[PATCH] powerpc/pseries/mobility: ignore ibm, platform-facilities updates
Tyrel Datwyler
tyreld at linux.ibm.com
Tue Oct 19 09:37:08 AEDT 2021
On 10/18/21 9:34 AM, Nathan Lynch wrote:
> 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
It always bothered me that the update from firmware here was an delete/add at
the entire '/ibm,platform-facilities' node level instead of update properties
calls. When I asked about it years ago the claim was it was just easier to pass
an entire sub-tree as a single node add.
>
> 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.
>
> 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.
The two props I would think maybe we would need to be most be concerned about
ensuring don't change are "ibm,resource-id" which gets used in the vio bus code
when configuring platform facilities nodes, and 'ibm,max-sync-cop' used by the
pseries-nx driver.
>
> We succeed in removing the original hierarchy from the device tree. But the
> drivers bound to the leaf nodes are ignorant of this, and do not relinquish
> their 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:
It is actually more subtle then the drivers themselves being ignorant. They
could register node update notifiers, but the real problem here is that the vio
bus device itself takes a reference to each child node registered to the bus.
I'm not sure we really want to unbind/rebind drivers as a result of LPM, but it
would be generic to the vio bus instead of updating each driver to ensure its
handling it device node references properly.
>
> 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.
I don't follow. If the code path you mention is truly broken in the way you say
then DLPAR operations involving nodes with child nodes should also be broken.
Last I had seen was that sysfs adds of the new nodes got renamed because the old
nodes still existed as a result of there reference count not going to zero. Has
this behavior changed, or am I misremembering the observed behavior?
>
> 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.
See my comments above about its the vio bus more at fault here then the drivers
themselves. I'm inclined to agree though that disrupting active operations with
a driver unbind/rebind is a little extreme.
This also brings me back to firmware removing and re-adding the whole
'/ibm,platform-facilities' node instead of simply updating changed properties
could avoid this whole fiasco.
>
> 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.
The reality is that '/ibm,platform-facilities' and 'cache' nodes are the only
LPM scoped device tree nodes that allow node delete/add. So, as a one-off
workaround to deal with what I consider a bad firmware approach I think this is
probably the best approach barring getting firmware to move to an update
properties approach.
An audit of the drivers is probably still a valid exercise to ensure any device
tree props they care about they pick up a new value should it change.
-Tyrel
>
> 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
> ---
> 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);
>
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