[PATCH net v2] team: fix hang in team_mode_get()

[PATCH net v2] team: fix hang in team_mode_get()

[Taehee Yoo]

When team mode is changed or set, the team_mode_get() is called to check
whether the mode module is inserted or not. If the mode module is not
inserted, it calls the request_module().
In the request_module(), it creates a child process, which is
the "modprobe" process and waits for the done of the child process.
At this point, the following locks were used.
down_read(&cb_lock()); by genl_rcv()
    genl_lock(); by genl_rcv_msc()
        rtnl_lock(); by team_nl_cmd_options_set()
            mutex_lock(&team->lock); by team_nl_team_get()

Concurrently, the team module could be removed by rmmod or "modprobe -r"
The __exit function of team module is team_module_exit(), which calls
team_nl_fini() and it tries to acquire following locks.
down_write(&cb_lock);
    genl_lock();
Because of the genl_lock() and cb_lock, this process can't be finished
earlier than request_module() routine.

The problem secenario.
CPU0                                     CPU1
team_mode_get
    request_module()
                                         modprobe -r team_mode_roundrobin
                                                     team <--(B)
        modprobe team <--(A)
            team_mode_roundrobin

By request_module(), the "modprobe team_mode_roundrobin" command
will be executed. At this point, the modprobe process will decide
that the team module should be inserted before team_mode_roundrobin.
Because the team module is being removed.

By the module infrastructure, the same module insert/remove operations
can't be executed concurrently.
So, (A) waits for (B) but (B) also waits for (A) because of locks.
So that the hang occurs at this point.

Test commands:
    while :
    do
        teamd -d &
	killall teamd &
	modprobe -rv team_mode_roundrobin &
    done

The approach of this patch is to hold the reference count of the team
module if the team module is compiled as a module. If the reference count
of the team module is not zero while request_module() is being called,
the team module will not be removed at that moment.
So that the above scenario could not occur.

Fixes: 3d249d4ca7d0 ("net: introduce ethernet teaming device")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
---

v1 -> v2:
 - Remove unnecessary #if statement and 'put' variable

 drivers/net/team/team.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/net/team/team.c b/drivers/net/team/team.c
index 4004f98e50d9..4f1ccbafb961 100644
--- a/drivers/net/team/team.c
+++ b/drivers/net/team/team.c
@@ -465,8 +465,11 @@ EXPORT_SYMBOL(team_mode_unregister);
 
 static const struct team_mode *team_mode_get(const char *kind)
 {
-	struct team_mode_item *mitem;
 	const struct team_mode *mode = NULL;
+	struct team_mode_item *mitem;
+
+	if (!try_module_get(THIS_MODULE))
+		return NULL;
 
 	spin_lock(&mode_list_lock);
 	mitem = __find_mode(kind);
@@ -483,6 +486,7 @@ static const struct team_mode *team_mode_get(const char *kind)
 	}
 
 	spin_unlock(&mode_list_lock);
+	module_put(THIS_MODULE);
 	return mode;
 }
 
-- 
2.17.1

Re: [PATCH net v2] team: fix hang in team_mode_get()

[Jiri Pirko]

Mon, Apr 20, 2020 at 03:11:45PM CEST, ap420073@gmail.com wrote:

[...]


>diff --git a/drivers/net/team/team.c b/drivers/net/team/team.c
>index 4004f98e50d9..4f1ccbafb961 100644
>--- a/drivers/net/team/team.c
>+++ b/drivers/net/team/team.c
>@@ -465,8 +465,11 @@ EXPORT_SYMBOL(team_mode_unregister);
> 
> static const struct team_mode *team_mode_get(const char *kind)
> {
>-	struct team_mode_item *mitem;
> 	const struct team_mode *mode = NULL;
>+	struct team_mode_item *mitem;

Remove this unrelated move from the patch.


>+
>+	if (!try_module_get(THIS_MODULE))
>+		return NULL;
> 
> 	spin_lock(&mode_list_lock);
> 	mitem = __find_mode(kind);
>@@ -483,6 +486,7 @@ static const struct team_mode *team_mode_get(const char *kind)
> 	}
> 
> 	spin_unlock(&mode_list_lock);
>+	module_put(THIS_MODULE);
> 	return mode;
> }
> 
>-- 
>2.17.1
>

Re: [PATCH net v2] team: fix hang in team_mode_get()

[Taehee Yoo]

On Mon, 20 Apr 2020 at 23:35, Jiri Pirko <jiri@resnulli.us> wrote:
>

Hi Jiri,
Thank you for the review.

> Mon, Apr 20, 2020 at 03:11:45PM CEST, ap420073@gmail.com wrote:
>
> [...]
>
>
> >diff --git a/drivers/net/team/team.c b/drivers/net/team/team.c
> >index 4004f98e50d9..4f1ccbafb961 100644
> >--- a/drivers/net/team/team.c
> >+++ b/drivers/net/team/team.c
> >@@ -465,8 +465,11 @@ EXPORT_SYMBOL(team_mode_unregister);
> >
> > static const struct team_mode *team_mode_get(const char *kind)
> > {
> >-      struct team_mode_item *mitem;
> >       const struct team_mode *mode = NULL;
> >+      struct team_mode_item *mitem;
>
> Remove this unrelated move from the patch.
>

Okay, I will send a v3 patch.
Thanks a lot!
Taehee Yoo