Re: [Qemu-devel] [PATCH] block: fix QEMU crash with scsi-hd and drive_del

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, May 16, 2018 at 01:21:54PM +0200, Greg Kurz 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()

I'm surprised you didn't hit another crash later on with this patch
applied.  What happens to this completion after you've set blk->root =
NULL?

> 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.

QEMU should drain I/O requests before making block driver graph changes.
I think the drained region in blk_remove_bs() needs to begin earlier so
that requests are completed before we begin to change things.

Stefan

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