Re: [PATCH v2] ipc/mqueue: Avoid relying on a stack reference past its expiry

[Varad Gautam]

On 5/6/21 11:18 AM, Hillf Danton wrote:
> On Thu,  6 May 2021 08:56:19 
>> do_mq_timedreceive calls wq_sleep with a stack local address. The
>> sender (do_mq_timedsend) uses this address to later call
>> pipelined_send.
>>
>> This leads to a very hard to trigger race where a do_mq_timedreceive call
>> might return and leave do_mq_timedsend to rely on an invalid address,
>> causing the following crash:
>>
>> [  240.739977] RIP: 0010:wake_q_add_safe+0x13/0x60
>> [  240.739991] Call Trace:
>> [  240.739999]  __x64_sys_mq_timedsend+0x2a9/0x490
>> [  240.740003]  ? auditd_test_task+0x38/0x40
>> [  240.740007]  ? auditd_test_task+0x38/0x40
>> [  240.740011]  do_syscall_64+0x80/0x680
>> [  240.740017]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>> [  240.740019] RIP: 0033:0x7f5928e40343
>>
>> The race occurs as:
>>
>> 1. do_mq_timedreceive calls wq_sleep with the address of
>> `struct ext_wait_queue` on function stack (aliased as `ewq_addr` here)
>> - it holds a valid `struct ext_wait_queue *` as long as the stack has
>> not been overwritten.
>>
>> 2. `ewq_addr` gets added to info->e_wait_q[RECV].list in wq_add, and
>> do_mq_timedsend receives it via wq_get_first_waiter(info, RECV) to call
>> __pipelined_op.
>>
>> 3. Sender calls __pipelined_op::smp_store_release(&this->state, STATE_READY=
>> ).
>> Here is where the race window begins. (`this` is `ewq_addr`.)
>>
>> 4. If the receiver wakes up now in do_mq_timedreceive::wq_sleep, it
>> will see `state =3D=3D STATE_READY` and break. `ewq_addr` gets removed from
>> info->e_wait_q[RECV].list.
>>
>> 5. do_mq_timedreceive returns, and `ewq_addr` is no longer guaranteed
>> to be a `struct ext_wait_queue *` since it was on do_mq_timedreceive's
>> stack. (Although the address may not get overwritten until another
>> function happens to touch it, which means it can persist around for an
>> indefinite time.)
>>
>> 6. do_mq_timedsend::__pipelined_op() still believes `ewq_addr` is a
>> `struct ext_wait_queue *`, and uses it to find a task_struct to pass
>> to the wake_q_add_safe call. In the lucky case where nothing has
>> overwritten `ewq_addr` yet, `ewq_addr->task` is the right task_struct.
>> In the unlucky case, __pipelined_op::wake_q_add_safe gets handed a
>> bogus address as the receiver's task_struct causing the crash.
>>
>> do_mq_timedsend::__pipelined_op() should not dereference `this` after
>> setting STATE_READY, as the receiver counterpart is now free to return.
>> Change __pipelined_op to call wake_q_add before setting STATE_READY
>> which ensures that the receiver's task_struct can still be found via
>> `this`.
>>
>> Fixes: c5b2cbdbdac563 ("ipc/mqueue.c: update/document memory barriers")
>> Signed-off-by: Varad Gautam <varad.gautam@xxxxxxxx>
>> Reported-by: Matthias von Faber <matthias.vonfaber@xxxxxxxxxxx>
>> Cc: <stable@xxxxxxxxxxxxxxx> # 5.6
>> Cc: Christian Brauner <christian.brauner@xxxxxxxxxx>
>> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
>> Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>> Cc: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Davidlohr Bueso <dbueso@xxxxxxx>
>> ---
>> v2: Call wake_q_add before smp_store_release, instead of using a
>>     get_task_struct/wake_q_add_safe combination across
>>     smp_store_release. (Davidlohr Bueso)
>>
>>  ipc/mqueue.c | 33 ++++++++++++++++++++++++---------
>>  1 file changed, 24 insertions(+), 9 deletions(-)
>>
>> diff --git a/ipc/mqueue.c b/ipc/mqueue.c
