[PULL 2/2] hw/usb/hcd-xhci: Fix unbounded loop in xhci_ring_chain_length() (CVE-2020-14394)

[Thomas Huth]

The loop condition in xhci_ring_chain_length() is under control of
the guest, and additionally the code does not check for failed DMA
transfers (e.g. if reaching the end of the RAM), so the loop there
could run for a very long time or even forever. Fix it by checking
the return value of dma_memory_read() and by introducing a maximum
loop length.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/646
Message-Id: <20220804131300.96368-1-thuth@redhat.com>
Reviewed-by: Mauro Matteo Cascella <mcascell@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 hw/usb/hcd-xhci.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/hw/usb/hcd-xhci.c b/hw/usb/hcd-xhci.c
index 296cc6c8e6..3c48b58dde 100644
--- a/hw/usb/hcd-xhci.c
+++ b/hw/usb/hcd-xhci.c
@@ -21,6 +21,7 @@
 
 #include "qemu/osdep.h"
 #include "qemu/timer.h"
+#include "qemu/log.h"
 #include "qemu/module.h"
 #include "qemu/queue.h"
 #include "migration/vmstate.h"
@@ -725,10 +726,14 @@ static int xhci_ring_chain_length(XHCIState *xhci, const 
XHCIRing *ring)
     bool control_td_set = 0;
     uint32_t link_cnt = 0;
 
-    while (1) {
+    do {
         TRBType type;
-        dma_memory_read(xhci->as, dequeue, &trb, TRB_SIZE,
-                        MEMTXATTRS_UNSPECIFIED);
+        if (dma_memory_read(xhci->as, dequeue, &trb, TRB_SIZE,
+                        MEMTXATTRS_UNSPECIFIED) != MEMTX_OK) {
+            qemu_log_mask(LOG_GUEST_ERROR, "%s: DMA memory access failed!\n",
+                          __func__);
+            return -1;
+        }
         le64_to_cpus(&trb.parameter);
         le32_to_cpus(&trb.status);
         le32_to_cpus(&trb.control);
@@ -762,7 +767,17 @@ static int xhci_ring_chain_length(XHCIState *xhci, const 
XHCIRing *ring)
         if (!control_td_set && !(trb.control & TRB_TR_CH)) {
             return length;
         }
-    }
+
+        /*
+         * According to the xHCI spec, Transfer Ring segments should have
+         * a maximum size of 64 kB (see chapter "6 Data Structures")
+         */
+    } while (length < TRB_LINK_LIMIT * 65536 / TRB_SIZE);
+
+    qemu_log_mask(LOG_GUEST_ERROR, "%s: exceeded maximum tranfer ring size!\n",
+                          __func__);
+
+    return -1;
 }
 
 static void xhci_er_reset(XHCIState *xhci, int v)
-- 
2.31.1