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From f006159bba058c873bf8c1a3684ec72f338f2a2c Mon Sep 17 00:00:00 2001
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From: Kevin Wolf <kwolf@redhat.com>
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Date: Fri, 14 Sep 2018 10:55:33 +0200
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Subject: [PATCH 42/49] block-backend: Fix potential double blk_delete()
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RH-Author: Kevin Wolf <kwolf@redhat.com>
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Message-id: <20180914105540.18077-36-kwolf@redhat.com>
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Patchwork-id: 82186
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O-Subject: [RHV-7.6 qemu-kvm-rhev PATCH 35/42] block-backend: Fix potential double blk_delete()
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Bugzilla: 1601212
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RH-Acked-by: John Snow <jsnow@redhat.com>
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RH-Acked-by: Max Reitz <mreitz@redhat.com>
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RH-Acked-by: Fam Zheng <famz@redhat.com>
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blk_unref() first decreases the refcount of the BlockBackend and calls
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blk_delete() if the refcount reaches zero. Requests can still be in
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flight at this point, they are only drained during blk_delete():
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At this point, arbitrary callbacks can run. If any callback takes a
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temporary BlockBackend reference, it will first increase the refcount to
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1 and then decrease it to 0 again, triggering another blk_delete(). This
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will cause a use-after-free crash in the outer blk_delete().
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Fix it by draining the BlockBackend before decreasing to refcount to 0.
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Assert in blk_ref() that it never takes the first refcount (which would
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mean that the BlockBackend is already being deleted).
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Reviewed-by: Fam Zheng <famz@redhat.com>
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Reviewed-by: Max Reitz <mreitz@redhat.com>
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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---
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block/block-backend.c | 9 ++++++++-
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1 file changed, 8 insertions(+), 1 deletion(-)
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diff --git a/block/block-backend.c b/block/block-backend.c
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index ea85770..1adf76b 100644
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--- a/block/block-backend.c
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+++ b/block/block-backend.c
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@@ -436,6 +436,7 @@ int blk_get_refcnt(BlockBackend *blk)
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*/
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void blk_ref(BlockBackend *blk)
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{
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+ assert(blk->refcnt > 0);
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blk->refcnt++;
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}
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@@ -448,7 +449,13 @@ void blk_unref(BlockBackend *blk)
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{
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if (blk) {
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assert(blk->refcnt > 0);
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- if (!--blk->refcnt) {
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+ if (blk->refcnt > 1) {
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+ blk->refcnt--;
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+ } else {
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+ blk_drain(blk);
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+ /* blk_drain() cannot resurrect blk, nobody held a reference */
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+ assert(blk->refcnt == 1);
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+ blk->refcnt = 0;
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blk_delete(blk);
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}
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}
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--
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1.8.3.1
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