On 09.01.19 15:32, Markus Armbruster wrote: > Max Reitz <mre...@redhat.com> writes: > >> On 08.01.19 11:36, Markus Armbruster wrote: >>> Copying block maintainers for help with assessing the bug's (non-)impact >>> on security. >>> >>> Christophe Fergeau <cferg...@redhat.com> writes: >>> >>>> On Mon, Jan 07, 2019 at 04:47:44PM +0100, Markus Armbruster wrote: >>>>> Eric Blake <ebl...@redhat.com> writes: >>>>> >>>>>> On 1/2/19 12:01 PM, Christophe Fergeau wrote: >>>>>>> Adding Markus to cc: list, I forgot to do it when sending the patch. >>>>>> >>>>>> Also worth backporting via qemu-stable, now in cc. >>>>>> >>>>>>> >>>>>>> Christophe >>>>>>> >>>>>>> On Wed, Jan 02, 2019 at 03:05:35PM +0100, Christophe Fergeau wrote: >>>>>>>> commit 8bca4613 added support for %% in json strings when >>>>>>>> interpolating, >>>>>>>> but in doing so, this broke handling of % when not interpolating as the >>>>>>>> '%' is skipped in both cases. >>>>>>>> This commit ensures we only try to handle %% when interpolating. >>>>> >>>>> Impact? >>>>> >>>>> If you're unable to assess, could you give us at least a reproducer? >>>> >>>> This all came from >>>> https://lists.freedesktop.org/archives/spice-devel/2018-December/046644.html >>>> Setting up a VM with libvirt with <graphics type='spice' autoport='yes' >>>> passwd='password%'/> >>>> fails to start with: >>>> qemu-system-x86_64: qobject/json-parser.c:146: parse_string: Assertion >>>> `*ptr' failed. >>>> >>>> If you use 'password%%' as the password instead, when trying to connect >>>> to the VM, you type 'password%' as the password instead of 'password%%' >>>> as configured in the domain XML. >>> >>> Thanks. >>> >>> As the commit message says, the bug bites when we parse a string >>> containing '%s' with !ctxt->ap. The parser then swallows a character. >>> If it swallows the terminating '"', it fails the assertion. >>> >>> We parse with !ctxt->ap in the following cases: >>> >>> * Tests (tests/check-qjson.c, tests/test-qobject-input-visitor.c, >>> tests/test-visitor-serialization.c) >>> >>> Plenty of tests, but we still failed to cover the buggy case :( >>> >>> * QMP input (monitor.c) >>> >>> * QGA input (qga/main.c) >>> >>> * qobject_from_json() >>> >>> - JSON pseudo-filenames (block.c) >>> >>> These are pseudo-filenames starting with "json:". >>> >>> - JSON key pairs (block/rbd.c) >>> >>> As far as I can tell, these can come only from pseudo-filenames >>> starting with "rbd:". >>> >>> - JSON command line option arguments of -display and -blockdev >>> (qobject-input-visitor.c) >>> >>> Reproducer: -blockdev '{"%"}' >>> >>> Command line, QMP and QGA input are trusted. >>> >>> Filenames are trusted when they come from command line, QMP or HMP. >>> They are untrusted when they come from from image file headers. >>> Example: QCOW2 backing file name. Note that this is *not* the security >>> boundary between host and guest. It's the boundary between host and an >>> image file from an untrusted source. >>> >>> I can't see how the bug could be exploited. Neither failing an >>> assertion nor skipping a character in a filename of your choice is >>> interesting. We don't support compiling with NDEBUG. >>> >>> Kevin, Max, do you agree? >> >> I wouldn't call it "not interesting" if adding an image to your VM at >> runtime can crash the whole thing. >> >> (qemu-img create -f qcow2 -u -b 'json:{"%"}' foo.qcow2 64M) > > "Not interesting" strictly from the point of view of exploiting the bug > to penetrate trust boundaries. > >> Whether this is a security issue... I don't know, but it is a DoS. > > I'm not sure whether feeding untrusted images to QEMU is a good idea in > general --- there's so much that could go wrong. How hardened against > abuse are out block drivers?
They are supposed to handle such cases gracefully, that's for sure. At least for qcow2 we do care about it. > I figure what distinguishes this case is how utterly trivial creating a > "bad" image is. I don't think an untrusted image should be able to crash qemu. > Anyway, you are the block layer maintainers, so you get to decide > whether to give this the full security bug treatment. I'm merely the > clown who broke it %-/ Er, then I suppose it is no security bug? O:-) Max
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