks,

tglx

-------------->
futex: Fix the write access fault problem for real
commit 64d1304a64 (futex: setup writeable mapping for futex ops which
modify user space data) did address only half of the problem of write
access faults.

The patch was made on two wrong assumptions:

1) access_ok(VERIFY_WRITE,...) would actually check write access.

On x86 it does _NOT_. It's a pure address range check.

2) a RW mapped region can not go away under us.

That's wrong as well. Nobody can prevent another thread to call
mprotect(PROT_READ) on that region where the futex resides. If that
call hits between the get_user_pages_fast() verification and the
actual write access in the atomic region we are toast again.

The solution is to not rely on access_ok and get_user() for any write
access related fault on private and shared futexes. Instead we need to
go through get_user_pages_fast() in the fault path to avoid any of the
above pitfalls. If get_user_pages_fast() returns -EFAULT we know that
we can not fix it anymore and need to bail out to user space.

Remove a bunch of confusing comments on this issue as well.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
kernel/futex.c | 48 +++++++++++++++++++++++++++++-------------------
1 file changed, 29 insertions(+), 19 deletions(-)
Index: linux-2.6-tip/kernel/futex.c
===================================================================
--- linux-2.6-tip.orig/kernel/futex.c
+++ linux-2.6-tip/kernel/futex.c
@@ -278,6 +278,31 @@ void put_futex_key(int fshared, union fu
drop_futex_key_refs(key);
}

+/*
+ * get_user_writeable - get user page and verify RW access
+ * @uaddr: pointer to faulting user space address
+ *
+ * We cannot write to the user space address and get_user just faults
+ * the page in, but does not tell us whether the mapping is writeable.
+ *
+ * We can not rely on access_ok() for private futexes as it is just a
+ * range check and we can neither rely on get_user_pages() as there
+ * might be a mprotect(PROT_READ) for that mapping after
+ * get_user_pages() and before the fault in the atomic write access.
+ */
+static int get_user_writeable(u32 __user *uaddr)
+{
+ unsigned long addr = (unsigned long)uaddr;
+ struct page *page;
+ int ret;
+
+ ret = get_user_pages_fast(addr, 1, 1, &page);
+ if (!ret)
+ put_page(page);
+
+ return ret;
+}
+
static u32 cmpxchg_futex_value_locked(u32 __user *uaddr, u32 uval, u32 newval)
{
u32 curval;
@@ -739,7 +764,6 @@ retry:
retry_private:
op_ret = futex_atomic_op_inuser(op, uaddr2);
if (unlikely(op_ret < 0)) {
- u32 dummy;

double_unlock_hb(hb1, hb2);

@@ -757,7 +781,7 @@ retry_private:
goto out_put_keys;
}

- ret = get_user(dummy, uaddr2);
+ ret = get_user_writeable(uaddr2);
if (ret)
goto out_put_keys;

@@ -1097,7 +1121,7 @@ retry:
handle_fault:
spin_unlock(q->lock_ptr);

- ret = get_user(uval, uaddr);
+ ret = get_user_writeable(uaddr);

spin_lock(q->lock_ptr);

@@ -1552,16 +1576,9 @@ out:
return ret;

uaddr_faulted:
- /*
- * We have to r/w *(int __user *)uaddr, and we have to modify it
- * atomically. Therefore, if we continue to fault after get_user()
- * below, we need to handle the fault ourselves, while still holding
- * the mmap_sem. This can occur if the uaddr is under contention as
- * we have to drop the mmap_sem in order to call get_user().
- */
queue_unlock(&q, hb);

- ret = get_user(uval, uaddr);
+ ret = get_user_writeable(uaddr);
if (ret)
goto out_put_key;

@@ -1657,17 +1674,10 @@ out:
return ret;

pi_faulted:
- /*
- * We have to r/w *(int __user *)uaddr, and we have to modify it
- * atomically. Therefore, if we continue to fault after get_user()
- * below, we need to handle the fault ourselves, while still holding
- * the mmap_sem. This can occur if the uaddr is under contention as
- * we have to drop the mmap_sem in order to call get_user().
- */
spin_unlock(&hb->lock);
put_futex_key(fshared, &key);

- ret = get_user(uval, uaddr);
+ ret = get_user_writeable(uaddr);
if (!ret)
goto retry;


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