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Message-ID: <20250905100454.685866-1-zhanghongru@xiaomi.com>
Date: Fri, 5 Sep 2025 18:04:54 +0800
From: Hongru Zhang <zhanghongru06@...il.com>
To: paul@...l-moore.com,
stephen.smalley.work@...il.com,
omosnace@...hat.com
Cc: linux-kernel@...r.kernel.org,
selinux@...r.kernel.org,
Hongru Zhang <zhanghongru@...omi.com>
Subject: [PATCH] selinux: Make avc cache slot size configurable during boot
From: Hongru Zhang <zhanghongru@...omi.com>
On mobile device high-load situations, permission check can happen
more than 90,000/s (8 core system). With default 512 cache nodes
configuration, avc cache miss happens more often and occasionally
leads to long time (>2ms) irqs off on both big and little cores,
which decreases system real-time capability.
An actual call stack is as follows:
=> avc_compute_av
=> avc_perm_nonode
=> avc_has_perm_noaudit
=> selinux_capable
=> security_capable
=> capable
=> __sched_setscheduler
=> do_sched_setscheduler
=> __arm64_sys_sched_setscheduler
=> invoke_syscall
=> el0_svc_common
=> do_el0_svc
=> el0_svc
=> el0t_64_sync_handler
=> el0t_64_sync
Although we can expand avc nodes through /sys/fs/selinux/cache_threshold
to mitigate long time irqs off, hash conflicts make the bucket average
length longer because of the fixed size of cache slots, leading to
avc_search_node latency increase.
Make avc cache slot size also configurable, and with fine tuning, we can
mitigate long time irqs off with slightly avc_search_node performance
regression.
Theoretically, the main overhead is memory consumption.
avc_search_node avg latency test results (about 100,000,000 times) on
Qcom SM8750, 6.6.30-android15-8:
Case 1:
+---------+---------------------+------------------------+
| | no-patch (512/512) | with-patch (512/512) |
+---------+---------------------+------------------------+
| latency | 85 ns | 87 ns |
+---------+---------------------+------------------------+
Case 2:
+---------+---------------------+------------------------+
| | no-patch (8192/512) | with-patch (8192/8192) |
+---------+---------------------+------------------------+
| latency | 277 ns | 106 ns |
+---------+---------------------+------------------------+
Case 1 shows 512 nodes configuration has ~2% performance regression
with patch.
Case 2 shows 8192 nodes configuration has ~61% latency benifit with
patch.
Signed-off-by: Hongru Zhang <zhanghongru@...omi.com>
---
.../admin-guide/kernel-parameters.txt | 4 ++
security/selinux/avc.c | 68 +++++++++++++------
2 files changed, 50 insertions(+), 22 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 747a55abf494..70dc6d659117 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6620,6 +6620,10 @@
1 -- enable.
Default value is 1.
+ selinux_avc_cache_slots= [SELINUX] Set the avc cache slot size.
+ Format: <int> (must be >0, power of 2)
+ Default: 512
+
serialnumber [BUGS=X86-32]
sev=option[,option...] [X86-64]
diff --git a/security/selinux/avc.c b/security/selinux/avc.c
index 430b0e23ee00..35f5436f5da0 100644
--- a/security/selinux/avc.c
+++ b/security/selinux/avc.c
@@ -34,7 +34,7 @@
#define CREATE_TRACE_POINTS
#include <trace/events/avc.h>
-#define AVC_CACHE_SLOTS 512
+int avc_cache_slots __ro_after_init = 512;
#define AVC_DEF_CACHE_THRESHOLD 512
#define AVC_CACHE_RECLAIM 16
@@ -68,9 +68,13 @@ struct avc_xperms_node {
struct list_head xpd_head; /* list head of extended_perms_decision */
};
+struct avc_slot {
+ struct hlist_head slot; /* head for avc_node->list */
+ spinlock_t slot_lock; /* lock for writes */
+};
+
struct avc_cache {
- struct hlist_head slots[AVC_CACHE_SLOTS]; /* head for avc_node->list */
- spinlock_t slots_lock[AVC_CACHE_SLOTS]; /* lock for writes */
+ struct avc_slot *slots;
atomic_t lru_hint; /* LRU hint for reclaim scan */
atomic_t active_nodes;
u32 latest_notif; /* latest revocation notification */
@@ -93,14 +97,34 @@ struct selinux_avc {
static struct selinux_avc selinux_avc;
+static int __init set_selinux_avc_cache_slots(char *str)
+{
+ int val;
+
+ if ((kstrtoint(str, 0, &val)) || !is_power_of_2(val)) {
