linux-kernel - Re: [PATCH -next 1/1] audit: ignore userspace log when backlog limit exceeded and backlog_wait

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Date:   Tue, 28 Dec 2021 20:20:51 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     luhuaxin <luhuaxin1@...wei.com>
Cc:     eparis@...hat.com, linux-audit@...hat.com,
        linux-kernel@...r.kernel.org, fangxiuning@...wei.com
Subject: Re: [PATCH -next 1/1] audit: ignore userspace log when backlog limit
 exceeded and backlog_wait_time is 0

On Tue, Dec 28, 2021 at 1:50 AM luhuaxin <luhuaxin1@...wei.com> wrote:
>
> When the backlog exceed the backlog_limit and backlog_wait_time is set
> to 0, the process will only sleep for a very short time (jiffies). The
> backlog may still exceed backlog_limit in extreme cases.
>
> The more reasonable way to fix this problem is:
>
> 1. If backlog_wait_time is set to zero, ignore the log;
> 2. If backlog_wait_time is set to non-zero, let process sleep for
> backlog_wait_time.
>
> The above log limit logic is also the same as that in the existing
> audit_log_start function.
>
> Fixes: 8f110f530635 ("[PATCH] audit: ensure userspace is penalized the
>   same as the kernel when under pressure")

One quick comment on the "Fixes" tag above: you shouldn't add the
"[PATCH]" string to the commit's subject, you should use the commit
subject that you would see if you typed `git log --oneline`.  It also
shouldn't be word-wrapped, it should be all on one line in your
patch/email.

Regardless of the above, I don't think this is a patch we want to
merge upstream.  I can understand the desire to improve performance,
but this doesn't seem appropriate to me; adjusting the
backlog_wait_time to very low values just so you can drop audit
records is not an approach we want to advocate, or support, upstream.

> Signed-off-by: luhuaxin <luhuaxin1@...wei.com>
> ---
>  kernel/audit.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/audit.c b/kernel/audit.c
> index 249e11628..70450f70a 100644
> --- a/kernel/audit.c
> +++ b/kernel/audit.c
> @@ -1545,7 +1545,8 @@ static void audit_receive(struct sk_buff  *skb)
>
>         /* can't block with the ctrl lock, so penalize the sender now */
>         if (audit_backlog_limit &&
> -           (skb_queue_len(&audit_queue) > audit_backlog_limit)) {
> +           (skb_queue_len(&audit_queue) > audit_backlog_limit) &&
> +           audit_backlog_wait_time) {
>                 DECLARE_WAITQUEUE(wait, current);
>
>                 /* wake kauditd to try and flush the queue */
> @@ -1842,9 +1843,8 @@ struct audit_buffer *audit_log_start(struct audit_context *ctx, gfp_t gfp_mask,
>          *    while holding the mutex, although we do penalize the sender
>          *    later in audit_receive() when it is safe to block
>          */
> +       long stime = audit_backlog_wait_time;
>         if (!(auditd_test_task(current) || audit_ctl_owner_current())) {
> -               long stime = audit_backlog_wait_time;
> -
>                 while (audit_backlog_limit &&
>                        (skb_queue_len(&audit_queue) > audit_backlog_limit)) {
>                         /* wake kauditd to try and flush the queue */
> @@ -1872,6 +1872,14 @@ struct audit_buffer *audit_log_start(struct audit_context *ctx, gfp_t gfp_mask,
>                                 return NULL;
>                         }
>                 }
> +       } else if (!stime && audit_backlog_limit &&
> +                  (skb_queue_len(&audit_queue) > audit_backlog_limit)) {
> +               if (audit_rate_check() && printk_ratelimit())
> +                       pr_warn("audit_backlog=%d > audit_backlog_limit=%d\n",
> +                               skb_queue_len(&audit_queue),
> +                               audit_backlog_limit);
> +               audit_log_lost("backlog limit exceeded");
> +               return NULL;
>         }
>
>         ab = audit_buffer_alloc(ctx, gfp_mask, type);
> --
> 2.23.0

-- 
paul moore
www.paul-moore.com