Re: [PATCH v5] futex: Fix regression with read only mappings

From: Shawn Bohrer
Date: Tue Jul 12 2011 - 11:27:47 EST

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On Thu, Jun 30, 2011 at 11:21:32AM -0500, Shawn Bohrer wrote:
> commit 7485d0d3758e8e6491a5c9468114e74dc050785d (futexes: Remove rw
> parameter from get_futex_key()) in 2.6.33 fixed two problems: First, It
> prevented a loop when encountering a ZERO_PAGE. Second, it fixed RW
> MAP_PRIVATE futex operations by forcing the COW to occur by
> unconditionally performing a write access get_user_pages_fast() to get
> the page. The commit also introduced a user-mode regression in that it
> broke futex operations on read-only memory maps. For example, this
> breaks workloads that have one or more reader processes doing a
> FUTEX_WAIT on a futex within a read only shared file mapping, and a
> writer processes that has a writable mapping issuing the FUTEX_WAKE.
>
> This fixes the regression for valid futex operations on RO mappings by
> trying a RO get_user_pages_fast() when the RW get_user_pages_fast()
> fails. This change makes it necessary to also check for invalid use
> cases, such as anonymous RO mappings (which can never change) and the
> ZERO_PAGE which the commit referenced above was written to address.
>
> This patch does restore the original behavior with RO MAP_PRIVATE
> mappings, which have inherent user-mode usage problems and don't really
> make sense. With this patch performing a FUTEX_WAIT within a RO
> MAP_PRIVATE mapping will be successfully woken provided another process
> updates the region of the underlying mapped file. However, the mmap()
> man page states that for a MAP_PRIVATE mapping:
>
> It is unspecified whether changes made to the file after
> the mmap() call are visible in the mapped region.
>
> So user-mode users attempting to use futex operations on RO MAP_PRIVATE
> mappings are depending on unspecified behavior. Additionally a
> RO MAP_PRIVATE mapping could fail to wake up in the following case.
>
> Thread-A: call futex(FUTEX_WAIT, memory-region-A).
> get_futex_key() return inode based key.
> sleep on the key
> Thread-B: call mprotect(PROT_READ|PROT_WRITE, memory-region-A)
> Thread-B: write memory-region-A.
> COW happen. This process's memory-region-A become related
> to new COWed private (ie PageAnon=1) page.
> Thread-B: call futex(FUETX_WAKE, memory-region-A).
> get_futex_key() return mm based key.
> IOW, we fail to wake up Thread-A.
>
> Once again doing something like this is just silly and users who do
> something like this get what they deserve.
>
> While RO MAP_PRIVATE mappings are nonsensical, checking for a private
> mapping requires walking the vmas and was deemed too costly to avoid a
> userspace hang.
>
> This Patch is based on Peter Zijlstra's initial patch with modifications to
> only allow RO mappings for futex operations that need VERIFY_READ access.
>
> Reported-by: David Oliver <david@xxxxxxxxxxxxxxx>
> Signed-off-by: Shawn Bohrer <sbohrer@xxxxxxxxxxxxxxx>
> Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Signed-off-by: Darren Hart <dvhart@xxxxxxxxxxxxxxx>
> ---
>
> Clairified commit description. Changed get_futex_key() back to having a
> fshared and rw parameter, per Darren's request.
>
> kernel/futex.c | 54 ++++++++++++++++++++++++++++++++++++++++++------------
> 1 files changed, 42 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/futex.c b/kernel/futex.c
> index fe28dc2..70bb54b 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -218,6 +218,8 @@ static void drop_futex_key_refs(union futex_key *key)
> * @uaddr: virtual address of the futex
> * @fshared: 0 for a PROCESS_PRIVATE futex, 1 for PROCESS_SHARED
> * @key: address where result is stored.
> + * @rw: mapping needs to be read/write (values: VERIFY_READ,
> + * VERIFY_WRITE)
> *
> * Returns a negative error code or 0
> * The key words are stored in *key on success.
> @@ -229,12 +231,12 @@ static void drop_futex_key_refs(union futex_key *key)
> * lock_page() might sleep, the caller should not hold a spinlock.
> */
> static int
> -get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
> +get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
> {
> unsigned long address = (unsigned long)uaddr;
> struct mm_struct *mm = current->mm;
> struct page *page, *page_head;
> - int err;
> + int err, ro = 0;
>
> /*
> * The futex address must be "naturally" aligned.
