OvmfPkg: Use heap memory for virtio-blk request

The storage space for virtio-blk request header being shared
with the host was from the stack as the request structure was
a local function variable.

A bug in the VMM can corrupt the stack space, and such issues
can be very hard to debug.

Note: This is only an issue with a normal guest VM (non-CCA).
A CCA guest VM would perform bounce buffering for sharing the
data and therefore not have this issue.

Instead of using the stack for sharing the data with the host,
memory can be allocated from the heap pool. However, pool
allocations are not any safer in terms of pages being shared
between different allocations, and so mapping a pool allocation
for DMA may expose it to potential corruption by the VMM in
exactly the same way. The only difference is the potential
impact on program behaviour, which is much higher with the
stack.

Additionally, for guest-side corruption heap allocations can
take advantage by turning on heap guard to help find the bug.

Therefore, minor improvement can be achieved by allocating
memory for the virtio-blk request header from the heap for
sharing with the host.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>

1 files changed, 20 insertions, 10 deletions

diff --git a/OvmfPkg/VirtioBlkDxe/VirtioBlk.c b/OvmfPkg/VirtioBlkDxe/VirtioBlk.c
index 74ed52f9da..f881cde4c5 100644
--- a/OvmfPkg/VirtioBlkDxe/VirtioBlk.c
+++ b/OvmfPkg/VirtioBlkDxe/VirtioBlk.c
@@ -13,6 +13,7 @@
   Copyright (C) 2012, Red Hat, Inc.

   Copyright (c) 2012 - 2018, Intel Corporation. All rights reserved.<BR>

   Copyright (c) 2017, AMD Inc, All rights reserved.<BR>

+  Copyright (c) 2024, Arm Limited. All rights reserved.<BR>

 

   SPDX-License-Identifier: BSD-2-Clause-Patent

 

@@ -240,7 +241,7 @@ SynchronousRequest (
   )

 {

   UINT32                   BlockSize;

-  volatile VIRTIO_BLK_REQ  Request;

+  volatile VIRTIO_BLK_REQ  *Request;

   volatile UINT8           *HostStatus;

   VOID                     *HostStatusBuffer;

   DESC_INDICES             Indices;

@@ -273,15 +274,20 @@ SynchronousRequest (
   //

   ASSERT (BufferSize % BlockSize == 0);

 

+  Request = AllocateZeroPool (sizeof (*Request));

+  if (Request == NULL) {

+    return EFI_DEVICE_ERROR;

+  }

+

   //

   // Prepare virtio-blk request header, setting zero size for flush.

   // IO Priority is homogeneously 0.

   //

-  Request.Type = RequestIsWrite ?

-                 (BufferSize == 0 ? VIRTIO_BLK_T_FLUSH : VIRTIO_BLK_T_OUT) :

-                 VIRTIO_BLK_T_IN;

-  Request.IoPrio = 0;

-  Request.Sector = MultU64x32 (Lba, BlockSize / 512);

+  Request->Type = RequestIsWrite ?

+                  (BufferSize == 0 ? VIRTIO_BLK_T_FLUSH : VIRTIO_BLK_T_OUT) :

+                  VIRTIO_BLK_T_IN;

+  Request->IoPrio = 0;

+  Request->Sector = MultU64x32 (Lba, BlockSize / 512);

 

   //

   // Host status is bi-directional (we preset with a value and expect the

@@ -294,7 +300,8 @@ SynchronousRequest (
                           &HostStatusBuffer

                           );

   if (EFI_ERROR (Status)) {

-    return EFI_DEVICE_ERROR;

+    Status = EFI_DEVICE_ERROR;

+    goto FreeBlkRequest;

   }

 

   HostStatus = HostStatusBuffer;

@@ -306,8 +313,8 @@ SynchronousRequest (
   Status = VirtioMapAllBytesInSharedBuffer (

              Dev->VirtIo,

              VirtioOperationBusMasterRead,

-             (VOID *)&Request,

-             sizeof Request,

+             (VOID *)Request,

+             sizeof (*Request),

              &RequestDeviceAddress,

              &RequestMapping

              );

@@ -372,7 +379,7 @@ SynchronousRequest (
   VirtioAppendDesc (

     &Dev->Ring,

     RequestDeviceAddress,

-    sizeof Request,

+    sizeof (*Request),

     VRING_DESC_F_NEXT,

     &Indices

     );

@@ -454,6 +461,9 @@ FreeHostStatusBuffer:
                  HostStatusBuffer

                  );

 

+FreeBlkRequest:

+  FreePool ((VOID *)Request);

+

   return Status;

 }