The previous commit d01ffb9eee4a ("ax25: add refcount in ax25_dev to
avoid UAF bugs") and commit feef318c855a ("ax25: fix UAF bugs of
net_device caused by rebinding operation") increase the refcounts of
ax25_dev and net_device in ax25_bind() and decrease the matching refcounts
in ax25_kill_by_device() in order to prevent UAF bugs, but there are
reference count leaks.
The root cause of refcount leaks is shown below:
(Thread 1) | (Thread 2)
ax25_bind() |
... |
ax25_addr_ax25dev() |
ax25_dev_hold() //(1) |
... |
dev_hold_track() //(2) |
... | ax25_destroy_socket()
| ax25_cb_del()
| ...
| hlist_del_init() //(3)
|
|
(Thread 3) |
ax25_kill_by_device() |
... |
ax25_for_each(s, &ax25_list) { |
if (s->ax25_dev == ax25_dev) //(4) |
... |
Firstly, we use ax25_bind() to increase the refcount of ax25_dev in
position (1) and increase the refcount of net_device in position (2).
Then, we use ax25_cb_del() invoked by ax25_destroy_socket() to delete
ax25_cb in hlist in position (3) before calling ax25_kill_by_device().
Finally, the decrements of refcounts in ax25_kill_by_device() will not
be executed, because no s->ax25_dev equals to ax25_dev in position (4).
This patch adds decrements of refcounts in ax25_release() and use
lock_sock() to do synchronization. If refcounts decrease in ax25_release(),
the decrements of refcounts in ax25_kill_by_device() will not be
executed and vice versa.
Fixes: d01ffb9eee4a ("ax25: add refcount in ax25_dev to avoid UAF bugs")
Fixes: 87563a043cef ("ax25: fix reference count leaks of ax25_dev")
Fixes: feef318c855a ("ax25: fix UAF bugs of net_device caused by rebinding operation")
Reported-by: Thomas Osterried <redacted>
Signed-off-by: Duoming Zhou <redacted>
---
Changes in V4:
- Add decrements of refcounts in ax25_release() instead of using any additional variables.
net/ax25/af_ax25.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)
From: Dan Carpenter <hidden> Date: 2022-03-15 10:27:33
On Tue, Mar 15, 2022 at 09:54:03AM +0800, Duoming Zhou wrote:
The previous commit d01ffb9eee4a ("ax25: add refcount in ax25_dev to
avoid UAF bugs") and commit feef318c855a ("ax25: fix UAF bugs of
net_device caused by rebinding operation") increase the refcounts of
ax25_dev and net_device in ax25_bind() and decrease the matching refcounts
in ax25_kill_by_device() in order to prevent UAF bugs, but there are
reference count leaks.
The root cause of refcount leaks is shown below:
(Thread 1) | (Thread 2)
ax25_bind() |
... |
ax25_addr_ax25dev() |
ax25_dev_hold() //(1) |
... |
dev_hold_track() //(2) |
... | ax25_destroy_socket()
| ax25_cb_del()
| ...
| hlist_del_init() //(3)
|
|
(Thread 3) |
ax25_kill_by_device() |
... |
ax25_for_each(s, &ax25_list) { |
if (s->ax25_dev == ax25_dev) //(4) |
... |
Firstly, we use ax25_bind() to increase the refcount of ax25_dev in
position (1) and increase the refcount of net_device in position (2).
Then, we use ax25_cb_del() invoked by ax25_destroy_socket() to delete
ax25_cb in hlist in position (3) before calling ax25_kill_by_device().
Finally, the decrements of refcounts in ax25_kill_by_device() will not
be executed, because no s->ax25_dev equals to ax25_dev in position (4).
This patch adds decrements of refcounts in ax25_release() and use
lock_sock() to do synchronization. If refcounts decrease in ax25_release(),
the decrements of refcounts in ax25_kill_by_device() will not be
executed and vice versa.
Fixes: d01ffb9eee4a ("ax25: add refcount in ax25_dev to avoid UAF bugs")
Fixes: 87563a043cef ("ax25: fix reference count leaks of ax25_dev")
Fixes: feef318c855a ("ax25: fix UAF bugs of net_device caused by rebinding operation")
Reported-by: Thomas Osterried <redacted>
Signed-off-by: Duoming Zhou <redacted>
---
Changes in V4:
- Add decrements of refcounts in ax25_release() instead of using any additional variables.
I'm happy that this is simpler. I'm not super happy about the
if (sk->sk_wq) check. That seems like a fragile side-effect condition
instead of something deliberate. But I don't know networking so maybe
this is something which we can rely on.
