From 95970127dcd1d5d2f365f87fa888e5f0baa3cd10 Mon Sep 17 00:00:00 2001
From: Thomas Huth <thuth@redhat.com>
Date: Thu, 4 Aug 2022 15:13:00 +0200
Subject: [PATCH] hw/usb/hcd-xhci: Fix unbounded loop in
 xhci_ring_chain_length() (CVE-2020-14394)

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 47fb79aa4d..ac02548dcf 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"
@@ -726,10 +727,14 @@ 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);
@@ -763,7 +768,17 @@ 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)
-- 
2.27.0