From: Thomas Huth <thuth@redhat.com>

Some machines like the microvm machine instantiate a "sysbus-xhci"
device with just 1 interrupt (by setting the "intrs" property to 1).
xhci_sysbus_realize() then only allocates the s->irq array with one
entry.

When the guest writes to the ERDP register of a corresponding XHCI
"interrupter", the generic XHCI code calls the xhci_sysbus_intr_raise()
function with n > 1, and this function then calls qemu_set_irq() with
s->irq[n] pointing to a bad heap address. The qemu_set_irq() then tries
to call an IRQ handler via a function pointer in that heap space. This
either causes QEMU to die with a segmentation fault (if it's a bad
address), or even worse runs some unexpected code if the destination
of the pointer is executable code.

Looking at the xHCI spec, it is up to the implementation of the host
controller how many interrupters are available. So if we only support
one or some few interrupters, the registers of the other interrupters
should not do anything, i.e. reads should result in zeros and writes
should be completely ignored. (big thanks to Peter Maydell for helping
with the analyzation of the correct way to fix this here)

This way, the xhci_sysbus_intr_raise() function cannot be called with
an invalid interrupt number anymore. But for good measure, also add an
assert() statement to the xhci_sysbus_intr_raise() function to prevent
that similar problems with calling arbitrary function pointers on the
heap could occur again.

Fixes: CVE-2026-16043
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/4001
Reported-by: Tristan Madani <tristan@talencesecurity.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 v3: Use correct word "nonexistent" and don't skip trace in read function

 hw/usb/hcd-xhci-sysbus.c |  1 +
 hw/usb/hcd-xhci.c        | 13 +++++++++++++
 2 files changed, 14 insertions(+)

diff --git a/hw/usb/hcd-xhci-sysbus.c b/hw/usb/hcd-xhci-sysbus.c
index 19664c5985e..bbdd5fd64ab 100644
--- a/hw/usb/hcd-xhci-sysbus.c
+++ b/hw/usb/hcd-xhci-sysbus.c
@@ -20,6 +20,7 @@ static bool xhci_sysbus_intr_raise(XHCIState *xhci, int n, bool level)
 {
     XHCISysbusState *s = container_of(xhci, XHCISysbusState, xhci);
 
+    assert(n < xhci->numintrs);
     qemu_set_irq(s->irq[n], level);
 
     return false;
diff --git a/hw/usb/hcd-xhci.c b/hw/usb/hcd-xhci.c
index ae8add4227e..2e02457d0ae 100644
--- a/hw/usb/hcd-xhci.c
+++ b/hw/usb/hcd-xhci.c
@@ -3044,6 +3044,12 @@ static uint64_t xhci_runtime_read(void *ptr, hwaddr reg,
         }
     } else {
         int v = (reg - 0x20) / 0x20;
+
+        if (v >= xhci->numintrs) {
+            qemu_log_mask(LOG_GUEST_ERROR,
+                          "xhci: read from nonexistent interrupter %i\n", v);
+            goto out_trace;
+        }
         XHCIInterrupter *intr = &xhci->intr[v];
         switch (reg & 0x1f) {
         case 0x00: /* IMAN */
@@ -3070,6 +3076,7 @@ static uint64_t xhci_runtime_read(void *ptr, hwaddr reg,
         }
     }
 
+out_trace:
     trace_usb_xhci_runtime_read(reg, ret);
     return ret;
 }
@@ -3087,7 +3094,13 @@ static void xhci_runtime_write(void *ptr, hwaddr reg,
         trace_usb_xhci_unimplemented("runtime write", reg);
         return;
     }
+
     v = (reg - 0x20) / 0x20;
+    if (v >= xhci->numintrs) {
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "xhci: write to nonexistent interrupter %i\n", v);
+        return;
+    }
     intr = &xhci->intr[v];
 
     switch (reg & 0x1f) {
-- 
2.55.0

On Sun, 19 Jul 2026 at 07:15, Thomas Huth <thuth@redhat.com> wrote:
>
> From: Thomas Huth <thuth@redhat.com>
>
> Some machines like the microvm machine instantiate a "sysbus-xhci"
> device with just 1 interrupt (by setting the "intrs" property to 1).
> xhci_sysbus_realize() then only allocates the s->irq array with one
> entry.
>
> When the guest writes to the ERDP register of a corresponding XHCI
> "interrupter", the generic XHCI code calls the xhci_sysbus_intr_raise()
> function with n > 1, and this function then calls qemu_set_irq() with
> s->irq[n] pointing to a bad heap address. The qemu_set_irq() then tries
> to call an IRQ handler via a function pointer in that heap space. This
> either causes QEMU to die with a segmentation fault (if it's a bad
> address), or even worse runs some unexpected code if the destination
> of the pointer is executable code.
>
> Looking at the xHCI spec, it is up to the implementation of the host
> controller how many interrupters are available. So if we only support
> one or some few interrupters, the registers of the other interrupters
> should not do anything, i.e. reads should result in zeros and writes
> should be completely ignored. (big thanks to Peter Maydell for helping
> with the analyzation of the correct way to fix this here)
>
> This way, the xhci_sysbus_intr_raise() function cannot be called with
> an invalid interrupt number anymore. But for good measure, also add an
> assert() statement to the xhci_sysbus_intr_raise() function to prevent
> that similar problems with calling arbitrary function pointers on the
> heap could occur again.
>
> Fixes: CVE-2026-16043
> Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/4001
> Reported-by: Tristan Madani <tristan@talencesecurity.com>
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  v3: Use correct word "nonexistent" and don't skip trace in read function

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM