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

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

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?


Thanks,

Lizhi

>
> Rob