Re: Abort in mch_update_pciexbar

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 5/11/20 8:19 AM, Philippe Mathieu-Daudé wrote:
> On 5/11/20 6:59 AM, Alexander Bulekov wrote:
>> Hello,
>> While fuzzing, I found an input that triggers an assertion failure in
>> mch_update_pciexbar:
>>
>> #6 0x7f38d387c55a in abort 
>> /build/glibc-GwnBeO/glibc-2.30/stdlib/abort.c:79:7
>> #7 0x55c27e94ffd0 in mch_update_pciexbar hw/pci-host/q35.c:331:9
>> #8 0x55c27e94db38 in mch_write_config hw/pci-host/q35.c:487:9
>> #9 0x55c27e9e3f4c in pci_host_config_write_common hw/pci/pci_host.c:81:5
>> #10 0x55c27e9e5307 in pci_data_write hw/pci/pci_host.c:118:5
>> #11 0x55c27e9e6601 in pci_host_data_write hw/pci/pci_host.c:165:9
>> #12 0x55c27ca3b17b in memory_region_write_accessor memory.c:496:5
>> #13 0x55c27ca3a5e4 in access_with_adjusted_size memory.c:557:18
>> #14 0x55c27ca38177 in memory_region_dispatch_write memory.c:1488:16
>> #15 0x55c27c721325 in flatview_write_continue exec.c:3174:23
>> #16 0x55c27c70994d in flatview_write exec.c:3214:14
>> #17 0x55c27c709462 in address_space_write exec.c:3305:18
> 
> These lines don't match QEMU v5.0.0.
> 
>>
>> I can reproduce it in a qemu 5.0 build using:
>> cat << EOF | ~/Development/qemu/build/i386-softmmu/qemu-system-i386 -M 
>> pc-q35-5.0 -display none -nodefaults -nographic -qtest stdio
>> outl 0xcf8 0xf2000060
>> outl 0xcfc 0x8400056e
> 
> The guest shouldn't ask for a reserved bar length (grep for 
> MCH_HOST_BRIDGE_PCIEXBAR_LENGTH_RVD). I suppose we should simply report 
> it as GUEST_ERROR and ignore it.

This patch prevent the crash:

-- >8 --
diff --git a/hw/pci-host/q35.c b/hw/pci-host/q35.c
index 2bbc90b28f..2b744aca93 100644
--- a/hw/pci-host/q35.c
+++ b/hw/pci-host/q35.c
@@ -36,6 +36,7 @@
  #include "qapi/error.h"
  #include "qapi/visitor.h"
  #include "qemu/module.h"
+#include "qemu/log.h"

 
/****************************************************************************
   * Q35 host
@@ -320,6 +321,9 @@ static void mch_update_pciexbar(MCHPCIState *mch)
          addr_mask |= MCH_HOST_BRIDGE_PCIEXBAR_64ADMSK;
          break;
      case MCH_HOST_BRIDGE_PCIEXBAR_LENGTH_RVD:
+        qemu_log_mask(LOG_GUEST_ERROR, "%s: Invalid use of reserved 
value\n",
+                                       __func__);
+        return;
      default:
          abort();
      }
---

But the real question is what would the real hardware do in this case.

> 
>> EOF
>>
>> I also uploaded the above trace, in case the formatting is broken:
>>
>> curl https://paste.debian.net/plain/1146095 | qemu-system-i386 -M 
>> pc-q35-5.0 -display none -nodefaults -nographic -qtest stdio
>>
>> Please let me know if I can provide any further info.
> 
> It would help the community if you fill your bug reports with Launchpad, 
> so they don't get lost in the high email flow, and we can track/update 
> them. See for example:
> https://bugs.launchpad.net/qemu/+bug/1835865 and
> https://lists.gnu.org/archive/html/qemu-devel/2020-03/msg06082.html 
> which refers it.