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Message-ID: <CAEjxPJ4-+WfGPLev5QU_+_NgBx68zdBBQ6x_+aonzbk4f9BNEw@mail.gmail.com>
Date: Mon, 8 Sep 2025 09:45:03 -0400
From: Stephen Smalley <stephen.smalley.work@...il.com>
To: Hongru Zhang <zhanghongru06@...il.com>
Cc: paul@...l-moore.com, omosnace@...hat.com, linux-kernel@...r.kernel.org,
selinux@...r.kernel.org, Hongru Zhang <zhanghongru@...omi.com>
Subject: Re: [PATCH] selinux: Make avc cache slot size configurable during boot
On Fri, Sep 5, 2025 at 6:05 AM Hongru Zhang <zhanghongru06@...il.com> wrote:
>
> 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);
If you are making the number of buckets adjustable, you should also
change the hash function to better deal with multiple numbers of
slots.
> }
>
> /**
> @@ -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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