[PATCH v2] powerpc/pseries/mobility: ignore ibm, platform-facilities updates

[Daniel Axtens]

Hi Nathan,

Thanks for the detailed explanation.

I've not really worked with the partition migration code before I was
able to follow your logic.

> On VMs with NX encryption, compression, and/or RNG offload, these
> capabilities are described by nodes in the ibm,platform-facilities device
> tree hierarchy:
>
>   $ tree -d /sys/firmware/devicetree/base/ibm,platform-facilities/
>   /sys/firmware/devicetree/base/ibm,platform-facilities/
>   ├── ibm,compression-v1
>   ├── ibm,random-v1
>   └── ibm,sym-encryption-v1
>
>   3 directories
>
> The acceleration functions that these nodes describe are not disrupted by
> live migration, not even temporarily.
>
> But the post-migration ibm,update-nodes sequence firmware always sends
> "delete" messages for this hierarchy, followed by an "add" directive to
> reconstruct it via ibm,configure-connector (log with debugging statements
> enabled in mobility.c):
>
>   mobility: removing node /ibm,platform-facilities/ibm,random-v1:4294967285
>   mobility: removing node /ibm,platform-facilities/ibm,compression-v1:4294967284
>   mobility: removing node /ibm,platform-facilities/ibm,sym-encryption-v1:4294967283
>   mobility: removing node /ibm,platform-facilities:4294967286
>   ...
>   mobility: added node /ibm,platform-facilities:4294967286
>
> Note we receive a single "add" message for the entire hierarchy, and what
> we receive from the ibm,configure-connector sequence is the top-level
> platform-facilities node along with its three children. The debug message
> simply reports the parent node and not the whole subtree.

If I understand correctly, (and again, this is not my area at all!) we
still have to go out to the firmware and call the
ibm,configure-connector sequence in order to figure out that the node
we're supposed to add is the ibm,platform-facilites node, right? We
can't save enough information at delete time to avoid the trip out to
firmware?

> Also, significantly, the nodes added are almost completely equivalent to
> the ones removed; even phandles are unchanged. ibm,shared-interrupt-pool in
> the leaf nodes is the only property I've observed to differ, and Linux does
> not use that. So in practice, the sum of update messages Linux receives for
> this hierarchy is equivalent to minor property updates.
>
> We succeed in removing the original hierarchy from the device tree. But the
> vio bus code is ignorant of this, and does not unbind or relinquish its
> references. The leaf nodes, still reachable through sysfs, of course still
> refer to the now-freed ibm,platform-facilities parent node, which makes
> use-after-free possible:
>
>   refcount_t: addition on 0; use-after-free.
>   WARNING: CPU: 3 PID: 1706 at lib/refcount.c:25 refcount_warn_saturate+0x164/0x1f0
>   refcount_warn_saturate+0x160/0x1f0 (unreliable)
>   kobject_get+0xf0/0x100
>   of_node_get+0x30/0x50
>   of_get_parent+0x50/0xb0
>   of_fwnode_get_parent+0x54/0x90
>   fwnode_count_parents+0x50/0x150
>   fwnode_full_name_string+0x30/0x110
>   device_node_string+0x49c/0x790
>   vsnprintf+0x1c0/0x4c0
>   sprintf+0x44/0x60
>   devspec_show+0x34/0x50
>   dev_attr_show+0x40/0xa0
>   sysfs_kf_seq_show+0xbc/0x200
>   kernfs_seq_show+0x44/0x60
>   seq_read_iter+0x2a4/0x740
>   kernfs_fop_read_iter+0x254/0x2e0
>   new_sync_read+0x120/0x190
>   vfs_read+0x1d0/0x240
>
> Moreover, the "new" replacement subtree is not correctly added to the
> device tree, resulting in ibm,platform-facilities parent node without the
> appropriate leaf nodes, and broken symlinks in the sysfs device hierarchy:
>
>   $ tree -d /sys/firmware/devicetree/base/ibm,platform-facilities/
>   /sys/firmware/devicetree/base/ibm,platform-facilities/
>
>   0 directories
>
>   $ cd /sys/devices/vio ; find . -xtype l -exec file {} +
>   ./ibm,sym-encryption-v1/of_node: broken symbolic link to
>     ../../../firmware/devicetree/base/ibm,platform-facilities/ibm,sym-encryption-v1
>   ./ibm,random-v1/of_node:         broken symbolic link to
>     ../../../firmware/devicetree/base/ibm,platform-facilities/ibm,random-v1
>   ./ibm,compression-v1/of_node:    broken symbolic link to
>     ../../../firmware/devicetree/base/ibm,platform-facilities/ibm,compression-v1
>
> This is because add_dt_node() -> dlpar_attach_node() attaches only the
> parent node returned from configure-connector, ignoring any children. This
> should be corrected for the general case, but fixing that won't help with
> the stale OF node references, which is the more urgent problem.
>
> One way to address that would be to make the drivers respond to node
> removal notifications, so that node references can be dropped
> appropriately. But this would likely force the drivers to disrupt active
> clients for no useful purpose: equivalent nodes are immediately re-added.
> And recall that the acceleration capabilities described by the nodes remain
> available throughout the whole process.
>
> The solution I believe to be robust for this situation is to convert
> remove+add of a node with an unchanged phandle to an update of the node's
> properties in the Linux device tree structure. That would involve changing
> and adding a fair amount of code, and may take several iterations to land.
>
> Until that can be realized we have a confirmed use-after-free and the
> possibility of memory corruption. So add a limited workaround that
> discriminates on the node type, ignoring adds and removes. This should be
> amenable to backporting in the meantime.

