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

[Rob Herring]

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.

Rob