From: Vladimir Sementsov-Ogievskiy Date: Mon, 16 Mar 2020 09:06:30 +0300 Subject: block: bdrv_set_backing_bs: fix use-after-free MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Git-commit: 6e57963a77df1e275a73dab4c6a7ec9a9d3468d4 There is a use-after-free possible: bdrv_unref_child() leaves bs->backing freed but not NULL. bdrv_attach_child may produce nested polling loop due to drain, than access of freed pointer is possible. I've produced the following crash on 30 iotest with modified code. It does not reproduce on master, but still seems possible: #0 __strcmp_avx2 () at /lib64/libc.so.6 #1 bdrv_backing_overridden (bs=0x55c9d3cc2060) at block.c:6350 #2 bdrv_refresh_filename (bs=0x55c9d3cc2060) at block.c:6404 #3 bdrv_backing_attach (c=0x55c9d48e5520) at block.c:1063 #4 bdrv_replace_child_noperm (child=child@entry=0x55c9d48e5520, new_bs=new_bs@entry=0x55c9d3cc2060) at block.c:2290 #5 bdrv_replace_child (child=child@entry=0x55c9d48e5520, new_bs=new_bs@entry=0x55c9d3cc2060) at block.c:2320 #6 bdrv_root_attach_child (child_bs=child_bs@entry=0x55c9d3cc2060, child_name=child_name@entry=0x55c9d241d478 "backing", child_role=child_role@entry=0x55c9d26ecee0 , ctx=, perm=, shared_perm=21, opaque=0x55c9d3c5a3d0, errp=0x7ffd117108e0) at block.c:2424 #7 bdrv_attach_child (parent_bs=parent_bs@entry=0x55c9d3c5a3d0, child_bs=child_bs@entry=0x55c9d3cc2060, child_name=child_name@entry=0x55c9d241d478 "backing", child_role=child_role@entry=0x55c9d26ecee0 , errp=errp@entry=0x7ffd117108e0) at block.c:5876 #8 in bdrv_set_backing_hd (bs=bs@entry=0x55c9d3c5a3d0, backing_hd=backing_hd@entry=0x55c9d3cc2060, errp=errp@entry=0x7ffd117108e0) at block.c:2576 #9 stream_prepare (job=0x55c9d49d84a0) at block/stream.c:150 #10 job_prepare (job=0x55c9d49d84a0) at job.c:761 #11 job_txn_apply (txn=, fn=) at job.c:145 #12 job_do_finalize (job=0x55c9d49d84a0) at job.c:778 #13 job_completed_txn_success (job=0x55c9d49d84a0) at job.c:832 #14 job_completed (job=0x55c9d49d84a0) at job.c:845 #15 job_completed (job=0x55c9d49d84a0) at job.c:836 #16 job_exit (opaque=0x55c9d49d84a0) at job.c:864 #17 aio_bh_call (bh=0x55c9d471a160) at util/async.c:117 #18 aio_bh_poll (ctx=ctx@entry=0x55c9d3c46720) at util/async.c:117 #19 aio_poll (ctx=ctx@entry=0x55c9d3c46720, blocking=blocking@entry=true) at util/aio-posix.c:728 #20 bdrv_parent_drained_begin_single (poll=true, c=0x55c9d3d558f0) at block/io.c:121 #21 bdrv_parent_drained_begin_single (c=c@entry=0x55c9d3d558f0, poll=poll@entry=true) at block/io.c:114 #22 bdrv_replace_child_noperm (child=child@entry=0x55c9d3d558f0, new_bs=new_bs@entry=0x55c9d3d27300) at block.c:2258 #23 bdrv_replace_child (child=child@entry=0x55c9d3d558f0, new_bs=new_bs@entry=0x55c9d3d27300) at block.c:2320 #24 bdrv_root_attach_child (child_bs=child_bs@entry=0x55c9d3d27300, child_name=child_name@entry=0x55c9d241d478 "backing", child_role=child_role@entry=0x55c9d26ecee0 , ctx=, perm=, shared_perm=21, opaque=0x55c9d3cc2060, errp=0x7ffd11710c60) at block.c:2424 #25 bdrv_attach_child (parent_bs=parent_bs@entry=0x55c9d3cc2060, child_bs=child_bs@entry=0x55c9d3d27300, child_name=child_name@entry=0x55c9d241d478 "backing", child_role=child_role@entry=0x55c9d26ecee0 , errp=errp@entry=0x7ffd11710c60) at block.c:5876 #26 bdrv_set_backing_hd (bs=bs@entry=0x55c9d3cc2060, backing_hd=backing_hd@entry=0x55c9d3d27300, errp=errp@entry=0x7ffd11710c60) at block.c:2576 #27 stream_prepare (job=0x55c9d495ead0) at block/stream.c:150 ... Signed-off-by: Vladimir Sementsov-Ogievskiy Message-Id: <20200316060631.30052-2-vsementsov@virtuozzo.com> Reviewed-by: Philippe Mathieu-Daudé Reviewed-by: John Snow Signed-off-by: Max Reitz Signed-off-by: Bruce Rogers --- block.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/block.c b/block.c index 8539f99ac47cdbf0e9b823751074..3d0134f00b55e0400efc70058d00 100644 --- a/block.c +++ b/block.c @@ -2577,10 +2577,10 @@ void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd, if (bs->backing) { bdrv_unref_child(bs, bs->backing); + bs->backing = NULL; } if (!backing_hd) { - bs->backing = NULL; goto out; }