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    Subject[PATCH 5.5 62/65] x86/resctrl: Fix invalid attempt at removing the default resource group
    Date
    From: Reinette Chatre <reinette.chatre@intel.com>

    commit b0151da52a6d4f3951ea24c083e7a95977621436 upstream.

    The default resource group ("rdtgroup_default") is associated with the
    root of the resctrl filesystem and should never be removed. New resource
    groups can be created as subdirectories of the resctrl filesystem and
    they can be removed from user space.

    There exists a safeguard in the directory removal code
    (rdtgroup_rmdir()) that ensures that only subdirectories can be removed
    by testing that the directory to be removed has to be a child of the
    root directory.

    A possible deadlock was recently fixed with

    334b0f4e9b1b ("x86/resctrl: Fix a deadlock due to inaccurate reference").

    This fix involved associating the private data of the "mon_groups"
    and "mon_data" directories to the resource group to which they belong
    instead of NULL as before. A consequence of this change was that
    the original safeguard code preventing removal of "mon_groups" and
    "mon_data" found in the root directory failed resulting in attempts to
    remove the default resource group that ends in a BUG:

    kernel BUG at mm/slub.c:3969!
    invalid opcode: 0000 [#1] SMP PTI

    Call Trace:
    rdtgroup_rmdir+0x16b/0x2c0
    kernfs_iop_rmdir+0x5c/0x90
    vfs_rmdir+0x7a/0x160
    do_rmdir+0x17d/0x1e0
    do_syscall_64+0x55/0x1d0
    entry_SYSCALL_64_after_hwframe+0x44/0xa9

    Fix this by improving the directory removal safeguard to ensure that
    subdirectories of the resctrl root directory can only be removed if they
    are a child of the resctrl filesystem's root _and_ not associated with
    the default resource group.

    Fixes: 334b0f4e9b1b ("x86/resctrl: Fix a deadlock due to inaccurate reference")
    Reported-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
    Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
    Signed-off-by: Borislav Petkov <bp@suse.de>
    Tested-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
    Cc: stable@vger.kernel.org
    Link: https://lkml.kernel.org/r/884cbe1773496b5dbec1b6bd11bb50cffa83603d.1584461853.git.reinette.chatre@intel.com
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/kernel/cpu/resctrl/rdtgroup.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
    +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
    @@ -3006,7 +3006,8 @@ static int rdtgroup_rmdir(struct kernfs_
    * If the rdtgroup is a mon group and parent directory
    * is a valid "mon_groups" directory, remove the mon group.
    */
    - if (rdtgrp->type == RDTCTRL_GROUP && parent_kn == rdtgroup_default.kn) {
    + if (rdtgrp->type == RDTCTRL_GROUP && parent_kn == rdtgroup_default.kn &&
    + rdtgrp != &rdtgroup_default) {
    if (rdtgrp->mode == RDT_MODE_PSEUDO_LOCKSETUP ||
    rdtgrp->mode == RDT_MODE_PSEUDO_LOCKED) {
    ret = rdtgroup_ctrl_remove(kn, rdtgrp);

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