Re: [BUG qemu 4.0] segfault when unplugging virtio-blk-pci device

[Julia Suvorova <jusual@redhat.com>]

On Tue, Jan 7, 2020 at 2:06 PM Eryu Guan <eguan@linux.alibaba.com> wrote:
>
> On Thu, Jan 02, 2020 at 10:08:50AM +0800, Eryu Guan wrote:
> > On Tue, Dec 31, 2019 at 11:51:35AM +0100, Igor Mammedov wrote:
> > > On Tue, 31 Dec 2019 18:34:34 +0800
> > > Eryu Guan <eguan@linux.alibaba.com> wrote:
> > >
> > > > Hi,
> > > >
> > > > I'm using qemu 4.0 and hit segfault when tearing down kata sandbox, I
> > > > think it's because io completion hits use-after-free when device is
> > > > already gone. Is this a known bug that has been fixed? (I went through
> > > > the git log but didn't find anything obvious).
> > > >
> > > > gdb backtrace is:
> > > >
> > > > Core was generated by `/usr/local/libexec/qemu-kvm -name sandbox-5b8df8c6c6901c3c0a9b02879be10fe8d69d6'.
> > > > Program terminated with signal 11, Segmentation fault.
> > > > #0 object_get_class (obj=obj@entry=0x0) at /usr/src/debug/qemu-4.0/qom/object.c:903
> > > > 903        return obj->class;
> > > > (gdb) bt
> > > > #0  object_get_class (obj=obj@entry=0x0) at /usr/src/debug/qemu-4.0/qom/object.c:903
> > > > #1  0x0000558a2c009e9b in virtio_notify_vector (vdev=0x558a2e7751d0,
> > > >     vector=<optimized out>) at /usr/src/debug/qemu-4.0/hw/virtio/virtio.c:1118
> > > > #2  0x0000558a2bfdcb1e in virtio_blk_discard_write_zeroes_complete (
> > > >     opaque=0x558a2f2fd420, ret=0)
> > > >     at /usr/src/debug/qemu-4.0/hw/block/virtio-blk.c:186
> > > > #3  0x0000558a2c261c7e in blk_aio_complete (acb=0x558a2eed7420)
> > > >     at /usr/src/debug/qemu-4.0/block/block-backend.c:1305
> > > > #4  0x0000558a2c3031db in coroutine_trampoline (i0=<optimized out>,
> > > >     i1=<optimized out>) at /usr/src/debug/qemu-4.0/util/coroutine-ucontext.c:116
> > > > #5  0x00007f45b2f8b080 in ?? () from /lib64/libc.so.6
> > > > #6  0x00007fff9ed75780 in ?? ()
> > > > #7  0x0000000000000000 in ?? ()
> > > >
> > > > It seems like qemu was completing a discard/write_zero request, but
> > > > parent BusState was already freed & set to NULL.
> > > >
> > > > Do we need to drain all pending request before unrealizing virtio-blk
> > > > device? Like the following patch proposed?
> > > >
> > > > https://lists.gnu.org/archive/html/qemu-devel/2017-06/msg02945.html
> > > >
> > > > If more info is needed, please let me know.
> > >
> > > may be this will help: https://patchwork.kernel.org/patch/11213047/
> >
> > Yeah, this looks promising! I'll try it out (though it's a one-time
> > crash for me). Thanks!
>
> After applying this patch, I don't see the original segfaut and
> backtrace, but I see this crash
>
> [Thread debugging using libthread_db enabled]
> Using host libthread_db library "/lib64/libthread_db.so.1".
> Core was generated by `/usr/local/libexec/qemu-kvm -name sandbox-a2f34a11a7e1449496503bbc4050ae040c0d3'.
> Program terminated with signal 11, Segmentation fault.
> #0  0x0000561216a57609 in virtio_pci_notify_write (opaque=0x5612184747e0, addr=0, val=<optimized out>, size=<optimized out>) at /usr/src/debug/qemu-4.0/hw/virtio/virtio-pci.c:1324
> 1324        VirtIOPCIProxy *proxy = VIRTIO_PCI(DEVICE(vdev)->parent_bus->parent);
