From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists1p.gnu.org (lists1p.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7E940CD6E79 for ; Mon, 8 Jun 2026 16:53:00 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wWdD8-00014n-Jr; Mon, 08 Jun 2026 12:52:26 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wWdD7-00013l-1o for qemu-devel@nongnu.org; Mon, 08 Jun 2026 12:52:25 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wWdD4-0007PR-ME for qemu-devel@nongnu.org; Mon, 08 Jun 2026 12:52:24 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1780937542; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=zvuuVNSZbz51c4hsITO2+CK9lFnoymnQfTg5fzme91o=; b=DUJWNDal4AyXx+PcuXITTLYqZ4TCzZjbext7QhVFqcZeBID+M/fd0GHPfHJJRzBnakoWzO YB69QI4xi3zfOSI7NinTTPOuCbyoYTKteG1UOp9NRAUbnGS3nL4FwKB1LSs9UGn07/YT8t 2cPVAlZqkNGPhAoaUmyGHtliUijt0p4= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-661-oZXfcTY6ORWaX5WsaKx03Q-1; Mon, 08 Jun 2026 12:52:20 -0400 X-MC-Unique: oZXfcTY6ORWaX5WsaKx03Q-1 X-Mimecast-MFC-AGG-ID: oZXfcTY6ORWaX5WsaKx03Q_1780937540 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id DB21219560A1; Mon, 8 Jun 2026 16:52:19 +0000 (UTC) Received: from merkur.fritz.box (unknown [10.44.50.32]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 7E8BC19540CD; Mon, 8 Jun 2026 16:52:18 +0000 (UTC) From: Kevin Wolf To: qemu-block@nongnu.org Cc: kwolf@redhat.com, stefanha@redhat.com, qemu-devel@nongnu.org Subject: [PULL 4/8] qcow2: Fix data loss on zero write with detect-zeroes=unmap Date: Mon, 8 Jun 2026 18:52:03 +0200 Message-ID: <20260608165207.307488-5-kwolf@redhat.com> In-Reply-To: <20260608165207.307488-1-kwolf@redhat.com> References: <20260608165207.307488-1-kwolf@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 Received-SPF: pass client-ip=170.10.129.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 8 X-Spam_score: 0.8 X-Spam_bar: / X-Spam_report: (0.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.445, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org From: Thomas Lamprecht Commit b8bfb1478d ("qcow2: Fix corruption on discard during write with COW") added a wait_for_dependencies() at the start of qcow2_subcluster_zeroize(). That fixes the inconsistency it set out to fix, but turns the lock-protected pre-check in the caller, qcow2_co_pwrite_zeroes(), into a stale one: the wait yields s->lock, so an in-flight allocating write whose QCowL2Meta is already on s->cluster_allocs (but whose L2 entry is not yet linked) gets to link its entry during the yield. When the zeroize wakes, the cluster is now NORMAL, and with BDRV_REQ_MAY_UNMAP the free path in zero_in_l2_slice() unmaps the just-written cluster, silently dropping the data write's payload. This is reachable with detect-zeroes=unmap (the default for VirtIO disks with discard on in Proxmox VE), under which the block layer auto-promotes all-zero buffers to BDRV_REQ_ZERO_WRITE | BDRV_REQ_MAY_UNMAP. A memory-constrained Debian guest running 'apt full-upgrade' on such a disk reproduces it as random SIGSEGVs: swapped-out code pages come back as zero. Wait for in-flight dependencies before the lock-protected check in qcow2_co_pwrite_zeroes(). If a write linked its L2 entry during the wait, the type check now fails and the block layer falls back to a bounce-buffered zero write that only touches the requested subrange, preserving the racing write's data. Promote wait_for_dependencies() to qcow2_wait_for_dependencies() so qcow2.c can call it. Fixes: b8bfb1478d ("qcow2: Fix corruption on discard during write with COW") Cc: qemu-stable@nongnu.org Tested-by: Fiona Ebner Reviewed-by: Fiona Ebner Signed-off-by: Thomas Lamprecht Message-ID: <20260522151318.238064-1-t.lamprecht@proxmox.com> [kwolf: Reverted unnecessary change to 'nr' assignment] Reviewed-by: Kevin Wolf Signed-off-by: Kevin Wolf --- block/qcow2.h | 4 ++++ block/qcow2-cluster.c | 10 +++++----- block/qcow2.c | 8 +++++++- tests/qemu-iotests/046 | 23 +++++++++++++++++++++++ tests/qemu-iotests/046.out | 10 ++++++++++ 5 files changed, 49 insertions(+), 6 deletions(-) diff --git a/block/qcow2.h b/block/qcow2.h index 192a45d596b..ce517040c47 100644 --- a/block/qcow2.h +++ b/block/qcow2.h @@ -966,6 +966,10 @@ int coroutine_fn GRAPH_RDLOCK qcow2_subcluster_zeroize(BlockDriverState *bs, uint64_t offset, uint64_t bytes, int flags); +void coroutine_mixed_fn +qcow2_wait_for_dependencies(BlockDriverState *bs, uint64_t guest_offset, + uint64_t bytes); + int GRAPH_RDLOCK qcow2_expand_zero_clusters(BlockDriverState *bs, BlockDriverAmendStatusCB *status_cb, diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c index 8b1e80bd0b3..e02fae6a0c6 100644 --- a/block/qcow2-cluster.c +++ b/block/qcow2-cluster.c @@ -1474,9 +1474,9 @@ static int coroutine_fn handle_dependencies(BlockDriverState *bs, return 0; } -static void coroutine_mixed_fn wait_for_dependencies(BlockDriverState *bs, - uint64_t guest_offset, - uint64_t bytes) +void coroutine_mixed_fn qcow2_wait_for_dependencies(BlockDriverState *bs, + uint64_t guest_offset, + uint64_t bytes) { BDRVQcow2State *s = bs->opaque; QCowL2Meta *m = NULL; @@ -2035,7 +2035,7 @@ int qcow2_cluster_discard(BlockDriverState *bs, uint64_t offset, * We don't need to allocate a QCowL2Meta for the discard operation because * s->lock is held for the duration of the whole operation. */ - wait_for_dependencies(bs, offset, bytes); + qcow2_wait_for_dependencies(bs, offset, bytes); /* Caller must pass aligned values, except at image end */ assert(QEMU_IS_ALIGNED(offset, s->cluster_size)); @@ -2204,7 +2204,7 @@ int coroutine_fn qcow2_subcluster_zeroize(BlockDriverState *bs, uint64_t offset, * We don't need to allocate a QCowL2Meta for the zeroize operation because * s->lock is held for the duration of the whole operation. */ - wait_for_dependencies(bs, offset, bytes); + qcow2_wait_for_dependencies(bs, offset, bytes); /* If we have to stay in sync with an external data file, zero out * s->data_file first. */ diff --git a/block/qcow2.c b/block/qcow2.c index 81fd299b4c7..19271b10a49 100644 --- a/block/qcow2.c +++ b/block/qcow2.c @@ -4234,10 +4234,16 @@ qcow2_co_pwrite_zeroes(BlockDriverState *bs, int64_t offset, int64_t bytes, } qemu_co_mutex_lock(&s->lock); - /* We can have new write after previous check */ offset -= head; bytes = s->subcluster_size; nr = s->subcluster_size; + /* + * Wait for in-flight allocating writes first: otherwise the type + * check below could pass on UNALLOCATED while a yet-to-link_l2 write + * completes during qcow2_subcluster_zeroize()'s own wait, letting the + * resumed MAY_UNMAP discard the just-written data. + */ + qcow2_wait_for_dependencies(bs, offset, bytes); ret = qcow2_get_host_offset(bs, offset, &nr, &off, &type); if (ret < 0 || (type != QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN && diff --git a/tests/qemu-iotests/046 b/tests/qemu-iotests/046 index e03dd401479..0d84b5c1c75 100755 --- a/tests/qemu-iotests/046 +++ b/tests/qemu-iotests/046 @@ -226,6 +226,26 @@ aio_write -z 0x140000 0x10000 resume A aio_flush EOF + +# Start an allocating write to a previously unallocated cluster and, before +# its L2 update is linked, issue a concurrent sub-cluster zero write with +# MAY_UNMAP that targets a disjoint range within the same cluster. The zero +# write's head/tail are zero (cluster is unallocated), so qcow2_co_pwrite_zeroes +# would expand it to the full subcluster. Without waiting for dependencies +# before the zero write's "unallocated" type check, that check passes, +# qcow2_subcluster_zeroize then yields in wait_for_dependencies, the allocating +# write links its L2 entry, and the resumed zeroize unmaps the cluster - +# silently discarding the just-written data. Waiting first makes the zero write +# fall back to a bounce-buffered real write, which only touches its own +# subrange. +cat <