[PATCH net v3] team: fix hang in team_mode_get()

[PATCH net v3] 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>
---

v2 -> v3:
 - Remove unrelated change

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

 drivers/net/team/team.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/team/team.c b/drivers/net/team/team.c
index 4004f98e50d9..04845a4017f9 100644
--- a/drivers/net/team/team.c
+++ b/drivers/net/team/team.c
@@ -468,6 +468,9 @@ static const struct team_mode *team_mode_get(const char *kind)
 	struct team_mode_item *mitem;
 	const struct team_mode *mode = NULL;
 
+	if (!try_module_get(THIS_MODULE))
+		return NULL;
+
 	spin_lock(&mode_list_lock);
 	mitem = __find_mode(kind);
 	if (!mitem) {
@@ -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 v3] team: fix hang in team_mode_get()

[Jiri Pirko]

Mon, Apr 20, 2020 at 05:01:33PM CEST, ap420073@gmail.com wrote:
>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>

Reviewed-by: Jiri Pirko <jiri@mellanox.com>

Re: [PATCH net v3] team: fix hang in team_mode_get()

[David Miller]

From: Taehee Yoo <ap420073@gmail.com>
Date: Mon, 20 Apr 2020 15:01:33 +0000

> 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>

Applied and queued up for -stable, thanks.