>> index 8031464ed4ae..bfcb6f81a824 100644
>> --- a/ipc/mqueue.c
>> +++ b/ipc/mqueue.c
>> @@ -78,11 +78,13 @@ struct posix_msg_tree_node {
>>   * MQ_BARRIER:
>>   * To achieve proper release/acquire memory barrier pairing, the state is =
>> set to
>>   * STATE_READY with smp_store_release(), and it is read with READ_ONCE fol=
>> lowed
>> - * by smp_acquire__after_ctrl_dep(). In addition, wake_q_add_safe() is use=
>> d.
>> + * by smp_acquire__after_ctrl_dep(). The state change to STATE_READY must =
>> be
>> + * the last write operation, after which the blocked task can immediately
>> + * return and exit.
>>   *
>>   * This prevents the following races:
>>   *
>> - * 1) With the simple wake_q_add(), the task could be gone already before
>> + * 1) With wake_q_add(), the task could be gone already before
>>   *    the increase of the reference happens
>>   * Thread A
>>   *				Thread B
>> @@ -97,10 +99,25 @@ struct posix_msg_tree_node {
>>   * sys_exit()
>>   *				get_task_struct() // UaF
>>   *
>> - * Solution: Use wake_q_add_safe() and perform the get_task_struct() befor=
>> e
>> - * the smp_store_release() that does ->state =3D STATE_READY.
>> + * 2) With wake_q_add(), the receiver task could have returned from the
>> + *    syscall and had its stack-allocated waiter overwritten before the
>> + *    sender could add it to the wake_q
>> + * Thread A
>> + *				Thread B
>> + * WRITE_ONCE(wait.state, STATE_NONE);
>> + * schedule_hrtimeout()
>> + *				->state =3D STATE_READY
>> + * <timeout returns>
>> + * if (wait.state =3D=3D STATE_READY) return;
>> + * sysret to user space
>> + * overwrite receiver's stack
>> + *				wake_q_add(A)
>> + *				if (cmpxchg()) // corrupted waiter
>>   *
>> - * 2) Without proper _release/_acquire barriers, the woken up task
>> + * Solution: Queue the task for wakeup before the smp_store_release() that
>> + * does ->state =3D STATE_READY.
>> + *
>> + * 3) Without proper _release/_acquire barriers, the woken up task
>>   *    could read stale data
>>   *
>>   * Thread A
>> @@ -116,7 +133,7 @@ struct posix_msg_tree_node {
>>   *
>>   * Solution: use _release and _acquire barriers.
>>   *
>> - * 3) There is intentionally no barrier when setting current->state
>> + * 4) There is intentionally no barrier when setting current->state
>>   *    to TASK_INTERRUPTIBLE: spin_unlock(&info->lock) provides the
>>   *    release memory barrier, and the wakeup is triggered when holding
>>   *    info->lock, i.e. spin_lock(&info->lock) provided a pairing
>> @@ -1005,11 +1022,9 @@ static inline void __pipelined_op(struct wake_q_head=
>>  *wake_q,
>>  				  struct ext_wait_queue *this)
>>  {
>>  	list_del(&this->list);
>> -	get_task_struct(this->task);
>> -
>> +	wake_q_add(wake_q, this->task);
>>  	/* see MQ_BARRIER for purpose/pairing */
>>  	smp_store_release(&this->state, STATE_READY);
>> -	wake_q_add_safe(wake_q, this->task);
>>  }
> 
> Given the smp_mb() in __wake_q_add(), and more crucially the position of
> wake_q_add() corrected in this work, that mb helps prevent waiter from
> reading stale data at least in case of pipelined_send(). Update comment
> if that is true.
> 

This race IIUC is covered by 3) under MQ_BARRIER after this patch. But instead
of the smp_mb() in __wake_q_add(), it is guarded by the use of
smp_store_release(STATE_READY), which ensures that the waiter only sees
STATE_READY after wake_q_add and after updating receiver->msg in
pipelined_send().

Thanks,
Varad

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