[RFC PATCH v4 15/25] vfio/pci: Check command enable changes before updating device state

["Aneesh Kumar K.V (Arm)" <aneesh.kumar@kernel.org>]

Add checks in vfio_basic_config_write() for changes to Bus Master Enable
and Memory Enable. Move the existing acquisition of vdev->memory_lock
before reading PCI_COMMAND so the read, checks and updates all occur
while the lock is held for write.

Read Bus Master Enable from hardware and Memory Enable from vconfig,
which also tracks the setting for VFs. If vfio_pci_bar_state_change_allowed()
returns false, reject changes to either bit with -EBUSY before updating
hardware or vconfig. Allow writes that leave both bits unchanged.

Memory Enable controls BAR decoding, while Bus Master Enable controls
device-initiated DMA. Later patches reject changes to both while the
device supplies guest_memfd backing. The check currently returns true,
preserving existing behavior.

Cc: Alex Williamson <alex@shazbot.org>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: David Hildenbrand <david@kernel.org>
Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
---
 drivers/vfio/pci/vfio_pci_config.c | 64 +++++++++++++++++++++---------
 1 file changed, 46 insertions(+), 18 deletions(-)

diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
index 9914f3ac69ae..5b97f64ab3ba 100644
--- a/drivers/vfio/pci/vfio_pci_config.c
+++ b/drivers/vfio/pci/vfio_pci_config.c
@@ -568,19 +568,49 @@ static int vfio_basic_config_write(struct vfio_pci_core_device *vdev, int pos,
 	struct pci_dev *pdev = vdev->pdev;
 	__le16 *virt_cmd;
 	u16 new_cmd = 0;
+	bool zap_bars_required = false;
 	int ret;
 
 	virt_cmd = (__le16 *)&vdev->vconfig[PCI_COMMAND];
 
 	if (offset == PCI_COMMAND) {
 		bool phys_mem, virt_mem, new_mem, phys_io, virt_io, new_io;
-		u16 phys_cmd;
+		bool enable_bits_changed;
+		u16 phys_cmd, old_cmd;
+
+		new_cmd = le32_to_cpu(val);
+		new_mem = !!(new_cmd & PCI_COMMAND_MEMORY);
+		zap_bars_required =
+			vfio_pci_bar_state_change_allowed(vdev) && !new_mem;
+
+		if (zap_bars_required) {
+			ret = vfio_pci_begin_bar_state_change(vdev);
+			if (ret)
+				return ret;
+			vfio_pci_dma_buf_move(vdev, true);
+		} else {
+			down_write(&vdev->memory_lock);
+		}
 
 		ret = pci_user_read_config_word(pdev, PCI_COMMAND, &phys_cmd);
-		if (ret)
-			return ret;
+		if (ret) {
+			count = ret;
+			goto out_unlock;
+		}
 
-		new_cmd = le32_to_cpu(val);
+		/*
+		 * Check BME and Memory Enable changes while holding memory_lock
+		 * for write, before updating hardware or vconfig. Return an
+		 * error rather than silently masking a rejected write.
+		 */
+		old_cmd = (phys_cmd & ~PCI_COMMAND_MEMORY) |
+			  (le16_to_cpu(*virt_cmd) & PCI_COMMAND_MEMORY);
+		enable_bits_changed = (old_cmd ^ new_cmd) &
+				      (PCI_COMMAND_MASTER | PCI_COMMAND_MEMORY);
+		if (enable_bits_changed && !vfio_pci_bar_state_change_allowed(vdev)) {
+			count = -EBUSY;
+			goto out_unlock;
+		}
 
 		phys_io = !!(phys_cmd & PCI_COMMAND_IO);
 		virt_io = !!(le16_to_cpu(*virt_cmd) & PCI_COMMAND_IO);
@@ -588,14 +618,6 @@ static int vfio_basic_config_write(struct vfio_pci_core_device *vdev, int pos,
 
 		phys_mem = !!(phys_cmd & PCI_COMMAND_MEMORY);
 		virt_mem = !!(le16_to_cpu(*virt_cmd) & PCI_COMMAND_MEMORY);
-		new_mem = !!(new_cmd & PCI_COMMAND_MEMORY);
-
-		if (!new_mem) {
-			vfio_pci_zap_and_down_write_memory_lock(vdev);
-			vfio_pci_dma_buf_move(vdev, true);
-		} else {
-			down_write(&vdev->memory_lock);
-		}
 
 		/*
 		 * If the user is writing mem/io enable (new_mem/io) and we
@@ -614,11 +636,8 @@ static int vfio_basic_config_write(struct vfio_pci_core_device *vdev, int pos,
 	}
 
 	count = vfio_default_config_write(vdev, pos, count, perm, offset, val);
-	if (count < 0) {
-		if (offset == PCI_COMMAND)
-			up_write(&vdev->memory_lock);
-		return count;
-	}
+	if (count < 0)
+		goto out_unlock;
 
 	/*
 	 * Save current memory/io enable bits in vconfig to allow for
@@ -632,9 +651,18 @@ static int vfio_basic_config_write(struct vfio_pci_core_device *vdev, int pos,
 
 		if (__vfio_pci_memory_enabled(vdev))
 			vfio_pci_dma_buf_move(vdev, false);
-		up_write(&vdev->memory_lock);
 	}
 
+out_unlock:
+	if (offset == PCI_COMMAND) {
+		if (zap_bars_required)
+			vfio_pci_end_bar_state_change(vdev);
+		else
+			up_write(&vdev->memory_lock);
+	}
+	if (count < 0)
+		return count;
+
 	/* Emulate INTx disable */
 	if (offset >= PCI_COMMAND && offset <= PCI_COMMAND + 1) {
 		bool virt_intx_disable;
-- 
2.43.0