[RFC PATCH-for-5.1] hw/isa/lpc_ich9: Ignore reserved/invalid SCI IRQ

[Philippe Mathieu-Daudé]

libFuzzer triggered the following assertion:

  cat << EOF | qemu-system-i386 -M pc-q35-5.0 \
    -nographic -monitor none -serial none \
    -qtest stdio -d guest_errors -trace pci\*
  outl 0xcf8 0x8400f841
  outl 0xcfc 0xebed205d
  outl 0x5d02 0xedf82049
  EOF
  pci_cfg_write ICH9-LPC 31:0 @0x41 <- 0xebed205d
  hw/pci/pci.c:268: int pci_bus_get_irq_level(PCIBus *, int): Assertion 
`irq_num < bus->nirq' failed.

This is because ich9_lpc_sci_irq() returns -1 for reserved
(illegal) values, but ich9_lpc_pmbase_sci_update() considers
it valid and store it in a 8-bit unsigned type. Then the 255
value is used as GSI IRQ, resulting in a PIRQ value of 247,
more than ICH9_LPC_NB_PIRQS (8).

Fix by simply ignoring the invalid access (and reporting it):

  pci_cfg_write ICH9-LPC 31:0 @0x41 <- 0xebed205d
  ICH9 LPC: SCI IRQ SEL #3 is reserved
  pci_cfg_read mch 00:0 @0x0 -> 0x8086
  pci_cfg_read mch 00:0 @0x0 -> 0x29c08086
  ...

Cc: qemu-stable@nongnu.org
Reported-by: Alexander Bulekov <alxndr@bu.edu>
Fixes: 8f242cb724 ("ich9: implement SCI_IRQ_SEL register")
BugLink: https://bugs.launchpad.net/qemu/+bug/1878642
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
RFC because I have no idea how to report the error. The LPC is on
Device 31 Function 0. Should it trigger some error to the PCI bridge?
I can't find that info in the ICH9 datasheet.
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
 include/hw/i386/ich9.h |  1 +
 hw/isa/lpc_ich9.c      | 14 +++++++++++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/hw/i386/ich9.h b/include/hw/i386/ich9.h
index a98d10b252..60483b16cd 100644
--- a/include/hw/i386/ich9.h
+++ b/include/hw/i386/ich9.h
@@ -144,6 +144,7 @@ typedef struct ICH9LPCState {
 #define ICH9_LPC_PMBASE_BASE_ADDRESS_MASK       Q35_MASK(32, 15, 7)
 #define ICH9_LPC_PMBASE_RTE                     0x1
 #define ICH9_LPC_PMBASE_DEFAULT                 0x1
+
 #define ICH9_LPC_ACPI_CTRL                      0x44
 #define ICH9_LPC_ACPI_CTRL_ACPI_EN              0x80
 #define ICH9_LPC_ACPI_CTRL_SCI_IRQ_SEL_MASK     Q35_MASK(8, 2, 0)
diff --git a/hw/isa/lpc_ich9.c b/hw/isa/lpc_ich9.c
index cd6e169d47..5ed28a726c 100644
--- a/hw/isa/lpc_ich9.c
+++ b/hw/isa/lpc_ich9.c
@@ -29,6 +29,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/log.h"
 #include "cpu.h"
 #include "qapi/visitor.h"
 #include "qemu/range.h"
@@ -312,10 +313,12 @@ void ich9_generate_smi(void)
     cpu_interrupt(first_cpu, CPU_INTERRUPT_SMI);
 }
 
+/* Returns -1 on error, IRQ number on success */
 static int ich9_lpc_sci_irq(ICH9LPCState *lpc)
 {
-    switch (lpc->d.config[ICH9_LPC_ACPI_CTRL] &
-            ICH9_LPC_ACPI_CTRL_SCI_IRQ_SEL_MASK) {
+    uint8_t sel = lpc->d.config[ICH9_LPC_ACPI_CTRL] &
+                  ICH9_LPC_ACPI_CTRL_SCI_IRQ_SEL_MASK;
+    switch (sel) {
     case ICH9_LPC_ACPI_CTRL_9:
         return 9;
     case ICH9_LPC_ACPI_CTRL_10:
@@ -328,6 +331,8 @@ static int ich9_lpc_sci_irq(ICH9LPCState *lpc)
         return 21;
     default:
         /* reserved */
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "ICH9 LPC: SCI IRQ SEL #%u is reserved\n", sel);
         break;
     }
     return -1;
@@ -450,7 +455,7 @@ ich9_lpc_pmbase_sci_update(ICH9LPCState *lpc)
 {
     uint32_t pm_io_base = pci_get_long(lpc->d.config + ICH9_LPC_PMBASE);
     uint8_t acpi_cntl = pci_get_long(lpc->d.config + ICH9_LPC_ACPI_CTRL);
-    uint8_t new_gsi;
+    int new_gsi;
 
     if (acpi_cntl & ICH9_LPC_ACPI_CTRL_ACPI_EN) {
         pm_io_base &= ICH9_LPC_PMBASE_BASE_ADDRESS_MASK;
@@ -461,6 +466,9 @@ ich9_lpc_pmbase_sci_update(ICH9LPCState *lpc)
     ich9_pm_iospace_update(&lpc->pm, pm_io_base);
 
     new_gsi = ich9_lpc_sci_irq(lpc);
+    if (new_gsi == -1) {
+        return;
+    }
     if (lpc->sci_level && new_gsi != lpc->sci_gsi) {
         qemu_set_irq(lpc->pm.irq, 0);
         lpc->sci_gsi = new_gsi;
-- 
2.21.3