> @@ -262,8 +264,18 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
>
> again:
> err = get_user_pages_fast(address, 1, 1, &page);
> + /*
> + * If write access is not required (eg. FUTEX_WAIT), try
> + * and get read-only access.
> + */
> + if (err == -EFAULT && rw == VERIFY_READ) {
> + err = get_user_pages_fast(address, 1, 0, &page);
> + ro = 1;
> + }
> if (err < 0)
> return err;
> + else
> + err = 0;
>
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> page_head = page;
> @@ -305,6 +317,13 @@ again:
> if (!page_head->mapping) {
> unlock_page(page_head);
> put_page(page_head);
> + /*
> + * ZERO_PAGE pages don't have a mapping. Avoid a busy loop
> + * trying to find one. RW mapping would have COW'd (and thus
> + * have a mapping) so this page is RO and won't ever change.
> + */
> + if ((page_head == ZERO_PAGE(address)))
> + return -EFAULT;
> goto again;
> }
>
> @@ -316,6 +335,15 @@ again:
> * the object not the particular process.
> */
> if (PageAnon(page_head)) {
> + /*
> + * A RO anonymous page will never change and thus doesn't make
> + * sense for futex operations.
> + */
> + if (ro) {
> + err = -EFAULT;
> + goto out;
> + }
> +
> key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
> key->private.mm = mm;
> key->private.address = address;
> @@ -327,9 +355,10 @@ again:
>
> get_futex_key_refs(key);
>
> +out:
> unlock_page(page_head);
> put_page(page_head);
> - return 0;
> + return err;
> }
>
> static inline void put_futex_key(union futex_key *key)
> @@ -940,7 +969,7 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
> if (!bitset)
> return -EINVAL;
>
> - ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key);
> + ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, VERIFY_READ);
> if (unlikely(ret != 0))
> goto out;
>
> @@ -986,10 +1015,10 @@ futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
> int ret, op_ret;
>
> retry:
> - ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1);
> + ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, VERIFY_READ);
> if (unlikely(ret != 0))
> goto out;
> - ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2);
> + ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, VERIFY_WRITE);
> if (unlikely(ret != 0))
> goto out_put_key1;
>
> @@ -1243,10 +1272,11 @@ retry:
> pi_state = NULL;
> }
>
> - ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1);
> + ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, VERIFY_READ);
> if (unlikely(ret != 0))
> goto out;
> - ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2);
> + ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2,
> + requeue_pi ? VERIFY_WRITE : VERIFY_READ);
> if (unlikely(ret != 0))
> goto out_put_key1;
>
> @@ -1790,7 +1820,7 @@ static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
> * while the syscall executes.
> */
> retry:
> - ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q->key);
> + ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q->key, VERIFY_READ);
> if (unlikely(ret != 0))
> return ret;
>
> @@ -1941,7 +1971,7 @@ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags, int detect,
> }
>
> retry:
> - ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q.key);
> + ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q.key, VERIFY_WRITE);
> if (unlikely(ret != 0))
> goto out;
>
> @@ -2060,7 +2090,7 @@ retry:
> if ((uval & FUTEX_TID_MASK) != vpid)
> return -EPERM;
>
> - ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key);
> + ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, VERIFY_WRITE);
> if (unlikely(ret != 0))
> goto out;
>
> @@ -2249,7 +2279,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
> debug_rt_mutex_init_waiter(&rt_waiter);
> rt_waiter.task = NULL;
>
> - ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2);
> + ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, VERIFY_WRITE);
> if (unlikely(ret != 0))
> goto out;
>

Any update on this getting applied? This fixes the regression in our
real world user-mode application so I'd really like to see it get
accepted upstream.

Thanks,
Shawn

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Next message: Stefan Hajnoczi: "Re: [PATCH 1/3] power_supply: scrub device pointer if registrationfails"
Previous message: Alan Stern: "Re: block: Check that queue is alive in blk_insert_cloned_request()"
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