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From 87f3b10dc600ac12272ee6cdc67571910ea722f6 Mon Sep 17 00:00:00 2001
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From: Stefan Hajnoczi <stefanha@redhat.com>
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Date: Tue, 11 Jan 2022 15:36:12 +0000
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Subject: [PATCH 10/12] block-backend: prevent dangling BDS pointers across
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aio_poll()
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RH-Author: Hanna Reitz <hreitz@redhat.com>
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RH-MergeRequest: 71: block-backend: prevent dangling BDS pointers across aio_poll()
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RH-Commit: [1/2] 1b4cab39bf8c933ab910293a29bfceaa9e821068 (hreitz/qemu-kvm-c-9-s)
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RH-Bugzilla: 2040123
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RH-Acked-by: Kevin Wolf <kwolf@redhat.com>
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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RH-Acked-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
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The BlockBackend root child can change when aio_poll() is invoked. This
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happens when a temporary filter node is removed upon blockjob
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completion, for example.
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Functions in block/block-backend.c must be aware of this when using a
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blk_bs() pointer across aio_poll() because the BlockDriverState refcnt
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may reach 0, resulting in a stale pointer.
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One example is scsi_device_purge_requests(), which calls blk_drain() to
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wait for in-flight requests to cancel. If the backup blockjob is active,
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then the BlockBackend root child is a temporary filter BDS owned by the
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blockjob. The blockjob can complete during bdrv_drained_begin() and the
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last reference to the BDS is released when the temporary filter node is
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removed. This results in a use-after-free when blk_drain() calls
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bdrv_drained_end(bs) on the dangling pointer.
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Explicitly hold a reference to bs across block APIs that invoke
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aio_poll().
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Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2021778
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Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2036178
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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Message-Id: <20220111153613.25453-2-stefanha@redhat.com>
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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(cherry picked from commit 1e3552dbd28359d35967b7c28dc86cde1bc29205)
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Signed-off-by: Hanna Reitz <hreitz@redhat.com>
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---
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block/block-backend.c | 19 +++++++++++++++++--
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1 file changed, 17 insertions(+), 2 deletions(-)
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diff --git a/block/block-backend.c b/block/block-backend.c
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index 12ef80ea17..23e727199b 100644
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--- a/block/block-backend.c
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+++ b/block/block-backend.c
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@@ -822,16 +822,22 @@ BlockBackend *blk_by_public(BlockBackendPublic *public)
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void blk_remove_bs(BlockBackend *blk)
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{
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ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
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- BlockDriverState *bs;
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BdrvChild *root;
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notifier_list_notify(&blk->remove_bs_notifiers, blk);
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if (tgm->throttle_state) {
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- bs = blk_bs(blk);
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+ BlockDriverState *bs = blk_bs(blk);
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+
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+ /*
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+ * Take a ref in case blk_bs() changes across bdrv_drained_begin(), for
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+ * example, if a temporary filter node is removed by a blockjob.
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+ */
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+ bdrv_ref(bs);
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bdrv_drained_begin(bs);
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throttle_group_detach_aio_context(tgm);
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throttle_group_attach_aio_context(tgm, qemu_get_aio_context());
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bdrv_drained_end(bs);
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+ bdrv_unref(bs);
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}
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blk_update_root_state(blk);
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@@ -1705,6 +1711,7 @@ void blk_drain(BlockBackend *blk)
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BlockDriverState *bs = blk_bs(blk);
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if (bs) {
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+ bdrv_ref(bs);
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bdrv_drained_begin(bs);
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}
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@@ -1714,6 +1721,7 @@ void blk_drain(BlockBackend *blk)
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if (bs) {
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bdrv_drained_end(bs);
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+ bdrv_unref(bs);
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}
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}
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@@ -2044,10 +2052,13 @@ static int blk_do_set_aio_context(BlockBackend *blk, AioContext *new_context,
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int ret;
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if (bs) {
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+ bdrv_ref(bs);
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+
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if (update_root_node) {
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ret = bdrv_child_try_set_aio_context(bs, new_context, blk->root,
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errp);
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if (ret < 0) {
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+ bdrv_unref(bs);
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return ret;
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}
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}
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@@ -2057,6 +2068,8 @@ static int blk_do_set_aio_context(BlockBackend *blk, AioContext *new_context,
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throttle_group_attach_aio_context(tgm, new_context);
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bdrv_drained_end(bs);
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}
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+
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+ bdrv_unref(bs);
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}
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blk->ctx = new_context;
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@@ -2326,11 +2339,13 @@ void blk_io_limits_disable(BlockBackend *blk)
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ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
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assert(tgm->throttle_state);
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if (bs) {
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+ bdrv_ref(bs);
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bdrv_drained_begin(bs);
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}
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throttle_group_unregister_tgm(tgm);
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if (bs) {
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bdrv_drained_end(bs);
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+ bdrv_unref(bs);
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}
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}
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--
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2.27.0
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