Re: Abort in mch_update_pciexbar

[Alexander Bulekov]

On 200511 0910, 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.

Right... I'll make sure the stack-trace comes from the same version in
the future.

> > > 
> > > 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.

OK, I'll use that from now on.
Thanks
-Alex