Yeah it's an unpleasant situation to find ourselves in. It's a bit icky
but as I think you said in a previous email, at least this isn't worse:
in the common case it should now succeed and and if properties change
significantly it will still fail.

My one question (from more of a security point of view) is:
 1) Say you start using the facilities with a particular set of
    parameters.

 2) Say you then get migrated and the parameters change.

 3) If you keep using the platform facilities as if the original
    properties are still valid, can this cause any Interesting,
    unexpected or otherwise Bad consequences? Are we going to end up
    (for example) scribbling over random memory somehow?

Apart from that, the code seems to do what it says, it seems to solve a
real problem, the error and memory handling makes sense, you _put the DT
nodes that you _get, the comments are helpful and descriptive, and it
passes the automated tests on patchwork/snowpatch.

Kind regards,
Daniel

> Signed-off-by: Nathan Lynch <nathanl at linux.ibm.com>
> Fixes: 410bccf97881 ("powerpc/pseries: Partition migration in the kernel")
> Cc: stable at vger.kernel.org
> ---
>
> Notes:
>     Changes since v1:
>     
>     * Clarify that the vio bus code maintains references to removed nodes, per
>       Tyrel.
>
>  arch/powerpc/platforms/pseries/mobility.c | 34 +++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
>
> diff --git a/arch/powerpc/platforms/pseries/mobility.c b/arch/powerpc/platforms/pseries/mobility.c
> index e83e0891272d..210a37a065fb 100644
> --- a/arch/powerpc/platforms/pseries/mobility.c
> +++ b/arch/powerpc/platforms/pseries/mobility.c
> @@ -63,6 +63,27 @@ static int mobility_rtas_call(int token, char *buf, s32 scope)
>  
>  static int delete_dt_node(struct device_node *dn)
>  {
> +	struct device_node *pdn;
> +	bool is_platfac;
> +
> +	pdn = of_get_parent(dn);
> +	is_platfac = of_node_is_type(dn, "ibm,platform-facilities") ||
> +		     of_node_is_type(pdn, "ibm,platform-facilities");
> +	of_node_put(pdn);
> +
> +	/*
> +	 * The drivers that bind to nodes in the platform-facilities
> +	 * hierarchy don't support node removal, and the removal directive
> +	 * from firmware is always followed by an add of an equivalent
> +	 * node. The capability (e.g. RNG, encryption, compression)
> +	 * represented by the node is never interrupted by the migration.
> +	 * So ignore changes to this part of the tree.
> +	 */
> +	if (is_platfac) {
> +		pr_notice("ignoring remove operation for %pOFfp\n", dn);
> +		return 0;
> +	}
> +
>  	pr_debug("removing node %pOFfp\n", dn);
>  	dlpar_detach_node(dn);
>  	return 0;
> @@ -222,6 +243,19 @@ static int add_dt_node(struct device_node *parent_dn, __be32 drc_index)
>  	if (!dn)
>  		return -ENOENT;
>  
> +	/*
> +	 * Since delete_dt_node() ignores this node type, this is the
> +	 * necessary counterpart. We also know that a platform-facilities
> +	 * node returned from dlpar_configure_connector() has children
> +	 * attached, and dlpar_attach_node() only adds the parent, leaking
> +	 * the children. So ignore these on the add side for now.
> +	 */
> +	if (of_node_is_type(dn, "ibm,platform-facilities")) {
> +		pr_notice("ignoring add operation for %pOF\n", dn);
> +		dlpar_free_cc_nodes(dn);
> +		return 0;
> +	}
> +
>  	rc = dlpar_attach_node(dn, parent_dn);
>  	if (rc)
>  		dlpar_free_cc_nodes(dn);
> -- 
> 2.31.1