Submission of requests on linux aio is a bit tricky and can lead to
requests completions on submission path:
44713c9e85 ("linux-aio: Handle io_submit() failure gracefully")
0ed93d84ed ("linux-aio: process completions from ioq_submit()")
That means that any coroutine which has been yielded in order to wait
for completion can be resumed from submission path and be eventually
terminated (freed).
The following use-after-free crash was observed when IO throttling
was enabled:
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7f5813dff700 (LWP 56417)]
virtqueue_unmap_sg (elem=0x7f5804009a30, len=1, vq=<optimized out>) at virtio.c:252
(gdb) bt
#0 virtqueue_unmap_sg (elem=0x7f5804009a30, len=1, vq=<optimized out>) at virtio.c:252
^^^^^^^^^^^^^^
remember the address
#1 virtqueue_fill (vq=0x5598b20d21b0, elem=0x7f5804009a30, len=1, idx=0) at virtio.c:282
#2 virtqueue_push (vq=0x5598b20d21b0, elem=elem@entry=0x7f5804009a30, len=<optimized out>) at virtio.c:308
#3 virtio_blk_req_complete (req=req@entry=0x7f5804009a30, status=status@entry=0 '\000') at virtio-blk.c:61
#4 virtio_blk_rw_complete (opaque=<optimized out>, ret=0) at virtio-blk.c:126
#5 blk_aio_complete (acb=0x7f58040068d0) at block-backend.c:923
#6 coroutine_trampoline (i0=<optimized out>, i1=<optimized out>) at coroutine-ucontext.c:78
(gdb) p * elem
$8 = {index = 77, out_num = 2, in_num = 1,
in_addr = 0x7f5804009ad8, out_addr = 0x7f5804009ae0,
in_sg = 0x0, out_sg = 0x7f5804009a50}
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
'in_sg' and 'out_sg' are invalid.
e.g. it is impossible that 'in_sg' is zero,
instead its value must be equal to:
(gdb) p/x 0x7f5804009ad8 + sizeof(elem->in_addr[0]) + 2 * sizeof(elem->out_addr[0])
$26 = 0x7f5804009af0
Seems 'elem' was corrupted. Meanwhile another thread raised an abort:
Thread 12 (Thread 0x7f57f2ffd700 (LWP 56426)):
#0 raise () from /lib/x86_64-linux-gnu/libc.so.6
#1 abort () from /lib/x86_64-linux-gnu/libc.so.6
#2 qemu_coroutine_enter (co=0x7f5804009af0) at qemu-coroutine.c:113
#3 qemu_co_queue_run_restart (co=0x7f5804009a30) at qemu-coroutine-lock.c:60
#4 qemu_coroutine_enter (co=0x7f5804009a30) at qemu-coroutine.c:119
^^^^^^^^^^^^^^^^^^
WTF?? this is equal to elem from crashed thread
#5 qemu_co_queue_run_restart (co=0x7f57e7f16ae0) at qemu-coroutine-lock.c:60
#6 qemu_coroutine_enter (co=0x7f57e7f16ae0) at qemu-coroutine.c:119
#7 qemu_co_queue_run_restart (co=0x7f5807e112a0) at qemu-coroutine-lock.c:60
#8 qemu_coroutine_enter (co=0x7f5807e112a0) at qemu-coroutine.c:119
#9 qemu_co_queue_run_restart (co=0x7f5807f17820) at qemu-coroutine-lock.c:60
#10 qemu_coroutine_enter (co=0x7f5807f17820) at qemu-coroutine.c:119
#11 qemu_co_queue_run_restart (co=0x7f57e7f18e10) at qemu-coroutine-lock.c:60
#12 qemu_coroutine_enter (co=0x7f57e7f18e10) at qemu-coroutine.c:119
#13 qemu_co_enter_next (queue=queue@entry=0x5598b1e742d0) at qemu-coroutine-lock.c:106
#14 timer_cb (blk=0x5598b1e74280, is_write=<optimized out>) at throttle-groups.c:419
Crash can be explained by access of 'co' object from the loop inside
qemu_co_queue_run_restart():
while ((next = QSIMPLEQ_FIRST(&co->co_queue_wakeup))) {
QSIMPLEQ_REMOVE_HEAD(&co->co_queue_wakeup, co_queue_next);
^^^^^^^^^^^^^^^^^^^^
on each iteration 'co' is accessed,
but 'co' can be already freed
qemu_coroutine_enter(next);
}
When 'next' coroutine is resumed (entered) it can in its turn resume
'co', and eventually free it. That's why we see 'co' (which was freed)
has the same address as 'elem' from the first backtrace.
