Message-ID: <76ade019-ee95-4ff9-aaff-37d49a6be195@canonical.com>
Date: Mon, 29 Jul 2024 15:27:11 +0200
From: Stefan Bader <stefan.bader@...onical.com>
To: linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
 devicetree@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 kvm-ppc@...r.kernel.org, kernel-team@...ts.ubuntu.com
Cc: Bjorn Helgaas <bhelgaas@...gle.com>,
 Saravana Kannan <saravanak@...gle.com>, Vaibhav Jain
 <vaibhav@...ux.ibm.com>, Nicholas Piggin <npiggin@...il.com>,
 Michael Ellerman <mpe@...erman.id.au>,
 Vaidyanathan Srinivasan <svaidy@...ux.ibm.com>,
 Kowshik Jois B S <kowsjois@...ux.ibm.com>, Lukas Wunner <lukas@...ner.de>,
 Rob Herring <robh@...nel.org>, Lizhi Hou <lizhi.hou@....com>
Subject: Re: [PATCH v2] PCI: Fix crash during pci_dev hot-unplug on pseries
 KVM guest

On 26.07.24 13:37, Rob Herring wrote:
> + Ubuntu kernel list, again
> 
> On Thu, Jul 25, 2024 at 11:15:39PM +0530, Amit Machhiwal wrote:
>> Hi Lizhi, Rob,
>>
>> Sorry for responding late. I got busy with some other things.
>>
>> On 2024/07/23 02:08 PM, Lizhi Hou wrote:
>>>
>>> On 7/23/24 12:54, Rob Herring wrote:
>>>> On Tue, Jul 23, 2024 at 12:21 PM Lizhi Hou <lizhi.hou@....com> wrote:
>>>>>
>>>>> On 7/23/24 09:21, Rob Herring wrote:
>>>>>> On Mon, Jul 15, 2024 at 01:52:30PM -0700, Lizhi Hou wrote:
>>>>>>> On 7/15/24 11:55, Rob Herring wrote:
>>>>>>>> On Mon, Jul 15, 2024 at 2:08 AM Amit Machhiwal <amachhiw@...ux.ibm.com> wrote:
>>>>>>>>> With CONFIG_PCI_DYNAMIC_OF_NODES [1], a hot-plug and hot-unplug sequence
>>>>>>>>> of a PCI device attached to a PCI-bridge causes following kernel Oops on
>>>>>>>>> a pseries KVM guest:
>>>>>>>>>
>>>>>>>>>      RTAS: event: 2, Type: Hotplug Event (229), Severity: 1
>>>>>>>>>      Kernel attempted to read user page (10ec00000048) - exploit attempt? (uid: 0)
>>>>>>>>>      BUG: Unable to handle kernel data access on read at 0x10ec00000048
>>>>>>>>>      Faulting instruction address: 0xc0000000012d8728
>>>>>>>>>      Oops: Kernel access of bad area, sig: 11 [#1]
>>>>>>>>>      LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA pSeries
>>>>>>>>> <snip>
>>>>>>>>>      NIP [c0000000012d8728] __of_changeset_entry_invert+0x10/0x1ac
>>>>>>>>>      LR [c0000000012da7f0] __of_changeset_revert_entries+0x98/0x180
>>>>>>>>>      Call Trace:
>>>>>>>>>      [c00000000bcc3970] [c0000000012daa60] of_changeset_revert+0x58/0xd8
>>>>>>>>>      [c00000000bcc39c0] [c000000000d0ed78] of_pci_remove_node+0x74/0xb0
>>>>>>>>>      [c00000000bcc39f0] [c000000000cdcfe0] pci_stop_bus_device+0xf4/0x138
>>>>>>>>>      [c00000000bcc3a30] [c000000000cdd140] pci_stop_and_remove_bus_device_locked+0x34/0x64
>>>>>>>>>      [c00000000bcc3a60] [c000000000cf3780] remove_store+0xf0/0x108
>>>>>>>>>      [c00000000bcc3ab0] [c000000000e89e04] dev_attr_store+0x34/0x78
>>>>>>>>>      [c00000000bcc3ad0] [c0000000007f8dd4] sysfs_kf_write+0x70/0xa4
>>>>>>>>>      [c00000000bcc3af0] [c0000000007f7248] kernfs_fop_write_iter+0x1d0/0x2e0
>>>>>>>>>      [c00000000bcc3b40] [c0000000006c9b08] vfs_write+0x27c/0x558
>>>>>>>>>      [c00000000bcc3bf0] [c0000000006ca168] ksys_write+0x90/0x170
>>>>>>>>>      [c00000000bcc3c40] [c000000000033248] system_call_exception+0xf8/0x290
>>>>>>>>>      [c00000000bcc3e50] [c00000000000d05c] system_call_vectored_common+0x15c/0x2ec
>>>>>>>>> <snip>
>>>>>>>>>
>>>>>>>>> A git bisect pointed this regression to be introduced via [1] that added
>>>>>>>>> a mechanism to create device tree nodes for parent PCI bridges when a
>>>>>>>>> PCI device is hot-plugged.
>>>>>>>>>
>>>>>>>>> The Oops is caused when `pci_stop_dev()` tries to remove a non-existing
