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

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()

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 groug@kaod.org

<formletter>

This is not the correct way to submit patches for inclusion in the stable kernel tree. Please read: https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html for how to do this properly.

</formletter>