qcow2: Fix data loss on zero write with detect-zeroes=unmap

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 <f.ebner@proxmox.com>
Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Message-ID: <20260522151318.238064-1-t.lamprecht@proxmox.com>
[kwolf: Reverted unnecessary change to 'nr' assignment]
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

3 files changed, 16 insertions, 6 deletions

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 8b1e80bd0b..e02fae6a0c 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 81fd299b4c..19271b10a4 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/block/qcow2.h b/block/qcow2.h
index 192a45d596..ce517040c4 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,