From: Nathan Lynch nathanl@linux.ibm.com
commit 319fa1a52e438a6e028329187783a25ad498c4e6 upstream.
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.
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.
Fixes: 410bccf97881 ("powerpc/pseries: Partition migration in the kernel") Cc: stable@vger.kernel.org Signed-off-by: Nathan Lynch nathanl@linux.ibm.com Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20211020194703.2613093-1-nathanl@linux.ibm.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/powerpc/platforms/pseries/mobility.c | 34 ++++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+)
--- 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,
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_nod 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);