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

From: Rob Herring
Date: Tue Jul 23 2024 - 15:54:53 EST

On Tue, Jul 23, 2024 at 12:21 PM Lizhi Hou <lizhi.hou@xxxxxxx> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> >>>> Signed-off-by: Lizhi Hou <lizhi.hou@xxxxxxx>
> >>>> Signed-off-by: Amit Machhiwal <amachhiw@xxxxxxxxxxxxx>
> >>>> ---
> >>>> 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@xxxxxxxxxxxxx/
> >>>>
> >>>> 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"
(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.

> 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.

Rob