linux-toradex.git/kernel/futex.c, branch v2.6.32.58

futex: Fix regression with read only mappings

2011-08-29T21:08:07+00:00

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 
Signed-off-by: Shawn Bohrer 
Acked-by: Peter Zijlstra 
Signed-off-by: Darren Hart 
Cc: KOSAKI Motohiro 
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
Signed-off-by: Thomas Gleixner 
Signed-off-by: Greg Kroah-Hartman

futex: Fix errors in nested key ref-counting

2010-11-22T18:47:31+00:00

commit 7ada876a8703f23befbb20a7465a702ee39b1704 upstream.

futex_wait() is leaking key references due to futex_wait_setup()
acquiring an additional reference via the queue_lock() routine. The
nested key ref-counting has been masking bugs and complicating code
analysis. queue_lock() is only called with a previously ref-counted
key, so remove the additional ref-counting from the queue_(un)lock()
functions.

Also futex_wait_requeue_pi() drops one key reference too many in
unqueue_me_pi(). Remove the key reference handling from
unqueue_me_pi(). This was paired with a queue_lock() in
futex_lock_pi(), so the count remains unchanged.

Document remaining nested key ref-counting sites.

Signed-off-by: Darren Hart 
Reported-and-tested-by: Matthieu Fertré
Reported-by: Louis Rilling
Cc: Peter Zijlstra 
Cc: Eric Dumazet 
Cc: John Kacur 
Cc: Rusty Russell 
LKML-Reference: <4CBB17A8.70401@linux.intel.com>
Signed-off-by: Thomas Gleixner 
Signed-off-by: Greg Kroah-Hartman

futex: futex_find_get_task remove credentails check

2010-08-02T17:21:24+00:00

commit 7a0ea09ad5352efce8fe79ed853150449903b9f5 upstream.

futex_find_get_task is currently used (through lookup_pi_state) from two
contexts, futex_requeue and futex_lock_pi_atomic.  None of the paths
looks it needs the credentials check, though.  Different (e)uids
shouldn't matter at all because the only thing that is important for
shared futex is the accessibility of the shared memory.

The credentail check results in glibc assert failure or process hang (if
glibc is compiled without assert support) for shared robust pthread
mutex with priority inheritance if a process tries to lock already held
lock owned by a process with a different euid:

pthread_mutex_lock.c:312: __pthread_mutex_lock_full: Assertion `(-(e)) != 3 || !robust' failed.

The problem is that futex_lock_pi_atomic which is called when we try to
lock already held lock checks the current holder (tid is stored in the
futex value) to get the PI state.  It uses lookup_pi_state which in turn
gets task struct from futex_find_get_task.  ESRCH is returned either
when the task is not found or if credentials check fails.

futex_lock_pi_atomic simply returns if it gets ESRCH.  glibc code,
however, doesn't expect that robust lock returns with ESRCH because it
should get either success or owner died.

Signed-off-by: Michal Hocko 
Acked-by: Darren Hart 
Cc: Ingo Molnar 
Cc: Thomas Gleixner 
Cc: Nick Piggin 
Cc: Alexey Kuznetsov 
Cc: Peter Zijlstra 
Signed-off-by: Linus Torvalds 
Signed-off-by: Greg Kroah-Hartman

futex: Handle futex value corruption gracefully

2010-02-23T15:37:43+00:00

commit 59647b6ac3050dd964bc556fe6ef22f4db5b935c upstream.

The WARN_ON in lookup_pi_state which complains about a mismatch
between pi_state->owner->pid and the pid which we retrieved from the
user space futex is completely bogus.

The code just emits the warning and then continues despite the fact
that it detected an inconsistent state of the futex. A conveniant way
for user space to spam the syslog.

Replace the WARN_ON by a consistency check. If the values do not match
return -EINVAL and let user space deal with the mess it created.

This also fixes the missing task_pid_vnr() when we compare the
pi_state->owner pid with the futex value.

Reported-by: Jermome Marchand 
Signed-off-by: Thomas Gleixner 
Acked-by: Darren Hart 
Acked-by: Peter Zijlstra 
Signed-off-by: Greg Kroah-Hartman

futex: Handle user space corruption gracefully

2010-02-23T15:37:43+00:00

commit 51246bfd189064079c54421507236fd2723b18f3 upstream.

If the owner of a PI futex dies we fix up the pi_state and set
pi_state->owner to NULL. When a malicious or just sloppy programmed
user space application sets the futex value to 0 e.g. by calling
pthread_mutex_init(), then the futex can be acquired again. A new
waiter manages to enqueue itself on the pi_state w/o damage, but on
unlock the kernel dereferences pi_state->owner and oopses.

Prevent this by checking pi_state->owner in the unlock path. If
pi_state->owner is not current we know that user space manipulated the
futex value. Ignore the mess and return -EINVAL.

This catches the above case and also the case where a task hijacks the
futex by setting the tid value and then tries to unlock it.

Reported-by: Jermome Marchand 
Signed-off-by: Thomas Gleixner 
Acked-by: Darren Hart 
Acked-by: Peter Zijlstra 
Signed-off-by: Greg Kroah-Hartman

futex_lock_pi() key refcnt fix

2010-02-23T15:37:43+00:00

commit 5ecb01cfdf96c5f465192bdb2a4fd4a61a24c6cc upstream.

This fixes a futex key reference count bug in futex_lock_pi(),
where a key's reference count is incremented twice but decremented
only once, causing the backing object to not be released.

If the futex is created in a temporary file in an ext3 file system,
this bug causes the file's inode to become an "undead" orphan,
which causes an oops from a BUG_ON() in ext3_put_super() when the
file system is unmounted. glibc's test suite is known to trigger this,
see .

