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Date: Mon, 14 May 2012 22:14:23 -0400
From: Paul Gortmaker <paul.gortmaker@...driver.com>
To: <stable@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: [34-longterm 167/179] futex: Fix regression with read only mappings
From: Shawn Bohrer <sbohrer@...advisors.com>
-------------------
This is a commit scheduled for the next v2.6.34 longterm release.
http://git.kernel.org/?p=linux/kernel/git/paulg/longterm-queue-2.6.34.git
If you see a problem with using this for longterm, please comment.
-------------------
commit 9ea71503a8ed9184d2d0b8ccc4d269d05f7940ae upstream.
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@...advisors.com>
Signed-off-by: Shawn Bohrer <sbohrer@...advisors.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Signed-off-by: Darren Hart <dvhart@...ux.intel.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc: peterz@...radead.org
Cc: eric.dumazet@...il.com
Cc: zvonler@...advisors.com
Cc: hughd@...gle.com
Link: http://lkml.kernel.org/r/1309450892-30676-1-git-send-email-sbohrer@rgmadvisors.com
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
[PG: in 34, the variable is "page"; in original 9ea71503a it is page_head]
Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
---
kernel/futex.c | 54 ++++++++++++++++++++++++++++++++++++++++++------------
1 file changed, 42 insertions(+), 12 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index e328f57..98a354d 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -203,6 +203,8 @@ 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.
@@ -214,12 +216,12 @@ 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;
- int err;
+ int err, ro = 0;
/*
* The futex address must be "naturally" aligned.
@@ -247,14 +249,31 @@ 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;
page = compound_head(page);
lock_page(page);
if (!page->mapping) {
unlock_page(page);
put_page(page);
+ /*
+ * 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 == ZERO_PAGE(address)))
+ return -EFAULT;
goto again;
}
@@ -266,6 +285,15 @@ again:
* the object not the particular process.
*/
if (PageAnon(page)) {
+ /*
+ * 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;
@@ -277,9 +305,10 @@ again:
get_futex_key_refs(key);
+out:
unlock_page(page);
put_page(page);
- return 0;
+ return err;
}
static inline
@@ -880,7 +909,7 @@ static int futex_wake(u32 __user *uaddr, int fshared, int nr_wake, u32 bitset)
if (!bitset)
return -EINVAL;
- ret = get_futex_key(uaddr, fshared, &key);
+ ret = get_futex_key(uaddr, fshared, &key, VERIFY_READ);
if (unlikely(ret != 0))
goto out;
@@ -926,10 +955,10 @@ futex_wake_op(u32 __user *uaddr1, int fshared, u32 __user *uaddr2,
int ret, op_ret;
retry:
- ret = get_futex_key(uaddr1, fshared, &key1);
+ ret = get_futex_key(uaddr1, fshared, &key1, VERIFY_READ);
if (unlikely(ret != 0))
goto out;
- ret = get_futex_key(uaddr2, fshared, &key2);
+ ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE);
if (unlikely(ret != 0))
goto out_put_key1;
@@ -1188,10 +1217,11 @@ retry:
pi_state = NULL;
}
- ret = get_futex_key(uaddr1, fshared, &key1);
+ ret = get_futex_key(uaddr1, fshared, &key1, VERIFY_READ);
if (unlikely(ret != 0))
goto out;
- ret = get_futex_key(uaddr2, fshared, &key2);
+ ret = get_futex_key(uaddr2, fshared, &key2,
+ requeue_pi ? VERIFY_WRITE : VERIFY_READ);
if (unlikely(ret != 0))
goto out_put_key1;
@@ -1746,7 +1776,7 @@ static int futex_wait_setup(u32 __user *uaddr, u32 val, int fshared,
*/
retry:
q->key = FUTEX_KEY_INIT;
- ret = get_futex_key(uaddr, fshared, &q->key);
+ ret = get_futex_key(uaddr, fshared, &q->key, VERIFY_READ);
if (unlikely(ret != 0))
return ret;
@@ -1912,7 +1942,7 @@ static int futex_lock_pi(u32 __user *uaddr, int fshared,
q.requeue_pi_key = NULL;
retry:
q.key = FUTEX_KEY_INIT;
- ret = get_futex_key(uaddr, fshared, &q.key);
+ ret = get_futex_key(uaddr, fshared, &q.key, VERIFY_WRITE);
if (unlikely(ret != 0))
goto out;
@@ -2031,7 +2061,7 @@ retry:
if ((uval & FUTEX_TID_MASK) != task_pid_vnr(current))
return -EPERM;
- ret = get_futex_key(uaddr, fshared, &key);
+ ret = get_futex_key(uaddr, fshared, &key, VERIFY_WRITE);
if (unlikely(ret != 0))
goto out;
@@ -2223,7 +2253,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
rt_waiter.task = NULL;
key2 = FUTEX_KEY_INIT;
- ret = get_futex_key(uaddr2, fshared, &key2);
+ ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE);
if (unlikely(ret != 0))
goto out;
--
1.7.9.6
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