Re: [PATCH v2] PCI: Fix crash during pci_dev hot-unplug on pseries KVM guest

[Lizhi Hou <lizhi.hou@amd.com>]

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@linux.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@linux.ibm.com>
>>>> Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
>>>> Signed-off-by: Amit Machhiwal <amachhiw@linux.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?

of_node->data can point to anything. And we do not know if it points a 
cset or not.  Do you mean to add a flag (e.g. OF_PCI_DYNAMIC) to 
indicate of_node->data points to cset?

I probably misunderstood. Could you explain more?


Thanks,

Lizhi

>
> Rob