[PATCH v2 3/3] memory: fix bh-initiated dma-reentracy issues

[Alexander Bulekov]

This patch leverages the DeviceState engaged_in_io flag to prevent
issues due to accesses similar to bh -> dma -> mmio.
e.g.  CVE-2021-3929

Signed-off-by: Alexander Bulekov <alx...@bu.edu>
---
 include/hw/pci/pci.h  | 13 +++++++++++--
 softmmu/dma-helpers.c | 12 ++++++++++++
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/include/hw/pci/pci.h b/include/hw/pci/pci.h
index 44dacfa224..ab1ad0f7a8 100644
--- a/include/hw/pci/pci.h
+++ b/include/hw/pci/pci.h
@@ -834,8 +834,17 @@ static inline MemTxResult pci_dma_rw(PCIDevice *dev, 
dma_addr_t addr,
                                      void *buf, dma_addr_t len,
                                      DMADirection dir, MemTxAttrs attrs)
 {
-    return dma_memory_rw(pci_get_address_space(dev), addr, buf, len,
-                         dir, attrs);
+    bool prior_engaged_state;
+    MemTxResult result;
+
+    prior_engaged_state = dev->qdev.engaged_in_io;
+
+    dev->qdev.engaged_in_io = true;
+    result = dma_memory_rw(pci_get_address_space(dev), addr, buf, len,
+                           dir, attrs);
+    dev->qdev.engaged_in_io = prior_engaged_state;
+
+    return result;
 }
 
 /**
diff --git a/softmmu/dma-helpers.c b/softmmu/dma-helpers.c
index 7820fec54c..7a4f1fb9b3 100644
--- a/softmmu/dma-helpers.c
+++ b/softmmu/dma-helpers.c
@@ -288,8 +288,16 @@ static MemTxResult dma_buf_rw(void *buf, dma_addr_t len, 
dma_addr_t *residual,
     uint8_t *ptr = buf;
     dma_addr_t xresidual;
     int sg_cur_index;
+    DeviceState *dev;
+    bool prior_engaged_state;
     MemTxResult res = MEMTX_OK;
 
+    dev = sg->dev;
+    if (dev) {
+        prior_engaged_state = dev->engaged_in_io;
+        dev->engaged_in_io = true;
+    }
+
     xresidual = sg->size;
     sg_cur_index = 0;
     len = MIN(len, xresidual);
@@ -302,6 +310,10 @@ static MemTxResult dma_buf_rw(void *buf, dma_addr_t len, 
dma_addr_t *residual,
         xresidual -= xfer;
     }
 
+    if (dev) {
+        dev->engaged_in_io = prior_engaged_state;
+    }
+
     if (residual) {
         *residual = xresidual;
     }
-- 
2.33.0