The bug is a regression from 2.6.28-git3, namely Peter Zijlstra's
38d47c1b7075bd7ec3881141bb3629da58f88dab "[PATCH] futex: rely on
get_user_pages() for shared futexes". That commit made get_futex_key()
also increment the reference count of the futex key, and updated its
callers to decrement the key's reference count before returning.
Unfortunately the normal exit path in futex_lock_pi() wasn't corrected:
the reference count is incremented by get_futex_key() and queue_lock(),
but the normal exit path only decrements once, via unqueue_me_pi().
The fix is to put_futex_key() after unqueue_me_pi(), since 2.6.31
this is easily done by 'goto out_put_key' rather than 'goto out'.

Signed-off-by: Mikael Pettersson 
Acked-by: Peter Zijlstra 
Acked-by: Darren Hart 
Signed-off-by: Thomas Gleixner 
Signed-off-by: Greg Kroah-Hartman

futexes: Remove rw parameter from get_futex_key()

2010-01-22T23:18:11+00:00

commit 7485d0d3758e8e6491a5c9468114e74dc050785d upstream.

Currently, futexes have two problem:

A) The current futex code doesn't handle private file mappings properly.

get_futex_key() uses PageAnon() to distinguish file and
anon, which can cause the following bad scenario:

  1) thread-A call futex(private-mapping, FUTEX_WAIT), it
     sleeps on file mapping object.
  2) thread-B writes a variable and it makes it cow.
  3) thread-B calls futex(private-mapping, FUTEX_WAKE), it
     wakes up blocked thread on the anonymous page. (but it's nothing)

B) Current futex code doesn't handle zero page properly.

Read mode get_user_pages() can return zero page, but current
futex code doesn't handle it at all. Then, zero page makes
infinite loop internally.

The solution is to use write mode get_user_page() always for
page lookup. It prevents the lookup of both file page of private
mappings and zero page.

Performance concerns:

Probaly very little, because glibc always initialize variables
for futex before to call futex(). It means glibc users never see
the overhead of this patch.

Compatibility concerns:

This patch has few compatibility issues. After this patch,
FUTEX_WAIT require writable access to futex variables (read-only
mappings makes EFAULT). But practically it's not a problem,
glibc always initalizes variables for futexes explicitly - nobody
uses read-only mappings.

Reported-by: Hugh Dickins 
Signed-off-by: KOSAKI Motohiro 
Acked-by: Peter Zijlstra 
Acked-by: Darren Hart 
Cc: Linus Torvalds 
Cc: KAMEZAWA Hiroyuki 
Cc: Nick Piggin 
Cc: Ulrich Drepper 
LKML-Reference: <20100105162633.45A2.A69D9226@jp.fujitsu.com>
Signed-off-by: Ingo Molnar 
Signed-off-by: Greg Kroah-Hartman

futex: Take mmap_sem for get_user_pages in fault_in_user_writeable

2009-12-18T22:04:15+00:00

commit 722d0172377a5697919b9f7e5beb95165b1dec4e upstream.

get_user_pages() must be called with mmap_sem held.

Signed-off-by: Andi Kleen 
Cc: Andrew Morton 
Cc: Nick Piggin 
Cc: Darren Hart 
Cc: Peter Zijlstra 
LKML-Reference: <20091208121942.GA21298@basil.fritz.box>
Signed-off-by: Thomas Gleixner 
Signed-off-by: Greg Kroah-Hartman

futex: Fix spurious wakeup for requeue_pi really

2009-10-28T19:34:34+00:00

The requeue_pi path doesn't use unqueue_me() (and the racy lock_ptr ==
NULL test) nor does it use the wake_list of futex_wake() which where
the reason for commit 41890f2 (futex: Handle spurious wake up)

See debugging discussing on LKML Message-ID: <4AD4080C.20703@us.ibm.com>

The changes in this fix to the wait_requeue_pi path were considered to
be a likely unecessary, but harmless safety net. But it turns out that
due to the fact that for unknown $@#!*( reasons EWOULDBLOCK is defined
as EAGAIN we built an endless loop in the code path which returns
correctly EWOULDBLOCK.

Spurious wakeups in wait_requeue_pi code path are unlikely so we do
the easy solution and return EWOULDBLOCK^WEAGAIN to user space and let
it deal with the spurious wakeup.

Cc: Darren Hart 
Cc: Peter Zijlstra 
Cc: Eric Dumazet 
Cc: John Stultz 
Cc: Dinakar Guniguntala 
LKML-Reference: <4AE23C74.1090502@us.ibm.com>
Cc: stable@kernel.org
Signed-off-by: Thomas Gleixner

futex: Move drop_futex_key_refs out of spinlock'ed region

2009-10-16T08:19:18+00:00

When requeuing tasks from one futex to another, the reference held
by the requeued task to the original futex location needs to be
dropped eventually.

Dropping the reference may ultimately lead to a call to
"iput_final" and subsequently call into filesystem- specific code -
which may be non-atomic.

It is therefore safer to defer this drop operation until after the
futex_hash_bucket spinlock has been dropped.

Originally-From: Helge Bahmann 
Signed-off-by: Darren Hart 
Cc: 
Cc: Peter Zijlstra 
Cc: Eric Dumazet 
Cc: Dinakar Guniguntala 
Cc: John Stultz 
Cc: Sven-Thorsten Dietrich 
Cc: John Kacur 
LKML-Reference: <4AD7A298.5040802@us.ibm.com>
Signed-off-by: Ingo Molnar