futex: Fix regression with read only mappings

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@rgmadvisors.com>
Signed-off-by: Shawn Bohrer <sbohrer@rgmadvisors.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: peterz@infradead.org
Cc: eric.dumazet@gmail.com
Cc: zvonler@rgmadvisors.com
Cc: hughd@google.com
Link: http://lkml.kernel.org/r/1309450892-30676-1-git-send-email-sbohrer@rgmadvisors.com
Cc: stable@kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 42 insertions, 12 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index fe28dc282eae..70bb54bd2468 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;