Re: [PATCH v3] PCI: Fix crash during pci_dev hot-unplug on pseries KVM guest

On Sat, 03 Aug 2024 00:03:25 +0530, Amit Machhiwal 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 introduces a new flag OF_CREATE_WITH_CSET
to keep track of device nodes created via `of_pci_make_dev_node()` and
later attempt to destroy only such device nodes which have this flag
set.

[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 <redacted>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Signed-off-by: Amit Machhiwal <redacted>
---
Changes since v2:
    * Drop v2 changes and introduce a different approach from Lizhi discussed
      over the v2 of this patch
    * V2: https://lore.kernel.org/all/20240715080726.2496198-1-amachhiw@linux.ibm.com/ (local)
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/ (local)

 drivers/pci/of.c   | 3 ++-
 include/linux/of.h | 1 +
 2 files changed, 3 insertions(+), 1 deletion(-)

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>