[RFC PATCH 4/4] audit: use kmem_cache to manage the audit_buffer cache
Paul Moore
paul at paul-moore.com
Tue Apr 11 20:07:03 UTC 2017
On Mon, Apr 10, 2017 at 12:04 AM, Richard Guy Briggs <rgb at redhat.com> wrote:
> On 2017-03-21 14:59, Paul Moore wrote:
>> From: Paul Moore <paul at paul-moore.com>
>> The audit subsystem implemented its own buffer cache mechanism which
>> is a bit silly these days when we could use the kmem_cache construct.
>>
>> Some credit is due to Florian Westphal for originally proposing that
>> we remove the audit cache implementation in favor of simple
>> kmalloc()/kfree() calls, but I would rather have a dedicated slab
>> cache to ease debugging and future stats/performance work.
>>
>> Cc: Florian Westphal <fw at strlen.de>
>> Signed-off-by: Paul Moore <paul at paul-moore.com>
>> ---
>> kernel/audit.c | 66 ++++++++++++++------------------------------------------
>> 1 file changed, 17 insertions(+), 49 deletions(-)
>>
>> diff --git a/kernel/audit.c b/kernel/audit.c
>> index b718bf3a73f8..f78cdd75a4d2 100644
>> --- a/kernel/audit.c
>> +++ b/kernel/audit.c
>> @@ -59,6 +59,7 @@
>> #include <linux/mutex.h>
>> #include <linux/gfp.h>
>> #include <linux/pid.h>
>> +#include <linux/slab.h>
>>
>> #include <linux/audit.h>
>>
>> @@ -152,12 +153,7 @@ static atomic_t audit_lost = ATOMIC_INIT(0);
>> /* Hash for inode-based rules */
>> struct list_head audit_inode_hash[AUDIT_INODE_BUCKETS];
>>
>> -/* The audit_freelist is a list of pre-allocated audit buffers (if more
>> - * than AUDIT_MAXFREE are in use, the audit buffer is freed instead of
>> - * being placed on the freelist). */
>> -static DEFINE_SPINLOCK(audit_freelist_lock);
>> -static int audit_freelist_count;
>> -static LIST_HEAD(audit_freelist);
>> +static struct kmem_cache *audit_buffer_cache;
>>
>> /* queue msgs to send via kauditd_task */
>> static struct sk_buff_head audit_queue;
>> @@ -193,17 +189,12 @@ DEFINE_MUTEX(audit_cmd_mutex);
>> * should be at least that large. */
>> #define AUDIT_BUFSIZ 1024
>>
>> -/* AUDIT_MAXFREE is the number of empty audit_buffers we keep on the
>> - * audit_freelist. Doing so eliminates many kmalloc/kfree calls. */
>> -#define AUDIT_MAXFREE (2*NR_CPUS)
>> -
>> /* The audit_buffer is used when formatting an audit record. The caller
>> * locks briefly to get the record off the freelist or to allocate the
>> * buffer, and locks briefly to send the buffer to the netlink layer or
>> * to place it on a transmit queue. Multiple audit_buffers can be in
>> * use simultaneously. */
>> struct audit_buffer {
>> - struct list_head list;
>> struct sk_buff *skb; /* formatted skb ready to send */
>> struct audit_context *ctx; /* NULL or associated context */
>> gfp_t gfp_mask;
>> @@ -1489,6 +1480,10 @@ static int __init audit_init(void)
>> if (audit_initialized == AUDIT_DISABLED)
>> return 0;
>>
>> + audit_buffer_cache = kmem_cache_create("audit_buffer",
>> + sizeof(struct audit_buffer),
>> + 0, SLAB_PANIC, NULL);
>> +
>> memset(&auditd_conn, 0, sizeof(auditd_conn));
>> spin_lock_init(&auditd_conn.lock);
>>
>> @@ -1557,60 +1552,33 @@ __setup("audit_backlog_limit=", audit_backlog_limit_set);
>>
>> static void audit_buffer_free(struct audit_buffer *ab)
>> {
>> - unsigned long flags;
>> -
>> if (!ab)
>> return;
>>
>> kfree_skb(ab->skb);
>> - spin_lock_irqsave(&audit_freelist_lock, flags);
>> - if (audit_freelist_count > AUDIT_MAXFREE)
>> - kfree(ab);
>> - else {
>> - audit_freelist_count++;
>> - list_add(&ab->list, &audit_freelist);
>> - }
>> - spin_unlock_irqrestore(&audit_freelist_lock, flags);
>> + kmem_cache_free(audit_buffer_cache, ab);
>> }
>>
>> -static struct audit_buffer * audit_buffer_alloc(struct audit_context *ctx,
>> - gfp_t gfp_mask, int type)
>> +static struct audit_buffer *audit_buffer_alloc(struct audit_context *ctx,
>> + gfp_t gfp_mask, int type)
>> {
>> - unsigned long flags;
>> - struct audit_buffer *ab = NULL;
>> - struct nlmsghdr *nlh;
>> -
>> - spin_lock_irqsave(&audit_freelist_lock, flags);
>> - if (!list_empty(&audit_freelist)) {
>> - ab = list_entry(audit_freelist.next,
>> - struct audit_buffer, list);
>> - list_del(&ab->list);
>> - --audit_freelist_count;
>> - }
>> - spin_unlock_irqrestore(&audit_freelist_lock, flags);
>> -
>> - if (!ab) {
>> - ab = kmalloc(sizeof(*ab), gfp_mask);
>> - if (!ab)
>> - goto err;
>> - }
>> + struct audit_buffer *ab;
>>
>> - ab->ctx = ctx;
>> - ab->gfp_mask = gfp_mask;
>> + ab = kmem_cache_alloc(audit_buffer_cache, gfp_mask);
>> + if (!ab)
>> + return NULL;
>>
>> ab->skb = nlmsg_new(AUDIT_BUFSIZ, gfp_mask);
>> if (!ab->skb)
>> goto err;
>> + if (!nlmsg_put(ab->skb, 0, 0, type, 0, 0))
>> + goto err;
>>
>> - nlh = nlmsg_put(ab->skb, 0, 0, type, 0, 0);
>> - if (!nlh)
>> - goto out_kfree_skb;
>
> Is there a reason to care about an error returned from nlmsg_put() if
> you aren't going to free the skb that was allocated? If you think
> nlmsg_put() can't fail due to extremely simple calling arguments then
> there is no need to check its return code.
>
> If nlmsg_new() succeeds, it has allocated an skb. If nlmsg_put() fails,
> you free the audit_buffer and the skb is now a memory leak.
>
> Have I read this correctly?
Check my math, but in the patched code if the nlmsg_put() call fails
then we jump to "err" which calls audit_buffer_free() which in turn
calls kfree_skb() on ab->skb so I don't believe we have a memory leak
on error ... I'll hold off on merging this in case I'm missing
something, but I'm pretty sure we're okay here.
> Otherwise, I like the intent of this simplification.
>
>> + ab->ctx = ctx;
>> + ab->gfp_mask = gfp_mask;
>>
>> return ab;
>>
>> -out_kfree_skb:
>> - kfree_skb(ab->skb);
>> - ab->skb = NULL;
>> err:
>> audit_buffer_free(ab);
>> return NULL;
>
> - RGB
>
> --
> Richard Guy Briggs <rgb at redhat.com>
> Sr. S/W Engineer, Kernel Security, Base Operating Systems
> Remote, Ottawa, Red Hat Canada
> IRC: rgb, SunRaycer
> Voice: +1.647.777.2635, Internal: (81) 32635
>
> --
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--
paul moore
www.paul-moore.com
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