From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:48698) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eMfC2-000869-Kh for qemu-devel@nongnu.org; Wed, 06 Dec 2017 14:17:20 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eMfC0-00008m-EU for qemu-devel@nongnu.org; Wed, 06 Dec 2017 14:17:18 -0500 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:33978) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1eMfC0-00007P-5c for qemu-devel@nongnu.org; Wed, 06 Dec 2017 14:17:16 -0500 Received: from pps.filterd (m0098396.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id vB6JFWBZ043798 for ; Wed, 6 Dec 2017 14:17:13 -0500 Received: from e16.ny.us.ibm.com (e16.ny.us.ibm.com [129.33.205.206]) by mx0a-001b2d01.pphosted.com with ESMTP id 2epnw5sruw-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Wed, 06 Dec 2017 14:17:13 -0500 Received: from localhost by e16.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 6 Dec 2017 14:17:12 -0500 From: Michael Roth Date: Wed, 6 Dec 2017 13:16:04 -0600 In-Reply-To: <20171206191648.18208-1-mdroth@linux.vnet.ibm.com> References: <20171206191648.18208-1-mdroth@linux.vnet.ibm.com> Message-Id: <20171206191648.18208-12-mdroth@linux.vnet.ibm.com> Subject: [Qemu-devel] [PATCH 11/55] memory: avoid "resurrection" of dead FlatViews List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: qemu-stable@nongnu.org, Paolo Bonzini From: Paolo Bonzini It's possible for address_space_get_flatview() as it currently stands to cause a use-after-free for the returned FlatView, if the reference count is incremented after the FlatView has been replaced by a writer: thread 1 thread 2 RCU thread ------------------------------------------------------------- rcu_read_lock read as->current_map set as->current_map flatview_unref '--> call_rcu flatview_ref [ref=1] rcu_read_unlock flatview_destroy Since FlatViews are not updated very often, we can just detect the situation using a new atomic op atomic_fetch_inc_nonzero, similar to Linux's atomic_inc_not_zero, which performs the refcount increment only if it hasn't already hit zero. This is similar to Linux commit de09a9771a53 ("CRED: Fix get_task_cred() and task_state() to not resurrect dead credentials", 2010-07-29). Signed-off-by: Paolo Bonzini (cherry picked from commit 447b0d0b9ee8a0ac216c3186e0f3c427a1001f0c) Conflicts: docs/devel/atomics.txt * drop documentation ref to atomic_fetch_xor * prereq for 166206845f Signed-off-by: Michael Roth --- docs/devel/atomics.txt | 1 + include/qemu/atomic.h | 8 ++++++++ memory.c | 12 ++++++++---- 3 files changed, 17 insertions(+), 4 deletions(-) diff --git a/docs/devel/atomics.txt b/docs/devel/atomics.txt index 3ef5d85b1b..3fbaf52140 100644 --- a/docs/devel/atomics.txt +++ b/docs/devel/atomics.txt @@ -63,6 +63,7 @@ operations: typeof(*ptr) atomic_fetch_sub(ptr, val) typeof(*ptr) atomic_fetch_and(ptr, val) typeof(*ptr) atomic_fetch_or(ptr, val) + typeof(*ptr) atomic_fetch_inc_nonzero(ptr) typeof(*ptr) atomic_xchg(ptr, val) typeof(*ptr) atomic_cmpxchg(ptr, old, new) diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h index b6b62fb771..d73c9e14d7 100644 --- a/include/qemu/atomic.h +++ b/include/qemu/atomic.h @@ -442,4 +442,12 @@ } while(0) #endif +#define atomic_fetch_inc_nonzero(ptr) ({ \ + typeof_strip_qual(*ptr) _oldn = atomic_read(ptr); \ + while (_oldn && atomic_cmpxchg(ptr, _oldn, _oldn + 1) != _oldn) { \ + _oldn = atomic_read(ptr); \ + } \ + _oldn; \ +}) + #endif /* QEMU_ATOMIC_H */ diff --git a/memory.c b/memory.c index de57a16ece..41e2e67301 100644 --- a/memory.c +++ b/memory.c @@ -300,9 +300,9 @@ static void flatview_destroy(FlatView *view) g_free(view); } -static void flatview_ref(FlatView *view) +static bool flatview_ref(FlatView *view) { - atomic_inc(&view->ref); + return atomic_fetch_inc_nonzero(&view->ref) > 0; } static void flatview_unref(FlatView *view) @@ -792,8 +792,12 @@ static FlatView *address_space_get_flatview(AddressSpace *as) FlatView *view; rcu_read_lock(); - view = atomic_rcu_read(&as->current_map); - flatview_ref(view); + do { + view = atomic_rcu_read(&as->current_map); + /* If somebody has replaced as->current_map concurrently, + * flatview_ref returns false. + */ + } while (!flatview_ref(view)); rcu_read_unlock(); return view; } -- 2.11.0