Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196

[Hanna Reitz]

On 23.11.21 14:49, Philippe Mathieu-Daudé wrote:
> On 11/23/21 14:42, Hanna Reitz wrote:
> > On 18.11.21 13:06, Philippe Mathieu-Daudé wrote:
> > > From: Alexander Bulekov <alxndr@bu.edu>
> > >
> > > Without the previous commit, when running 'make check-qtest-i386'
> > > with QEMU configured with '--enable-sanitizers' we get:
> > >
> > >     AddressSanitizer:DEADLYSIGNAL
> > >     =================================================================
> > >     ==287878==ERROR: AddressSanitizer: SEGV on unknown address
> > > 0x000000000344
> > >     ==287878==The signal is caused by a WRITE memory access.
> > >     ==287878==Hint: address points to the zero page.
> > >         #0 0x564b2e5bac27 in blk_inc_in_flight
> > > block/block-backend.c:1346:5
> > >         #1 0x564b2e5bb228 in blk_pwritev_part block/block-backend.c:1317:5
> > >         #2 0x564b2e5bcd57 in blk_pwrite block/block-backend.c:1498:11
> > >         #3 0x564b2ca1cdd3 in fdctrl_write_data hw/block/fdc.c:2221:17
> > >         #4 0x564b2ca1b2f7 in fdctrl_write hw/block/fdc.c:829:9
> > >         #5 0x564b2dc49503 in portio_write softmmu/ioport.c:201:9
> > >
> > > Add the reproducer for CVE-2021-20196.
> > >
> > > Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
> > > Message-Id: <20210319050906.14875-2-alxndr@bu.edu>
> > > [PMD: Rebased, use global test_image]
> > > Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
> > > Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> > > ---
> > >    tests/qtest/fdc-test.c | 21 +++++++++++++++++++++
> > >    1 file changed, 21 insertions(+)
> > Not sure if I’m doing something wrong, but:
> >
> > Using the global test_image brings a problem, namely that this test
> > fails unconditionally (for me at least...?), with the reason being that
> > the global QEMU instance (launched by qtest_start(), quit by
> > qtest_end()) still has that image open, so by launching a second
> > instance concurrently, I get this:
> >
> > qemu-system-x86_64: Failed to get "write" lock
> > Is another process using the image [/tmp/qtest.xV4IxX]?
> Hmm I had too many odd problems running qtests in parallel so I
> switched to 'make check-qtest -j1' more than 1 year ago, which
> is probably why I haven't noticed that issue.

I’ve run the test with

QTEST_QEMU_BINARY=$PWD/qemu-system-x86_64 tests/qtest/fdc-test

so there definitely weren’t any other tests running at the same time.  I 
don’t know why you don’t encounter this problem, but it’s caused by the 
concurrent QEMU instance launched in the very same test (qtest_start() 
in main(), and cleaned up by qtest_end() after g_test_run()).

> Using another 'test_image' seems against code reuse principle,
> but at this point in release, duplicating it is simpler. Someone
> will clean that later =)
>
> > So either we need to use a different image file, or we need to quit the
> > global instance before using it (e.g. putting a qtest_end() at the
> > beginning of test_cve_*()), although the latter just seems wrong.
> >
> > Second, I can’t make this test fail.  When I apply this patch first (to
> > master) and run it, I don’t get a SIGSEGV.
> Is your QEMU built with --enable-sanitizers ?

Uh, no.  I had (wrongly) assumed there’d be no need, given that the 
SIGSEGV access address is below 4 kB...

I now did configure and compile it with --enable-sanitizers but still 
can’t reproduce it.

Hanna