From bf4c15a3debbe68b6eb25c52174843470a9c014f Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Tue, 11 Jan 2022 15:36:12 +0000 Subject: [PATCH 3/6] block-backend: prevent dangling BDS pointers across aio_poll() RH-Author: Stefan Hajnoczi RH-MergeRequest: 109: block-backend: prevent dangling BDS pointers across aio_poll() RH-Commit: [1/2] da5a59eddff0dc10be7de8e291fa675143d11d73 RH-Bugzilla: 2021778 2036178 RH-Acked-by: Hanna Reitz RH-Acked-by: Emanuele Giuseppe Esposito RH-Acked-by: Kevin Wolf The BlockBackend root child can change when aio_poll() is invoked. This happens when a temporary filter node is removed upon blockjob completion, for example. Functions in block/block-backend.c must be aware of this when using a blk_bs() pointer across aio_poll() because the BlockDriverState refcnt may reach 0, resulting in a stale pointer. One example is scsi_device_purge_requests(), which calls blk_drain() to wait for in-flight requests to cancel. If the backup blockjob is active, then the BlockBackend root child is a temporary filter BDS owned by the blockjob. The blockjob can complete during bdrv_drained_begin() and the last reference to the BDS is released when the temporary filter node is removed. This results in a use-after-free when blk_drain() calls bdrv_drained_end(bs) on the dangling pointer. Explicitly hold a reference to bs across block APIs that invoke aio_poll(). Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2021778 Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2036178 Signed-off-by: Stefan Hajnoczi Message-Id: <20220111153613.25453-2-stefanha@redhat.com> Signed-off-by: Kevin Wolf (cherry picked from commit 1e3552dbd28359d35967b7c28dc86cde1bc29205) Signed-off-by: Stefan Hajnoczi --- block/block-backend.c | 19 +++++++++++++++++-- 1 file changed, 17 insertions(+), 2 deletions(-) diff --git a/block/block-backend.c b/block/block-backend.c index 12ef80ea17..23e727199b 100644 --- a/block/block-backend.c +++ b/block/block-backend.c @@ -822,16 +822,22 @@ BlockBackend *blk_by_public(BlockBackendPublic *public) void blk_remove_bs(BlockBackend *blk) { ThrottleGroupMember *tgm = &blk->public.throttle_group_member; - BlockDriverState *bs; BdrvChild *root; notifier_list_notify(&blk->remove_bs_notifiers, blk); if (tgm->throttle_state) { - bs = blk_bs(blk); + BlockDriverState *bs = blk_bs(blk); + + /* + * Take a ref in case blk_bs() changes across bdrv_drained_begin(), for + * example, if a temporary filter node is removed by a blockjob. + */ + bdrv_ref(bs); bdrv_drained_begin(bs); throttle_group_detach_aio_context(tgm); throttle_group_attach_aio_context(tgm, qemu_get_aio_context()); bdrv_drained_end(bs); + bdrv_unref(bs); } blk_update_root_state(blk); @@ -1705,6 +1711,7 @@ void blk_drain(BlockBackend *blk) BlockDriverState *bs = blk_bs(blk); if (bs) { + bdrv_ref(bs); bdrv_drained_begin(bs); } @@ -1714,6 +1721,7 @@ void blk_drain(BlockBackend *blk) if (bs) { bdrv_drained_end(bs); + bdrv_unref(bs); } } @@ -2044,10 +2052,13 @@ static int blk_do_set_aio_context(BlockBackend *blk, AioContext *new_context, int ret; if (bs) { + bdrv_ref(bs); + if (update_root_node) { ret = bdrv_child_try_set_aio_context(bs, new_context, blk->root, errp); if (ret < 0) { + bdrv_unref(bs); return ret; } } @@ -2057,6 +2068,8 @@ static int blk_do_set_aio_context(BlockBackend *blk, AioContext *new_context, throttle_group_attach_aio_context(tgm, new_context); bdrv_drained_end(bs); } + + bdrv_unref(bs); } blk->ctx = new_context; @@ -2326,11 +2339,13 @@ void blk_io_limits_disable(BlockBackend *blk) ThrottleGroupMember *tgm = &blk->public.throttle_group_member; assert(tgm->throttle_state); if (bs) { + bdrv_ref(bs); bdrv_drained_begin(bs); } throttle_group_unregister_tgm(tgm); if (bs) { bdrv_drained_end(bs); + bdrv_unref(bs); } } -- 2.27.0