From 7de5d186bd03511cd5fa4dcd32876efb034f1dd0 Mon Sep 17 00:00:00 2001 From: David Gibson Date: Thu, 16 Nov 2017 03:07:13 +0100 Subject: [PATCH 09/30] memory: avoid "resurrection" of dead FlatViews RH-Author: David Gibson Message-id: <20171116030732.8560-4-dgibson@redhat.com> Patchwork-id: 77691 O-Subject: [PATCH 03/22] memory: avoid "resurrection" of dead FlatViews Bugzilla: 1481593 RH-Acked-by: Thomas Huth RH-Acked-by: Paolo Bonzini RH-Acked-by: Eduardo Habkost RH-Acked-by: Laurent Vivier 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to cause a use-after-free for the returned FlatView, if the reference count is incremented after the FlatView has been replaced by a writer: thread 1 thread 2 RCU thread ------------------------------------------------------------- rcu_read_lock read as-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) Signed-off-by: David Gibson Signed-off-by: Miroslav Rezanina --- 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 048e5f2..10c5fa3 100644 --- a/docs/devel/atomics.txt +++ b/docs/devel/atomics.txt @@ -64,6 +64,7 @@ operations: typeof(*ptr) atomic_fetch_and(ptr, val) typeof(*ptr) atomic_fetch_or(ptr, val) typeof(*ptr) atomic_fetch_xor(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 b6b62fb..d73c9e1 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 c0adc35..ca160de 100644 --- a/memory.c +++ b/memory.c @@ -294,9 +294,9 @@ 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) @@ -772,8 +772,12 @@ 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; } -- 1.8.3.1