[PATCH] hw/usb/hcd-xhci: Set reentrancy guard in timer functions (CVE-2026-17588)

[Thomas Huth <thuth@redhat.com>]

The xHCI controller processes USB events from timer callbacks
(xhci_mfwrap_timer and xhci_ep_kick_timer) outside of any MMIO handler
context. The device's mem_reentrancy_guard is therefore not engaged
during this processing.

A malicious guest can exploit this by pointing the event ring base
address (er_start) at the xHCI doorbell MMIO region. When
xhci_write_event() performs a DMA write to deliver a transfer
completion event, the write lands on doorbell register 0. If the
written value is 0, this triggers xhci_process_commands() reentrantly.
A CR_DISABLE_SLOT command prepared on the command ring then frees
endpoint and transfer objects via xhci_disable_ep() / g_free() while
the outer call stack still holds references to them, causing a
heap use-after-free.

Fix this by setting engaged_in_io on the device's mem_reentrancy_guard
around the processing in these non-MMIO entry points. This mirrors
the protection that EHCI, DWC2, and UHCI already have via
qemu_bh_new_guarded(). With the guard active, the memory subsystem's
automatic reentrancy check (system/memory.c) blocks DMA writes that
would dispatch into the device's own MMIO handlers.

CVE: CVE-2026-17588
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3926
Reported-by: XlabAI Team of Tencent Xuanwu Lab <xlabai@tencent.com>,
    Guannan Wang <wgnbuaa@gmail.com>, Zhanpeng Liu <pkugenuine@gmail.com>,
    Jiashuo Liang <761232680@qq.com>, Guancheng Li <lgcpku@gmail.com>
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/4195
Reported By: Ken Hsu and Royce Lu of Palo Alto Networks
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 hw/usb/hcd-xhci.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/hw/usb/hcd-xhci.c b/hw/usb/hcd-xhci.c
index d342aa2739e..12f8ffece0f 100644
--- a/hw/usb/hcd-xhci.c
+++ b/hw/usb/hcd-xhci.c
@@ -456,9 +456,15 @@ static void xhci_mfwrap_timer(void *opaque)
 {
     XHCIState *xhci = opaque;
     XHCIEvent wrap = { ER_MFINDEX_WRAP, CC_SUCCESS };
+    MemReentrancyGuard *guard = &xhci->parent.mem_reentrancy_guard;
+
+    assert(!guard->engaged_in_io);
+    guard->engaged_in_io = true;
 
     xhci_event(xhci, &wrap, 0);
     xhci_mfwrap_update(xhci);
+
+    guard->engaged_in_io = false;
 }
 
 static void xhci_die(XHCIState *xhci)
@@ -1086,7 +1092,14 @@ static void xhci_set_ep_state(XHCIState *xhci, XHCIEPContext *epctx,
 static void xhci_ep_kick_timer(void *opaque)
 {
     XHCIEPContext *epctx = opaque;
+    MemReentrancyGuard *guard = &epctx->xhci->parent.mem_reentrancy_guard;
+
+    assert(!guard->engaged_in_io);
+    guard->engaged_in_io = true;
+
     xhci_kick_epctx(epctx, 0);
+
+    guard->engaged_in_io = false;
 }
 
 static XHCIEPContext *xhci_alloc_epctx(XHCIState *xhci,
-- 
2.55.0