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

From: Hanna Reitz
Subject: Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196
Date: Tue, 23 Nov 2021 15:14:53 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.3.0

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:

    ==287878==ERROR: AddressSanitizer: SEGV on unknown address
    ==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
        #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.


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