Date:   Mon, 9 Jan 2023 17:20:23 +0000
From:   Chuck Lever III <chuck.lever@...cle.com>
To:     Trond Myklebust <trondmy@...merspace.com>
CC:     Anna Schumaker <Anna.Schumaker@...app.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Linux NFS Mailing List <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 Jan 9, 2023, at 12:11 PM, Chuck Lever III <chuck.lever@...cle.com> wrote:
> 
> 
> 
>> On Jan 9, 2023, at 10:38 AM, Trond Myklebust <trondmy@...merspace.com> wrote:
>> 
>> 
>> 
>>> On Jan 9, 2023, at 10:07, Chuck Lever III <chuck.lever@...cle.com> wrote:
>>> 
>>> 
>>> 
>>>> On Jan 9, 2023, at 9:44 AM, Trond Myklebust <trondmy@...merspace.com> wrote:
>>>> 
>>>> 
>>>> 
>>>>> On Jan 9, 2023, at 03:42, Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org> 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@...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...
>>>>> 
>>>> 
>>>> The nfsd READ_PLUS code is borked up and until 6.2-rc1. I thought we had a server option to disable that code, but it seems as if that is not the case.
>>>> Chuck + Anna, can we please send a stable patch to rip out that code altogether from all the older versions? If we want to enable READ_PLUS by default on the client, then we cannot do so if the majority of NFSv4.2 servers out there are running a borked implementation.
>>>> 
>>>> I do agree that we cannot allow buggy
>>> 
>>> or malicious, or non-Linux,
>>> 
>>> 
>>>> servers to cause memory corruption in the client code, so I propose that we revert the Kconfig default setting change again until both the client code and the legacy servers have been fixed.
>>> 
>>> I stand ready to receive and apply server-side fixes, as you suggested.
>>> 
>>> However IMO it would be most responsible to go straight for a client code fix. The Kconfig setting is a red herring (as delicious as that might sound). Any thoughts about how difficult that fix might be?
>>> 
>> 
>> A client fix is necessary, but not sufficient.
> 
> No argument from me. There is a problem on both sides.
> 
> 
>> The older server READ_PLUS code is slower than ordinary read for several filesystems, since it relies upon the (often poor) performance of lseek(SEEK_HOLE). Leaving legacy servers as is, would therefore still cause regressions for clients that default to trying READ_PLUS, even if the code is fixed to not Oops.
> 
> Backporting eeadcb757945 ("NFSD: Simplify READ_PLUS") to stable kernels would address both concerns.

And by "both concerns" I mean it addresses the possible performance
regression and the server's malformed RPC reply.

The client code still needs to be fixed.

--
Chuck Lever

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