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

[Amit Machhiwal <amachhiw@linux.ibm.com>]

Hi Stefan,

On 2024/07/29 03:27 PM, Stefan Bader wrote:
> 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@amd.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@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?
> > > > > 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.

We have mirrored the internal bugzilla here:

https://bugs.launchpad.net/ubuntu/+bug/2075721

Thanks,
Amit

> Thanks,
> 
> Stefan
> > 
> > Rob
> > 
> 
> -- 
> - Stefan
>