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

[Anna Schumaker]

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?
- Is there a specific workload that triggers the bug, or is it just
during boot when using nfsroot?
- 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).
- Can you share your export options from /etc/exports and the mount
options that the client uses?

Thanks,
Anna

>
> Best regards,
> Krzysztof
>