Re: Abort in mch_update_pciexbar

[Philippe Mathieu-Daudé]

On 5/11/20 9:39 AM, Michael S. Tsirkin wrote:
> 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.

This is a 2 bit field, all are covered except 0b11 which is the RESERVED 
case, and the abort() now will never fire. It is unclear why the 
compiler doesn't notice that.

> > ---
> >
> > 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.

OK, thanks for checking. I'll submit formal patch with proper description.

> > > > 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.