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    Subject[PATCH 5.14 841/849] powerpc/pseries/mobility: ignore ibm, platform-facilities updates
    Date
    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);

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