> Missing separate debuginfos, use: debuginfo-install glib2-2.42.2-5.1.alios7.x86_64 glibc-2.17-260.alios7.x86_64 libgcc-4.8.5-28.alios7.1.x86_64 libseccomp-2.3.1-3.alios7.x86_64 libstdc++-4.8.5-28.alios7.1.x86_64 numactl-libs-2.0.9-5.1.alios7.x86_64 pixman-0.32.6-3.1.alios7.x86_64 zlib-1.2.7-16.2.alios7.x86_64
> (gdb) bt
> #0  0x0000561216a57609 in virtio_pci_notify_write (opaque=0x5612184747e0, addr=0, val=<optimized out>, size=<optimized out>) at /usr/src/debug/qemu-4.0/hw/virtio/virtio-pci.c:1324
> #1  0x0000561216835b22 in memory_region_write_accessor (mr=<optimized out>, addr=<optimized out>, value=<optimized out>, size=<optimized out>, shift=<optimized out>, mask=<optimized out>, attrs=...) at /usr/src/debug/qemu-4.0/memory.c:502
> #2  0x0000561216833c5d in access_with_adjusted_size (addr=addr@entry=0, value=value@entry=0x7fcdeab1b8a8, size=size@entry=2, access_size_min=<optimized out>, access_size_max=<optimized out>, access_fn=0x561216835ac0 <memory_region_write_accessor>, mr=0x56121846d340, attrs=...)
>     at /usr/src/debug/qemu-4.0/memory.c:568
> #3  0x0000561216837c66 in memory_region_dispatch_write (mr=mr@entry=0x56121846d340, addr=0, data=<optimized out>, size=2, attrs=attrs@entry=...) at /usr/src/debug/qemu-4.0/memory.c:1503
> #4  0x00005612167e036f in flatview_write_continue (fv=fv@entry=0x56121852edd0, addr=addr@entry=841813602304, attrs=..., buf=buf@entry=0x7fce7dd97028 <Address 0x7fce7dd97028 out of bounds>, len=len@entry=2, addr1=<optimized out>, l=<optimized out>, mr=0x56121846d340)
>     at /usr/src/debug/qemu-4.0/exec.c:3279
> #5  0x00005612167e0506 in flatview_write (fv=0x56121852edd0, addr=841813602304, attrs=..., buf=0x7fce7dd97028 <Address 0x7fce7dd97028 out of bounds>, len=2) at /usr/src/debug/qemu-4.0/exec.c:3318
> #6  0x00005612167e4a1b in address_space_write (as=<optimized out>, addr=<optimized out>, attrs=..., buf=<optimized out>, len=<optimized out>) at /usr/src/debug/qemu-4.0/exec.c:3408
> #7  0x00005612167e4aa5 in address_space_rw (as=<optimized out>, addr=<optimized out>, attrs=..., attrs@entry=..., buf=buf@entry=0x7fce7dd97028 <Address 0x7fce7dd97028 out of bounds>, len=<optimized out>, is_write=<optimized out>) at /usr/src/debug/qemu-4.0/exec.c:3419
> #8  0x0000561216849da1 in kvm_cpu_exec (cpu=cpu@entry=0x56121849aa00) at /usr/src/debug/qemu-4.0/accel/kvm/kvm-all.c:2034
> #9  0x000056121682255e in qemu_kvm_cpu_thread_fn (arg=arg@entry=0x56121849aa00) at /usr/src/debug/qemu-4.0/cpus.c:1281
> #10 0x0000561216b794d6 in qemu_thread_start (args=<optimized out>) at /usr/src/debug/qemu-4.0/util/qemu-thread-posix.c:502
> #11 0x00007fce7bef6e25 in start_thread () from /lib64/libpthread.so.0
> #12 0x00007fce7bc1ef1d in clone () from /lib64/libc.so.6
>
> And I searched and found
> https://bugzilla.redhat.com/show_bug.cgi?id=1706759 , which has the same
> backtrace as above, and it seems commit 7bfde688fb1b ("virtio-blk: Add
> blk_drain() to virtio_blk_device_unrealize()") is to fix this particular
> bug.
>
> But I can still hit the bug even after applying the commit. Do I miss
> anything?

Hi Eryu,
This backtrace seems to be caused by this bug (there were two bugs in
1706759): https://bugzilla.redhat.com/show_bug.cgi?id=1708480
Although the solution hasn't been tested on virtio-blk yet, you may
want to apply this patch:
    https://lists.nongnu.org/archive/html/qemu-devel/2019-12/msg05197.html
Let me know if this works.

Best regards, Julia Suvorova.