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 ---

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); }

/*