Re: Abort in mch_update_pciexbar

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, May 11, 2020 at 09:10:48AM +0200, Philippe Mathieu-Daudé wrote:
> 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();
>      }

Maybe report the value too.

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

Spec doesn't say unfortunately. All it says is:
	Designers must not rely on the absence or characteristics of any
	features or instructions marked "reserved" or "undefined." Intel
	reserves these for

	future definition and shall have no responsibility whatsoever
	for conflicts or incompatibilities arising from future changes to them.



> > 
> > > 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.