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

[Greg Kurz <groug@xxxxxxxx>]

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@xxxxxxxx>
---

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);
 }
 
 /*