Messages in this thread | | | Subject | Re: Bug: fio traps into kernel without exiting because futex has a deadloop | From | "Zhang, Yanmin" <> | Date | Mon, 15 Jun 2009 14:03:37 +0800 |
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On Fri, 2009-06-12 at 10:39 +0200, Thomas Gleixner wrote: > 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. I tested it with my test case and it doesn't hang again. But I still have considerations.
> > 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); I checked function get_user_pages_fast. It might return negative, 0, or positive value. 0 means it doesn't pin any page. So why does below statement use (!ret) to put_page?
I changed my test case and run it for unlimited times. It seems memory leak is big.
get_user_pages_fast is used by get_futex_key with the similiar issue.
> + 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); X86 pte entry has no READABLE flag. Other platforms might have. If their pte only set WRITE flag, Is it poosible to create a similiar DOS attack with WRITEONLY area on such platforms?
> 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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