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Message-Id: <20250407124637.2433230-1-james.morse@arm.com>
Date: Mon, 7 Apr 2025 13:46:37 +0100
From: James Morse <james.morse@....com>
To: x86@...nel.org,
linux-kernel@...r.kernel.org
Cc: Tony Luck <tony.luck@...el.com>,
Reinette Chatre <reinette.chatre@...el.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
babu.moger@....com,
James Morse <james.morse@....com>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH] x86/resctrl: Fix rdtgroup_mkdir()'s unlocked use of kernfs_node::name
Since commit 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+0x6c/0xa0
| lockdep_rcu_suspicious.cold+0x4e/0x96
| is_mon_groups+0xba/0xd0
| rdtgroup_mkdir+0x118/0x1970
| kernfs_iop_mkdir+0xfa/0x1a0
| vfs_mkdir+0x456/0x760
| do_mkdirat+0x257/0x310
| __x64_sys_mkdir+0xd4/0x120
| do_syscall_64+0x6d/0x150
| entry_SYSCALL_64_after_hwframe+0x76/0x7e
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.
CC: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Fixes: 741c10b096bc ("kernfs: Use RCU to access kernfs_node::name.")
Signed-off-by: James Morse <james.morse@....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 93ec829015f1..a81a42cf6656 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 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 @@ static int rdtgroup_mkdir_ctrl_mon(struct kernfs_node *parent_kn,
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;
--
2.39.5
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