[tip:x86/cache] x86/intel_rdt: Respect read and write access

[tip-bot for Reinette Chatre <tipbot@zytor.com>]

Commit-ID:  21220bb199f7d65c8f0a63ac7d3209e40fbdd706
Gitweb:     https://git.kernel.org/tip/21220bb199f7d65c8f0a63ac7d3209e40fbdd706
Author:     Reinette Chatre <reinette.chatre@intel.com>
AuthorDate: Fri, 22 Jun 2018 15:42:09 -0700
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Sat, 23 Jun 2018 13:03:45 +0200

x86/intel_rdt: Respect read and write access

By default, if the opener has CAP_DAC_OVERRIDE, a kernfs file can be opened
regardless of RW permissions. Writing to a kernfs file will thus succeed
even if permissions are 0000.

It's required to restrict the actions that can be performed on a resource
group from userspace based on the mode of the resource group.  This
restriction will be done through a modification of the file
permissions. That is, for example, if a resource group is locked then the
user cannot add tasks to the resource group.

For this restriction through file permissions to work it has to be ensured
that the permissions are always respected. To do so the resctrl filesystem
is created with the KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK flag that will result
in open(2) failing with -EACCESS regardless of CAP_DAC_OVERRIDE if the
permission does not have the respective read or write access.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: fenghua.yu@intel.com
Cc: tony.luck@intel.com
Cc: vikas.shivappa@linux.intel.com
Cc: gavin.hindman@intel.com
Cc: jithu.joseph@intel.com
Cc: dave.hansen@intel.com
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/26f4fc25f110bfc07c2d2c8b2c4ee904922fedf7.1529706536.git.reinette.chatre@intel.com

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

diff --git a/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c b/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
index fef92b939f0f..6eb716765a3f 100644
--- a/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
+++ b/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
@@ -2545,7 +2545,8 @@ static int __init rdtgroup_setup_root(void)
 	int ret;
 
 	rdt_root = kernfs_create_root(&rdtgroup_kf_syscall_ops,
-				      KERNFS_ROOT_CREATE_DEACTIVATED,
+				      KERNFS_ROOT_CREATE_DEACTIVATED |
+				      KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK,
 				      &rdtgroup_default);
 	if (IS_ERR(rdt_root))
 		return PTR_ERR(rdt_root);