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Re: [PATCH] iscsi: Cap block count from GET LBA STATUS (CVE-2020-1711)

From: Felipe Franciosi
Subject: Re: [PATCH] iscsi: Cap block count from GET LBA STATUS (CVE-2020-1711)
Date: Wed, 5 Feb 2020 14:22:13 +0000

Hi and sorry for the delay, this got lost in my inbox.

> On Jan 28, 2020, at 12:42 PM, Kevin Wolf <address@hidden> wrote:
> Am 28.01.2020 um 13:30 hat Philippe Mathieu-Daudé geschrieben:
>> Hi guys,
>> (Cc'ing Jon)
>> On 1/23/20 5:59 PM, Kevin Wolf wrote:
>>> Am 23.01.2020 um 13:44 hat Felipe Franciosi geschrieben:
>>>> When querying an iSCSI server for the provisioning status of blocks (via
>>>> GET LBA STATUS), Qemu only validates that the response descriptor zero's
>>>> LBA matches the one requested. Given the SCSI spec allows servers to
>>>> respond with the status of blocks beyond the end of the LUN, Qemu may
>>>> have its heap corrupted by clearing/setting too many bits at the end of
>>>> its allocmap for the LUN.
>>>> A malicious guest in control of the iSCSI server could carefully program
>>>> Qemu's heap (by selectively setting the bitmap) and then smash it.
>>>> This limits the number of bits that iscsi_co_block_status() will try to
>>>> update in the allocmap so it can't overflow the bitmap.
>>>> Signed-off-by: Felipe Franciosi <address@hidden>
>>>> Signed-off-by: Peter Turschmid <address@hidden>
>>>> Signed-off-by: Raphael Norwitz <address@hidden>
>>> Thanks, applied to the block branch.
>> We are trying to reproduce this, do you already have some code that
>> triggered this issue?
> I don't, maybe Felipe has a reproducer that would crash QEMU.

It's not hard.

1) Attach an iSCSI LUN to Qemu. Do not read data from it (so Qemu
   won't populate the bitmap).
2) Issue a read larger than 64 blocks towards the end of the LUN.
   Qemu will attempt a GET LBA STATUS to find out if the provisioning
   status of the blocks.
3) Get your iSCSI server to respond with more blocks than is available
   on the LUN for that LBA.

We did that with an iSCSI server written exclusively for such tests.
All the responses can be controlled or fuzzed. But it should be easy
to modify any existing server (eg. iet).


>> I am new to the block API, I noticed the block/blkdebug.c file with
>> 'blkdebug' option, is it helpful to reproduce this issue via HMP?
>> Any suggestion what would be the easier/quicker way to test this?
> On the QEMU side, you just need to connect to an iscsi backend. The
> malicious response must come from the server, which is not part of QEMU.
> So no, blkdebug won't help you.
>> Looking for iotests examples I see tests/qemu-iotests/147 providing a
>> BuiltinNBD class. Is it the recommended way to go, to mock a iSCSI server?
> That BuiltinNBD class doesn't implement an NBD server, but it just
> starts the built-in NBD server in QEMU and runs some tests against it.
> QEMU doesn't have a built-in iscsi server.
> Kevin

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