Re: [Patch net] net_sched: fix a memory leak of filter chain

[Cong Wang]

On Wed, Sep 6, 2017 at 12:38 AM, Jiri Pirko  wrote:
> Wed, Sep 06, 2017 at 07:03:10AM CEST, xiyou.wangc...@gmail.com wrote:
>>tcf_chain_destroy() is called by tcf_block_put() and tcf_chain_put().
>>tcf_chain_put() is refcn'ed and paired with tcf_chain_get(),
>>but tcf_block_put() is not, it should be paired with tcf_block_get()
>>and we still need to decrease the refcnt. However, tcf_block_put()
>>is special, it stores the chains too, we have to detach them if
>>it is not the last user.
>
> You don't describe the original issue, or I am missing that from your
> description.

The original issue is the mismatch of tcf_block_put() and tcf_block_get()
w.r.t. refcnt. Think it in this way: if you call tcf_bock_put() immediately
after tcf_block_get(), would you get effectively a nop?


>
>
>>
>>What's more, index 0 is not special at all, it should be treated
>>like other chains. This also makes the code more readable.
>
> [...]
>
>
>>@@ -246,10 +246,7 @@ EXPORT_SYMBOL(tcf_chain_get);
>>
>> void tcf_chain_put(struct tcf_chain *chain)
>> {
>>-  /* Destroy unused chain, with exception of chain 0, which is the
>>-   * default one and has to be always present.
>>-   */
>>-  if (--chain->refcnt == 0 && !chain->filter_chain && chain->index != 0)
>>+  if (--chain->refcnt == 0)
>
> The refcounting is only done for actions holding reference to the chain.
> You still need to check is the filter chain is not empty.
> See tc_ctl_tfilter.

With my patch refcnt is done for block too, if you notice the
tcf_chain_put() in tcf_block_put().


>
> Also, chain 0 is created by default on a block creation. It has to be
> present always for a reason. Please see tcf_block_get. The pointer to
> chain 0 is assigned to the qdisc filter list pointer.

Sure, this is why block holds a refcnt to chain (not just chain 0) with
my patch, aka why the initial refcnt is 1 rather than 0.

Re: [Patch net] net_sched: fix a memory leak of filter chain

[Jiri Pirko]

Wed, Sep 06, 2017 at 07:03:10AM CEST, xiyou.wangc...@gmail.com wrote:
>tcf_chain_destroy() is called by tcf_block_put() and tcf_chain_put().
>tcf_chain_put() is refcn'ed and paired with tcf_chain_get(),
>but tcf_block_put() is not, it should be paired with tcf_block_get()
>and we still need to decrease the refcnt. However, tcf_block_put()
>is special, it stores the chains too, we have to detach them if
>it is not the last user.

You don't describe the original issue, or I am missing that from your
description.


>
>What's more, index 0 is not special at all, it should be treated
>like other chains. This also makes the code more readable.

[...]


>@@ -246,10 +246,7 @@ EXPORT_SYMBOL(tcf_chain_get);
> 
> void tcf_chain_put(struct tcf_chain *chain)
> {
>-  /* Destroy unused chain, with exception of chain 0, which is the
>-   * default one and has to be always present.
>-   */
>-  if (--chain->refcnt == 0 && !chain->filter_chain && chain->index != 0)
>+  if (--chain->refcnt == 0)

The refcounting is only done for actions holding reference to the chain.
You still need to check is the filter chain is not empty.
See tc_ctl_tfilter.

Also, chain 0 is created by default on a block creation. It has to be
present always for a reason. Please see tcf_block_get. The pointer to
chain 0 is assigned to the qdisc filter list pointer.



>   tcf_chain_destroy(chain);
> }
> EXPORT_SYMBOL(tcf_chain_put);
>@@ -296,8 +293,11 @@ void tcf_block_put(struct tcf_block *block)
> 
>   list_for_each_entry_safe(chain, tmp, >chain_list, list) {
>   tcf_chain_flush(chain);
>-  tcf_chain_destroy(chain);
>+  tcf_chain_put(chain);
>   }
>+  /* If tc actions still hold the chain, just detach it. */
>+  list_for_each_entry_safe(chain, tmp, >chain_list, list)
>+  tcf_chain_detach(chain);
>   kfree(block);
> }
> EXPORT_SYMBOL(tcf_block_put);
>-- 
>2.13.0
>

[Patch net] net_sched: fix a memory leak of filter chain

[Cong Wang]

tcf_chain_destroy() is called by tcf_block_put() and tcf_chain_put().
tcf_chain_put() is refcn'ed and paired with tcf_chain_get(),
but tcf_block_put() is not, it should be paired with tcf_block_get()
and we still need to decrease the refcnt. However, tcf_block_put()
is special, it stores the chains too, we have to detach them if
it is not the last user.

What's more, index 0 is not special at all, it should be treated
like other chains. This also makes the code more readable.

Fixes: 744a4cf63e52 ("net: sched: fix use after free when tcf_chain_destroy is 
called multiple times")
Reported-by: Jakub Kicinski 
Cc: Jiri Pirko 
Signed-off-by: Cong Wang 
---
 net/sched/cls_api.c | 22 +++---
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
index 6c5ea84d2682..c6d25b29bcd4 100644
--- a/net/sched/cls_api.c
+++ b/net/sched/cls_api.c
@@ -213,17 +213,17 @@ static void tcf_chain_flush(struct tcf_chain *chain)
}
 }
 
-static void tcf_chain_destroy(struct tcf_chain *chain)
+static void tcf_chain_detach(struct tcf_chain *chain)
 {
/* May be already removed from the list by the previous call. */
if (!list_empty(>list))
list_del_init(>list);
+}
 
-   /* There might still be a reference held when we got here from
-* tcf_block_put. Wait for the user to drop reference before free.
-*/
-   if (!chain->refcnt)
-   kfree(chain);
+static void tcf_chain_destroy(struct tcf_chain *chain)
+{
+   tcf_chain_detach(chain);
+   kfree(chain);
 }
 
 struct tcf_chain *tcf_chain_get(struct tcf_block *block, u32 chain_index,
@@ -246,10 +246,7 @@ EXPORT_SYMBOL(tcf_chain_get);
 
 void tcf_chain_put(struct tcf_chain *chain)
 {
-   /* Destroy unused chain, with exception of chain 0, which is the
-* default one and has to be always present.
-*/
-   if (--chain->refcnt == 0 && !chain->filter_chain && chain->index != 0)
+   if (--chain->refcnt == 0)
tcf_chain_destroy(chain);
 }
 EXPORT_SYMBOL(tcf_chain_put);
@@ -296,8 +293,11 @@ void tcf_block_put(struct tcf_block *block)
 
list_for_each_entry_safe(chain, tmp, >chain_list, list) {
tcf_chain_flush(chain);
-   tcf_chain_destroy(chain);
+   tcf_chain_put(chain);
}
+   /* If tc actions still hold the chain, just detach it. */
+   list_for_each_entry_safe(chain, tmp, >chain_list, list)
+   tcf_chain_detach(chain);
kfree(block);
 }
 EXPORT_SYMBOL(tcf_block_put);
-- 
2.13.0