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

[Bjorn Helgaas <helgaas@kernel.org>]

On Sat, Aug 03, 2024 at 12:03:25AM +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:

What is unique about pseries here?  There's nothing specific to
pseries in the patch, so I would expect this to be a generic problem
on any arch.

>  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

Weird address.  I would expect NULL or something.  Where did this
non-NULL pointer come from?

>  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.

I'm sure this description is 100% correct, but it's at such a low
level that it doesn't really help understand the underlying design
problem.

Will need an ack from Rob.

> 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 <kowsjois@linux.ibm.com>
> Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
> 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/
> 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/pci/of.c   | 3 ++-
>  include/linux/of.h | 1 +
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> index dacea3fc5128..bc455370143e 100644
> --- a/drivers/pci/of.c
> +++ b/drivers/pci/of.c
> @@ -653,7 +653,7 @@ void of_pci_remove_node(struct pci_dev *pdev)
>  	struct device_node *np;
>  
>  	np = pci_device_to_OF_node(pdev);
> -	if (!np || !of_node_check_flag(np, OF_DYNAMIC))
> +	if (!np || !of_node_check_flag(np, OF_CREATE_WITH_CSET))
>  		return;
>  	pdev->dev.of_node = NULL;

of_pci_remove_node() goes on to call of_changeset_revert() and
of_changeset_destroy().  of_pci_remove_node() has nothing PCI-specific
in it.

There are a few other callers of of_changeset_destroy(), but they
don't look anything like of_pci_remove_node().  Seems like there
should be some similarity across the callers, so it makes me a little
nervous that there isn't.

> @@ -712,6 +712,7 @@ void of_pci_make_dev_node(struct pci_dev *pdev)
>  	if (ret)
>  		goto out_free_node;
>  
> +	of_node_set_flag(np, OF_CREATE_WITH_CSET);
>  	np->data = cset;
>  	pdev->dev.of_node = np;
>  	kfree(name);
> diff --git a/include/linux/of.h b/include/linux/of.h
> index 85b60ac9eec5..5faa5a1198c6 100644
> --- a/include/linux/of.h
> +++ b/include/linux/of.h
> @@ -153,6 +153,7 @@ extern struct device_node *of_stdout;
>  #define OF_POPULATED_BUS	4 /* platform bus created for children */
>  #define OF_OVERLAY		5 /* allocated for an overlay */
>  #define OF_OVERLAY_FREE_CSET	6 /* in overlay cset being freed */
> +#define OF_CREATE_WITH_CSET	7 /* Created by of_changeset_create_node */

Follow existing capitalization style.