linux-kernel - Re: Regression: NULL pointer dereference after NFS_V4_2_READ

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Date:   Mon, 9 Jan 2023 09:42:30 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Trond Myklebust <trondmy@...merspace.com>
Cc:     Anna Schumaker <Anna.Schumaker@...app.com>,
        linux-nfs <linux-nfs@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "regressions@...ts.linux.dev" <regressions@...ts.linux.dev>
Subject: Re: Regression: NULL pointer dereference after NFS_V4_2_READ_PLUS
 (commit 7fd461c47)

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@...aro.org> wrote:
>>>
>>> [You don't often get email from krzysztof.kozlowski@...aro.org. 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@...app.com>
>>> 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@...app.com>
>>>    Signed-off-by: Trond Myklebust <trond.myklebust@...merspace.com>
>>>
>>> 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...

Best regards,
Krzysztof