[tip: x86/urgent] x86/resctrl: Fix rdtgroup_mkdir()'s unlocked use of kernfs_node::name

tip-bot2 for James Morse
2025-04-09T10:11:23Z

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: 45c2e30bbd64d75559b99f3a5c455129a7a34b06
Gitweb: https://git.kernel.org/tip/45c2e30bbd64d75559b99f3a5c455129a7a34b06
Author: James Morse <james.morse@arm.com>
AuthorDate: Mon, 07 Apr 2025 13:46:37 +01:00
Committer: Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Wed, 09 Apr 2025 11:35:08 +02:00

x86/resctrl: Fix rdtgroup_mkdir()'s unlocked use of kernfs_node::name

Since

741c10b096bc ("kernfs: Use RCU to access kernfs_node::name.")

a helper rdt_kn_name() that checks that rdtgroup_mutex is held has been used
for all accesses to the kernfs node name.

rdtgroup_mkdir() uses the name to determine if a valid monitor group is being
created by checking the parent name is "mon_groups". This is done without
holding rdtgroup_mutex, and now triggers the following warning:

| WARNING: suspicious RCU usage
| 6.15.0-rc1 #4465 Tainted: G E
| -----------------------------
| arch/x86/kernel/cpu/resctrl/internal.h:408 suspicious rcu_dereference_check() usage!
[...]
| Call Trace:
| <TASK>
| dump_stack_lvl
| lockdep_rcu_suspicious.cold
| is_mon_groups
| rdtgroup_mkdir
| kernfs_iop_mkdir
| vfs_mkdir
| do_mkdirat
| __x64_sys_mkdir
| do_syscall_64
| entry_SYSCALL_64_after_hwframe

Creating a control or monitor group calls mkdir_rdt_prepare(), which uses
rdtgroup_kn_lock_live() to take the rdtgroup_mutex.

To avoid taking and dropping the lock, move the check for the monitor group
name and position into mkdir_rdt_prepare() so that it occurs under
rdtgroup_mutex. Hoist is_mon_groups() earlier in the file.

[ bp: Massage. ]

Fixes: 741c10b096bc ("kernfs: Use RCU to access kernfs_node::name.")
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250407124637.2433230-1-james.morse@arm.com
---
arch/x86/kernel/cpu/resctrl/rdtgroup.c | 48 ++++++++++++++-----------
1 file changed, 27 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index 93ec829..cc4a541 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -3553,6 +3553,22 @@ static void mkdir_rdt_prepare_rmid_free(struct rdtgroup *rgrp)
free_rmid(rgrp->closid, rgrp->mon.rmid);
}

+/*
+ * We allow creating mon groups only with in a directory called "mon_groups"
+ * which is present in every ctrl_mon group. Check if this is a valid
+ * "mon_groups" directory.
+ *
+ * 1. The directory should be named "mon_groups".
+ * 2. The mon group itself should "not" be named "mon_groups".
+ * This makes sure "mon_groups" directory always has a ctrl_mon group
+ * as parent.
+ */
+static bool is_mon_groups(struct kernfs_node *kn, const char *name)
+{
+ return (!strcmp(rdt_kn_name(kn), "mon_groups") &&
+ strcmp(name, "mon_groups"));
+}
+
static int mkdir_rdt_prepare(struct kernfs_node *parent_kn,
const char *name, umode_t mode,
enum rdt_group_type rtype, struct rdtgroup **r)
@@ -3568,6 +3584,15 @@ static int mkdir_rdt_prepare(struct kernfs_node *parent_kn,
goto out_unlock;
}

+ /*
+ * Check that the parent directory for a monitor group is a "mon_groups"
+ * directory.
+ */
+ if (rtype == RDTMON_GROUP && !is_mon_groups(parent_kn, name)) {
+ ret = -EPERM;
+ goto out_unlock;
+ }
+
if (rtype == RDTMON_GROUP &&
(prdtgrp->mode == RDT_MODE_PSEUDO_LOCKSETUP ||
prdtgrp->mode == RDT_MODE_PSEUDO_LOCKED)) {
@@ -3751,22 +3776,6 @@ out_unlock:
return ret;
}

-/*
- * We allow creating mon groups only with in a directory called "mon_groups"
- * which is present in every ctrl_mon group. Check if this is a valid
- * "mon_groups" directory.
- *
- * 1. The directory should be named "mon_groups".
- * 2. The mon group itself should "not" be named "mon_groups".
- * This makes sure "mon_groups" directory always has a ctrl_mon group
- * as parent.
- */
-static bool is_mon_groups(struct kernfs_node *kn, const char *name)
-{
- return (!strcmp(rdt_kn_name(kn), "mon_groups") &&
- strcmp(name, "mon_groups"));
-}
-
static int rdtgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
umode_t mode)
{
@@ -3782,11 +3791,8 @@ static int rdtgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
if (resctrl_arch_alloc_capable() && parent_kn == rdtgroup_default.kn)
return rdtgroup_mkdir_ctrl_mon(parent_kn, name, mode);

- /*
- * If RDT monitoring is supported and the parent directory is a valid
- * "mon_groups" directory, add a monitoring subdirectory.
- */
- if (resctrl_arch_mon_capable() && is_mon_groups(parent_kn, name))
+ /* Else, attempt to add a monitoring subdirectory. */
+ if (resctrl_arch_mon_capable())
return rdtgroup_mkdir_mon(parent_kn, name, mode);

return -EPERM;