>>>>>>>>> device-tree node associated with the pci_dev that was earlier
>>>>>>>>> hot-plugged and was attached under a pci-bridge. The PCI dev header
>>>>>>>>> `dev->hdr_type` being 0, results a conditional check done with
>>>>>>>>> `pci_is_bridge()` into false. Consequently, a call to
>>>>>>>>> `of_pci_make_dev_node()` to create a device node is never made. When at
>>>>>>>>> a later point in time, in the device node removal path, a memcpy is
>>>>>>>>> attempted in `__of_changeset_entry_invert()`; since the device node was
>>>>>>>>> never created, results in an Oops due to kernel read access to a bad
>>>>>>>>> address.
>>>>>>>>>
>>>>>>>>> To fix this issue, the patch updates `of_changeset_create_node()` to
>>>>>>>>> allocate a new node only when the device node doesn't exist and init it
>>>>>>>>> in case it does already. Also, introduce `of_pci_free_node()` to be
>>>>>>>>> called to only revert and destroy the changeset device node that was
>>>>>>>>> created via a call to `of_changeset_create_node()`.
>>>>>>>>>
>>>>>>>>> [1] commit 407d1a51921e ("PCI: Create device tree node for bridge")
>>>>>>>>>
>>>>>>>>> Fixes: 407d1a51921e ("PCI: Create device tree node for bridge")
>>>>>>>>> Reported-by: Kowshik Jois B S <kowsjois@...ux.ibm.com>
>>>>>>>>> Signed-off-by: Lizhi Hou <lizhi.hou@....com>
>>>>>>>>> Signed-off-by: Amit Machhiwal <amachhiw@...ux.ibm.com>
>>>>>>>>> ---
>>>>>>>>> Changes since v1:
>>>>>>>>>         * Included Lizhi's suggested changes on V1
>>>>>>>>>         * Fixed below two warnings from Lizhi's changes and rearranged the cleanup
>>>>>>>>>           part a bit in `of_pci_make_dev_node`
>>>>>>>>>             drivers/pci/of.c:611:6: warning: no previous prototype for ‘of_pci_free_node’ [-Wmissing-prototypes]
>>>>>>>>>               611 | void of_pci_free_node(struct device_node *np)
>>>>>>>>>                   |      ^~~~~~~~~~~~~~~~
>>>>>>>>>             drivers/pci/of.c: In function ‘of_pci_make_dev_node’:
>>>>>>>>>             drivers/pci/of.c:696:1: warning: label ‘out_destroy_cset’ defined but not used [-Wunused-label]
>>>>>>>>>               696 | out_destroy_cset:
>>>>>>>>>                   | ^~~~~~~~~~~~~~~~
>>>>>>>>>         * V1: https://lore.kernel.org/all/20240703141634.2974589-1-amachhiw@linux.ibm.com/
>>>>>>>>>
>>>>>>>>>      drivers/of/dynamic.c  | 16 ++++++++++++----
>>>>>>>>>      drivers/of/unittest.c |  2 +-
>>>>>>>>>      drivers/pci/bus.c     |  3 +--
>>>>>>>>>      drivers/pci/of.c      | 39 ++++++++++++++++++++++++++-------------
>>>>>>>>>      drivers/pci/pci.h     |  2 ++
>>>>>>>>>      include/linux/of.h    |  1 +
>>>>>>>>>      6 files changed, 43 insertions(+), 20 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/drivers/of/dynamic.c b/drivers/of/dynamic.c
>>>>>>>>> index dda6092e6d3a..9bba5e82a384 100644
>>>>>>>>> --- a/drivers/of/dynamic.c
>>>>>>>>> +++ b/drivers/of/dynamic.c
>>>>>>>>> @@ -492,21 +492,29 @@ struct device_node *__of_node_dup(const struct device_node *np,
>>>>>>>>>       * a given changeset.
>>>>>>>>>       *
>>>>>>>>>       * @ocs: Pointer to changeset
>>>>>>>>> + * @np: Pointer to device node. If null, allocate a new node. If not, init an
>>>>>>>>> + *     existing one.
>>>>>>>>>       * @parent: Pointer to parent device node
>>>>>>>>>       * @full_name: Node full name
>>>>>>>>>       *
>>>>>>>>>       * Return: Pointer to the created device node or NULL in case of an error.
>>>>>>>>>       */
>>>>>>>>>      struct device_node *of_changeset_create_node(struct of_changeset *ocs,
>>>>>>>>> +                                            struct device_node *np,