The fix is obvious: use temporary queue and do not touch coroutine after
first qemu_coroutine_enter() is invoked.
The issue is quite rare and happens every ~12 hours on very high IO
and CPU load (building linux kernel with -j512 inside guest) when IO
throttling is enabled. With the fix applied guest is running ~35 hours
and is still alive so far.
Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170601160847.23720-1-roman.penyaev@profitbricks.com
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Fam Zheng <famz@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-devel@nongnu.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
QEMU README
===========
QEMU is a generic and open source machine & userspace emulator and
virtualizer.
QEMU is capable of emulating a complete machine in software without any
need for hardware virtualization support. By using dynamic translation,
it achieves very good performance. QEMU can also integrate with the Xen
and KVM hypervisors to provide emulated hardware while allowing the
hypervisor to manage the CPU. With hypervisor support, QEMU can achieve
near native performance for CPUs. When QEMU emulates CPUs directly it is
capable of running operating systems made for one machine (e.g. an ARMv7
board) on a different machine (e.g. an x86_64 PC board).
QEMU is also capable of providing userspace API virtualization for Linux
and BSD kernel interfaces. This allows binaries compiled against one
architecture ABI (e.g. the Linux PPC64 ABI) to be run on a host using a
different architecture ABI (e.g. the Linux x86_64 ABI). This does not
involve any hardware emulation, simply CPU and syscall emulation.
QEMU aims to fit into a variety of use cases. It can be invoked directly
by users wishing to have full control over its behaviour and settings.
It also aims to facilitate integration into higher level management
layers, by providing a stable command line interface and monitor API.
It is commonly invoked indirectly via the libvirt library when using
open source applications such as oVirt, OpenStack and virt-manager.
QEMU as a whole is released under the GNU General Public License,
version 2. For full licensing details, consult the LICENSE file.
Building
========
QEMU is multi-platform software intended to be buildable on all modern
Linux platforms, OS-X, Win32 (via the Mingw64 toolchain) and a variety
of other UNIX targets. The simple steps to build QEMU are:
mkdir build
cd build
../configure
make
Additional information can also be found online via the QEMU website:
http://qemu-project.org/Hosts/Linux
http://qemu-project.org/Hosts/Mac
http://qemu-project.org/Hosts/W32
Submitting patches
==================
The QEMU source code is maintained under the GIT version control system.
git clone git://git.qemu-project.org/qemu.git
When submitting patches, the preferred approach is to use 'git
format-patch' and/or 'git send-email' to format & send the mail to the
qemu-devel@nongnu.org mailing list. All patches submitted must contain
a 'Signed-off-by' line from the author. Patches should follow the
guidelines set out in the HACKING and CODING_STYLE files.
Additional information on submitting patches can be found online via
the QEMU website
http://qemu-project.org/Contribute/SubmitAPatch
http://qemu-project.org/Contribute/TrivialPatches
Bug reporting
=============
The QEMU project uses Launchpad as its primary upstream bug tracker. Bugs
found when running code built from QEMU git or upstream released sources
should be reported via:
https://bugs.launchpad.net/qemu/
If using QEMU via an operating system vendor pre-built binary package, it
is preferable to report bugs to the vendor's own bug tracker first. If
the bug is also known to affect latest upstream code, it can also be
reported via launchpad.
For additional information on bug reporting consult:
http://qemu-project.org/Contribute/ReportABug
Contact
=======
The QEMU community can be contacted in a number of ways, with the two
main methods being email and IRC
- qemu-devel@nongnu.org
http://lists.nongnu.org/mailman/listinfo/qemu-devel
- #qemu on irc.oftc.net
Information on additional methods of contacting the community can be
found online via the QEMU website:
http://qemu-project.org/Contribute/StartHere
-- End