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Re: [Qemu-devel] [PATCH] json: Fix % handling when not interpolating

From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH] json: Fix % handling when not interpolating
Date: Wed, 09 Jan 2019 17:20:17 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)

Max Reitz <address@hidden> writes:

> On 09.01.19 15:32, Markus Armbruster wrote:
>> Max Reitz <address@hidden> 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 <address@hidden> writes:
>>>>> On Mon, Jan 07, 2019 at 04:47:44PM +0100, Markus Armbruster wrote:
>>>>>> Eric Blake <address@hidden> 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.

"Should" in the sense of "if they don't, it's a bug, and we'll do what
we can to fix it", or "if they don't, I'll be surprised"?

>> 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:-)

I'm not charging toll for the bridge I built for you ;)

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