>>>>>>>>>                                                  struct device_node *parent,
>>>>>>>>>                                                  const char *full_name)
>>>>>>>>>      {
>>>>>>>>> -       struct device_node *np;
>>>>>>>>>             int ret;
>>>>>>>>>
>>>>>>>>> -       np = __of_node_dup(NULL, full_name);
>>>>>>>>> -       if (!np)
>>>>>>>>> -               return NULL;
>>>>>>>>> +       if (!np) {
>>>>>>>>> +               np = __of_node_dup(NULL, full_name);
>>>>>>>>> +               if (!np)
>>>>>>>>> +                       return NULL;
>>>>>>>>> +       } else {
>>>>>>>>> +               of_node_set_flag(np, OF_DYNAMIC);
>>>>>>>>> +               of_node_set_flag(np, OF_DETACHED);
>>>>>>>> Are we going to rename the function to
>>>>>>>> of_changeset_create_or_maybe_modify_node()? No. The functions here are
>>>>>>>> very clear in that they allocate new objects and don't reuse what's
>>>>>>>> passed in.
>>>>>>> Ok. How about keeping of_changeset_create_node unchanged.
>>>>>>>
>>>>>>> Instead, call kzalloc(), of_node_init() and of_changeset_attach_node()
>>>>>>>
>>>>>>> in of_pci_make_dev_node() directly.
>>>>>>>
>>>>>>> A similar example is dlpar_parse_cc_node().
>>>>>>>
>>>>>>>
>>>>>>> Does this sound better?
>>>>>> No, because really that code should be re-written using of_changeset
>>>>>> API.
>>>>>>
>>>>>> My suggestion is add a data pointer to struct of_changeset and then set
>>>>>> that to something to know the data ptr is a changeset and is your
>>>>>> changeset.
>>>>> I do not fully understand the point. I think the issue is that we do not
>>>>> know if a given of_node is created by of_pci_make_dev_node(), correct?
>>>> Yes.
>>>>
>>>>> of_node->data can point to anything. And we do not know if it points a
>>>>> cset or not.
>>>> Right. But instead of checking "of_node->data == of_pci_free_node",
>>>> you would just be checking "*(of_node->data) == of_pci_free_node"
>>>
>>> if of_node->data is a char* pointer, it would be panic. So I used
>>> of_node->data == of_pci_free_node.
>>>
>>>> (omitting a NULL check and cast for simplicity). I suppose in theory
>>>> that could have a false match, but that could happen in this patch
>>>> already.
>>>
>>> I think if any other kernel code  put of_pci_free_node to of_node->data, it
>>> can be fixed over there.
>>>
>>>>
>>>>> Do you mean to add a flag (e.g. OF_PCI_DYNAMIC) to
>>>>> indicate of_node->data points to cset?
>>>> That would be another option, but OF_PCI_DYNAMIC would not be a good
>>>> name because that would be a flag bit for every single caller needing
>>>> similar functionality. Name it just what it indicates: of_node->data
>>>> points to cset
>>>>
>>>> If we have that flag, then possibly the DT core can handle more
>>>> clean-up itself like calling detach and freeing the changeset.
>>>> Ideally, the flags should be internal to the DT code.
>>>
>>> Sure. If you prefer this option, I will propose another fix.
>>>
>>
>> The crash in question is a critical issue that we would want to have a fix for
>> soon. And while this is still being figured out, is it okay to go with the fix I
>> proposed in the V1 of this patch?
> 
> No, sorry but this is not critical. This config option should be off.
> Fix the distro. They turned it on. If hiding it behind CONFIG_EXPERT or
> something would work, that would be fine for upstream. But I suspect
> Ubuntu turns that on because they turn on everything...

Likely that was the case. Noted and we will turn it off in one of the 
next updates.
Thanks,

Stefan
> 
> Rob
> 

-- 
- Stefan


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