Heh, of course I meant qemu-sta...@nongnu.org ;)
On Wed, 16 May 2018 13:21:54 +0200
Greg Kurz <gr...@kaod.org> wrote:
> Removing a drive with drive_del while it is being used to run an I/O
> intensive workload can cause QEMU to crash.
>
> An AIO flush can yield at some point:
>
> blk_aio_flush_entry()
> blk_co_flush(blk)
> bdrv_co_flush(blk->root->bs)
> ...
> qemu_coroutine_yield()
>
> and let the HMP command to run, free blk->root and give control
> back to the AIO flush:
>
> hmp_drive_del()
> blk_remove_bs()
> bdrv_root_unref_child(blk->root)
> child_bs = blk->root->bs
> bdrv_detach_child(blk->root)
> bdrv_replace_child(blk->root, NULL)
> blk->root->bs = NULL
> g_free(blk->root) <============== blk->root becomes stale
> bdrv_unref(child_bs)
> bdrv_delete(child_bs)
> bdrv_close()
> bdrv_drained_begin()
> bdrv_do_drained_begin()
> bdrv_drain_recurse()
> aio_poll()
> ...
> qemu_coroutine_switch()
>
> and the AIO flush completion ends up dereferencing blk->root:
>
> blk_aio_complete()
> scsi_aio_complete()
> blk_get_aio_context(blk)
> bs = blk_bs(blk)
> ie, bs = blk->root ? blk->root->bs : NULL
> ^^^^^
> stale
>
> The solution to this user-after-free situation is is to clear
> blk->root before calling bdrv_unref() in bdrv_detach_child(),
> and let blk_get_aio_context() fall back to the main loop context
> since the BDS has been removed.
>
> Signed-off-by: Greg Kurz <gr...@kaod.org>
> ---
>
> The use-after-free condition is easy to reproduce with a stress-ng
> run in the guest:
>
> -device virtio-scsi-pci,id=scsi1 \
> -drive file=/home/greg/images/scratch.qcow2,format=qcow2,if=none,id=drive1 \
> -device scsi-hd,bus=scsi1.0,drive=drive1,id=scsi-hd1
>
> # stress-ng --hdd 0 --aggressive
>
> and doing drive_del from the QEMU monitor while stress-ng is still running:
>
> (qemu) drive_del drive1
>
> The crash is less easy to hit though, as it depends on the bs field
> of the stale blk->root to have a non-NULL value that eventually breaks
> something when it gets dereferenced. The following patch simulates
> that, and allows to validate the fix:
>
> --- a/block.c
> +++ b/block.c
> @@ -2127,6 +2127,8 @@ BdrvChild *bdrv_attach_child(BlockDriverState
> *parent_bs,
>
> static void bdrv_detach_child(BdrvChild *child)
> {
> + BlockDriverState *bs = child->bs;
> +
> if (child->next.le_prev) {
> QLIST_REMOVE(child, next);
> child->next.le_prev = NULL;
> @@ -2135,7 +2137,15 @@ static void bdrv_detach_child(BdrvChild *child)
> bdrv_replace_child(child, NULL);
>
> g_free(child->name);
> - g_free(child);
> + /* Poison the BdrvChild instead of freeing it, in order to break blk_bs()
> + * if the blk still has a pointer to this BdrvChild in blk->root.
> + */
> + if (atomic_read(&bs->in_flight)) {
> + child->bs = (BlockDriverState *) -1;
> + fprintf(stderr, "\nPoisonned BdrvChild %p\n", child);
> + } else {
> + g_free(child);
> + }
> }
>
> void bdrv_root_unref_child(BdrvChild *child)
> ---
> block/block-backend.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/block/block-backend.c b/block/block-backend.c
> index 681b240b1268..ed9434e236b9 100644
> --- a/block/block-backend.c
> +++ b/block/block-backend.c
> @@ -756,6 +756,7 @@ 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) {
> @@ -768,8 +769,9 @@ void blk_remove_bs(BlockBackend *blk)
>
> blk_update_root_state(blk);
>
> - bdrv_root_unref_child(blk->root);
> + root = blk->root;
> blk->root = NULL;
> + bdrv_root_unref_child(root);
> }
>
> /*
>
>