When you sent the earlier patch then I asked if the devices in
ax25_kill_by_device() were always bound and if we could just use a local
variable instead of something tied to the ax25_dev struct. I still
wonder about that. In other words, could we just do this?
regards,
dan carpenter
Hello,
On Tue, 15 Mar 2022 13:26:57 +0300, Dan Carpenter wrote:
I'm happy that this is simpler. I'm not super happy about the
if (sk->sk_wq) check. That seems like a fragile side-effect condition
instead of something deliberate. But I don't know networking so maybe
this is something which we can rely on.
The variable sk->sk_wq is the address of waiting queue of sock, it is initialized to the
address of sock->wq through the following path:
sock_create->__sock_create->ax25_create()->sock_init_data()->RCU_INIT_POINTER(sk->sk_wq, &sock->wq).
Because we have used sock_alloc() to allocate the socket in __sock_create(), sock or the address of
sock->wq is not null.
What`s more, sk->sk_wq is set to null only in sock_orphan().
Another solution:
We could also use sk->sk_socket to check. We set sk->sk_socket to sock in the following path:
sock_create()->__sock_create()->ax25_create()->sock_init_data()->sk_set_socket(sk, sock).
Because we have used sock_alloc() to allocate the socket in __sock_create(), sock or sk->sk_socket
is not null.
What`s more, sk->sk_socket is set to null only in sock_orphan().
I will change the if (sk->sk_wq) check to if(sk->sk_socket) check, because I think it is
easier to understand.
quoted hunk
When you sent the earlier patch then I asked if the devices in
ax25_kill_by_device() were always bound and if we could just use a local
variable instead of something tied to the ax25_dev struct. I still
wonder about that. In other words, could we just do this?
If we just use ax25_dev_device_up() to bring device up without using ax25_bind(),
the "found" flag could be false when we enter ax25_kill_by_device() and the refcounts
underflow will happen. So we should use two additional variables.
If we use additional variables to fix the bug, I think there is a problem.
In the real world, the device could be detached only once. If the following
race condition happens, we could not deallocate ax25_dev and net_device anymore,
because we could not call ax25_kill_by_device() again.
(Thread 1) | (Thread 2)
ax25_bind() |
| ax25_kill_by_device() //decrease refcounts
(Thread 3) |
ax25_bind() |
... | ...
ax25_dev_hold() //(1) |
dev_hold_track() //(2) |
| ax25_dev_device_down()
In patch "[PATCH net V4 1/2] ax25: Fix refcount leaks caused by ax25_cb_del()",
even the device has been detached, we could also decrease the refcouns by using
ax25_release(), which could ensure ax25_dev and net_device could be deallocated.
So I think "[PATCH net V4 1/2]" is better.
Best wishes,
Duoming Zhou
From: Dan Carpenter <hidden> Date: 2022-03-15 14:19:45
On Tue, Mar 15, 2022 at 10:11:10PM +0800, 周多明 wrote:
Hello,
On Tue, 15 Mar 2022 13:26:57 +0300, Dan Carpenter wrote:
quoted
I'm happy that this is simpler. I'm not super happy about the
if (sk->sk_wq) check. That seems like a fragile side-effect condition
instead of something deliberate. But I don't know networking so maybe
this is something which we can rely on.
The variable sk->sk_wq is the address of waiting queue of sock, it is initialized to the
address of sock->wq through the following path:
sock_create->__sock_create->ax25_create()->sock_init_data()->RCU_INIT_POINTER(sk->sk_wq, &sock->wq).
Because we have used sock_alloc() to allocate the socket in __sock_create(), sock or the address of
sock->wq is not null.
What`s more, sk->sk_wq is set to null only in sock_orphan().
Another solution:
We could also use sk->sk_socket to check. We set sk->sk_socket to sock in the following path:
sock_create()->__sock_create()->ax25_create()->sock_init_data()->sk_set_socket(sk, sock).
Because we have used sock_alloc() to allocate the socket in __sock_create(), sock or sk->sk_socket
is not null.
What`s more, sk->sk_socket is set to null only in sock_orphan().
I will change the if (sk->sk_wq) check to if(sk->sk_socket) check, because I think it is
easier to understand.
quoted
When you sent the earlier patch then I asked if the devices in
ax25_kill_by_device() were always bound and if we could just use a local
variable instead of something tied to the ax25_dev struct. I still
wonder about that. In other words, could we just do this?
If we just use ax25_dev_device_up() to bring device up without using ax25_bind(),
the "found" flag could be false when we enter ax25_kill_by_device() and the refcounts
underflow will happen. So we should use two additional variables.
That answers my question. Thank you.
If we use additional variables to fix the bug, I think there is a problem.
So the v3 patch was buggy?
Why was this not explained under the --- cut off line?
regards,
dan carpenter