+ pr_warn("Unable to set selinux_avc_cache_slots, use default value\n");
+ return 1;
+ }
+
+ avc_cache_slots = val;
+
+ return 1;
+}
+__setup("selinux_avc_cache_slots=", set_selinux_avc_cache_slots);
+
void selinux_avc_init(void)
{
int i;
+ selinux_avc.avc_cache.slots =
+ kmalloc_array(avc_cache_slots, sizeof(struct avc_slot), GFP_KERNEL);
+ if (!selinux_avc.avc_cache.slots)
+ panic("SELinux: No memory to alloc avc cache slots\n");
+
selinux_avc.avc_cache_threshold = AVC_DEF_CACHE_THRESHOLD;
- for (i = 0; i < AVC_CACHE_SLOTS; i++) {
- INIT_HLIST_HEAD(&selinux_avc.avc_cache.slots[i]);
- spin_lock_init(&selinux_avc.avc_cache.slots_lock[i]);
+ for (i = 0; i < avc_cache_slots; i++) {
+ INIT_HLIST_HEAD(&selinux_avc.avc_cache.slots[i].slot);
+ spin_lock_init(&selinux_avc.avc_cache.slots[i].slot_lock);
}
atomic_set(&selinux_avc.avc_cache.active_nodes, 0);
atomic_set(&selinux_avc.avc_cache.lru_hint, 0);
@@ -124,7 +148,7 @@ static struct kmem_cache *avc_xperms_cachep __ro_after_init;
static inline u32 avc_hash(u32 ssid, u32 tsid, u16 tclass)
{
- return (ssid ^ (tsid<<2) ^ (tclass<<4)) & (AVC_CACHE_SLOTS - 1);
+ return (ssid ^ (tsid<<2) ^ (tclass<<4)) & (avc_cache_slots - 1);
}
/**
@@ -150,8 +174,8 @@ int avc_get_hash_stats(char *page)
slots_used = 0;
max_chain_len = 0;
- for (i = 0; i < AVC_CACHE_SLOTS; i++) {
- head = &selinux_avc.avc_cache.slots[i];
+ for (i = 0; i < avc_cache_slots; i++) {
+ head = &selinux_avc.avc_cache.slots[i].slot;
if (!hlist_empty(head)) {
slots_used++;
chain_len = 0;
@@ -167,7 +191,7 @@ int avc_get_hash_stats(char *page)
return scnprintf(page, PAGE_SIZE, "entries: %d\nbuckets used: %d/%d\n"
"longest chain: %d\n",
atomic_read(&selinux_avc.avc_cache.active_nodes),
- slots_used, AVC_CACHE_SLOTS, max_chain_len);
+ slots_used, avc_cache_slots, max_chain_len);
}
/*
@@ -463,11 +487,11 @@ static inline int avc_reclaim_node(void)
struct hlist_head *head;
spinlock_t *lock;
- for (try = 0, ecx = 0; try < AVC_CACHE_SLOTS; try++) {
+ for (try = 0, ecx = 0; try < avc_cache_slots; try++) {
hvalue = atomic_inc_return(&selinux_avc.avc_cache.lru_hint) &
- (AVC_CACHE_SLOTS - 1);
- head = &selinux_avc.avc_cache.slots[hvalue];
- lock = &selinux_avc.avc_cache.slots_lock[hvalue];
+ (avc_cache_slots - 1);
+ head = &selinux_avc.avc_cache.slots[hvalue].slot;
+ lock = &selinux_avc.avc_cache.slots[hvalue].slot_lock;
if (!spin_trylock_irqsave(lock, flags))
continue;
@@ -524,7 +548,7 @@ static inline struct avc_node *avc_search_node(u32 ssid, u32 tsid, u16 tclass)
struct hlist_head *head;
hvalue = avc_hash(ssid, tsid, tclass);
- head = &selinux_avc.avc_cache.slots[hvalue];
+ head = &selinux_avc.avc_cache.slots[hvalue].slot;
hlist_for_each_entry_rcu(node, head, list) {
if (ssid == node->ae.ssid &&
tclass == node->ae.tclass &&
@@ -625,8 +649,8 @@ static void avc_insert(u32 ssid, u32 tsid, u16 tclass,
}
hvalue = avc_hash(ssid, tsid, tclass);
- head = &selinux_avc.avc_cache.slots[hvalue];
- lock = &selinux_avc.avc_cache.slots_lock[hvalue];
+ head = &selinux_avc.avc_cache.slots[hvalue].slot;
+ lock = &selinux_avc.avc_cache.slots[hvalue].slot_lock;
spin_lock_irqsave(lock, flag);
hlist_for_each_entry(pos, head, list) {
if (pos->ae.ssid == ssid &&
@@ -846,8 +870,8 @@ static int avc_update_node(u32 event, u32 perms, u8 driver, u8 base_perm,
/* Lock the target slot */
hvalue = avc_hash(ssid, tsid, tclass);
- head = &selinux_avc.avc_cache.slots[hvalue];
- lock = &selinux_avc.avc_cache.slots_lock[hvalue];
+ head = &selinux_avc.avc_cache.slots[hvalue].slot;
+ lock = &selinux_avc.avc_cache.slots[hvalue].slot_lock;
spin_lock_irqsave(lock, flag);
@@ -929,9 +953,9 @@ static void avc_flush(void)
unsigned long flag;
int i;
- for (i = 0; i < AVC_CACHE_SLOTS; i++) {
- head = &selinux_avc.avc_cache.slots[i];
- lock = &selinux_avc.avc_cache.slots_lock[i];
+ for (i = 0; i < avc_cache_slots; i++) {
+ head = &selinux_avc.avc_cache.slots[i].slot;
+ lock = &selinux_avc.avc_cache.slots[i].slot_lock;
spin_lock_irqsave(lock, flag);
/*
--
2.43.0
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