Commit e6cada92 authored by Greg Kurz's avatar Greg Kurz Committed by Kevin Wolf
Browse files

block: Avoid stale pointer dereference in blk_get_aio_context() 



It is possible for blk_remove_bs() to race with blk_drain_all(), causing
the latter to dereference a stale blk->root pointer:

  blk_remove_bs(blk)
   bdrv_root_unref_child(blk->root)
    child_bs = blk->root->bs
    bdrv_detach_child(blk->root)
     ...
     g_free(blk->root) <============== blk->root becomes stale
    bdrv_unref(child_bs) <============ yield at some point

A blk_drain_all() can be triggered by some guest action in the
meantime, eg. on POWER, SLOF might disable bus mastering on
a virtio-scsi-pci device:

  virtio_write_config()
   virtio_pci_stop_ioeventfd()
    virtio_bus_stop_ioeventfd()
     virtio_scsi_dataplane_stop()
      blk_drain_all()
       blk_get_aio_context()
       bs = blk->root ? blk->root->bs : NULL
            ^^^^^^^^^
              stale

Then, depending on one's luck, QEMU either crashes with SEGV or
hits the assertion in blk_get_aio_context().

blk->root is set by blk_insert_bs() which calls bdrv_root_attach_child()
first. The blk_remove_bs() function should rollback the changes made
by blk_insert_bs() in the opposite order (or it should be documented
somewhere why this isn't the case). Clear blk->root before calling
bdrv_root_unref_child() in blk_remove_bs().

Signed-off-by: default avatarGreg Kurz <groug@kaod.org>
Message-Id: <159430264541.389456.11925072456012783045.stgit@bahia.lan>
Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
parent d9f059aa

block/block-backend.c

+3 −1
Original line number Diff line number Diff line
@@ -808,6 +808,7 @@ 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) {
@@ -825,8 +826,9 @@ void blk_remove_bs(BlockBackend *blk) 
     * to avoid that and a potential QEMU crash.

     */

    blk_drain(blk);

    bdrv_root_unref_child(blk->root);

    root = blk->root;

    blk->root = NULL;

    bdrv_root_unref_child(root);

}



/*