Re: Regression: NULL pointer dereference after NFS_V4_2_READ_PLUS (commit 7fd461c47)

[Krzysztof Kozlowski]

On 22/01/2023 23:25, Anna Schumaker wrote:
> Hi Krzysztof,
> 
> On Mon, Jan 9, 2023 at 3:46 AM Krzysztof Kozlowski
> <krzysztof.kozlowski@xxxxxxxxxx> wrote:
>>
>> On 09/01/2023 09:14, Krzysztof Kozlowski wrote:
>>> On 08/01/2023 18:09, Trond Myklebust wrote:
>>>> Hi Krzysztof,
>>>>
>>>>> On Jan 8, 2023, at 08:25, Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx> wrote:
>>>>>
>>>>> [You don't often get email from krzysztof.kozlowski@xxxxxxxxxx. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification]
>>>>>
>>>>> On 07/01/2023 16:44, Krzysztof Kozlowski wrote:
>>>>>> Hi,
>>>>>>
>>>>>> Bisect identified commit 7fd461c47c6c ("NFSv4.2: Change the default
>>>>>> KConfig value for READ_PLUS") as one leading to NULL pointer exception
>>>>>> when mounting NFS root on NFSv4 client:
>>>>>>
>>>>>> [   25.739003] systemd[1]: Set hostname to <odroidhc1>.
>>>>>> [   25.771714] systemd[1]: Failed to bump fs.file-max, ignoring: Invalid
>>>>>> argument
>>>>>> [   26.199478] 8<--- cut here ---
>>>>>> [   26.201366] Unable to handle kernel NULL pointer dereference at
>>>>>> virtual address 00000004
>>>>>> ...
>>>>>> [   26.555522]  mmiocpy from xdr_inline_decode+0xec/0x16c
>>>>>> [   26.560628]  xdr_inline_decode from nfs4_xdr_dec_read_plus+0x178/0x358
>>>>>> [   26.567130]  nfs4_xdr_dec_read_plus from call_decode+0x204/0x304
>>>>>>
>>>>>> Full OOPS attached. Full log available here:
>>>>>> https://krzk.eu/#/builders/21/builds/3901/steps/15/logs/serial0
>>>>>>
>>>>>> Disabling NFS_V4_2_READ_PLUS fixes the issue, so obviously the commit is
>>>>>> not the cause, but rather making it default caused the regression.
>>>>>>
>>>>>> I did not make the bisect yet which commit introduced it, if every
>>>>>> config includes NFS_V4_2_READ_PLUS.
>>>>>
>>>>> When every kernel is built with NFS_V4_2_READ_PLUS, bisect pointed to:
>>>>> d3b00a802c84 ("NFS: Replace the READ_PLUS decoding code")
>>>>>
>>>>> commit d3b00a802c845a6021148ce2e669b5a0b5729959
>>>>> Author: Anna Schumaker <Anna.Schumaker@xxxxxxxxxx>
>>>>> Date:   Thu Jul 21 14:21:34 2022 -0400
>>>>>
>>>>>    NFS: Replace the READ_PLUS decoding code
>>>>>
>>>>>    We now take a 2-step process that allows us to place data and hole
>>>>>    segments directly at their final position in the xdr_stream without
>>>>>    needing to do a bunch of redundant copies to expand holes. Due to the
>>>>>    variable lengths of each segment, the xdr metadata might cross page
>>>>>    boundaries which I account for by setting a small scratch buffer so
>>>>>    xdr_inline_decode() won't fail.
>>>>>
>>>>>    Signed-off-by: Anna Schumaker <Anna.Schumaker@xxxxxxxxxx>
>>>>>    Signed-off-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
>>>>>
>>>>> With a trace:
>>>>> [   25.898462] systemd[1]: Set hostname to <odroidhc1>.
>>>>> [   25.933746] systemd[1]: Failed to bump fs.file-max, ignoring: Invalid
>>>>> argument
>>>>> [   25.986237] random: crng init done
>>>>> [   26.264564] 8<--- cut here ---
>>>>> [   26.266823] Unable to handle kernel NULL pointer dereference at
>>>>> virtual address 00000fe8
>>>>> ...
>>>>> [   26.597263]  nfs4_xdr_dec_read_plus from call_decode+0x204/0x304
>>>>> [   26.603222]  call_decode from __rpc_execute+0xd0/0x890
>>>>> [   26.608328]  __rpc_execute from rpc_async_schedule+0x1c/0x34
>>>>> [   26.613960]  rpc_async_schedule from process_one_work+0x294/0x790
>>>>> [   26.620030]  process_one_work from worker_thread+0x54/0x518
>>>>> [   26.625570]  worker_thread from kthread+0xf4/0x128
>>>>> [   26.630336]  kthread from ret_from_fork+0x14/0x2c
>>>>>
>>>>
>>>> Is this test being run against a 6.2-rc2 server, or is it an older server platform? We know there were bugs in older server implementations, so the question is whether this might be a problem with handling a bad/corrupt RPC reply from the server, or whether it is happening against code that is supposed to have been fixed?
>>>
>>> I would say that buggy server should not cause NULL pointer dereferences
>>> on the client. Otherwise this is a perfect recipe for a rogue server in
>>> the network to start crashing clients and running exploits... Imagine a
>>> compromised machine (through some other means) in a local company
>>> network running now a server with NFS share "HR salary data" or "HR
>>> planned layoffs", where unsuspected people in that network access it
>>> leading to exploit of NFS code on their side...
>>>
>>> Server is Raspberry Pi 3 kernel: 5.10.92-2-rpi-legacy-ARCH
>>>
>>> Which points that it is not latest stable, so anyway I need to update.
>>
>> I updated the kernel to 5.15.84-3-rpi-ARCH which is pretty close to
>> latest stable and I can reproduce the issue. Therefore:
>> 1. It is visible on two stable (one new, one old) kernels on the server,
>> 2. Buggy or rogue server should not cause NULL pointer on remote devices...
> 
> I'm still working on this issue, but I haven't been able to reproduce
> it with my setup at all yet. I was hoping I could ask you a couple of
> questions?
> 
> - Are both client and server run on a Raspberry Pi 3?

No, server is Rpi3, client is Odroid HC1.

> - Is there a specific workload that triggers the bug, or is it just
> during boot when using nfsroot?

No specific workload - mounting system with nfsroot. You have there full
logs as it is reproducible every time.

> - Would it be possible to get a Wireshark network trace of the crash
> (you'll have to run this on the server due to nfsroot).

I'll check.

> - Can you share your export options from /etc/exports and the mount
> options that the client uses?

exports are the same since like 5 years or more:

/srv/nfs/odroidhc1	192.168.1.0/24(rw,async,no_root_squash,no_subtree_check)
/srv/nfs/odroidhc1-home
192.168.1.0/24(rw,async,no_root_squash,no_subtree_check)


> 
> Thanks,
> Anna
> 
>>
>> Best regards,
>> Krzysztof
>>

Best regards,
Krzysztof