On 2021-05-05 23:56, Varad Gautam wrote:
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 == STATE_READY` and break. `ewq_addr` gets removed from
info->e_wait_q[RECV].list.
So when the blocked task sees the lockless STATE_READY and returns it
won't remove the list entry, instead the waker is in charge of doing so.
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)
LGTM, with some additional nits below:
Acked-by: Davidlohr Bueso <dbueso@xxxxxxx>
+ * 2) With wake_q_add(), the receiver task could have returned from
the
^^^^^^
s/receiver/blocked
+ * syscall and had its stack-allocated waiter overwritten before
the
+ * sender could add it to the wake_q
^^^^^
s/sender/waker
+ * Thread A
+ * Thread B
+ * WRITE_ONCE(wait.state, STATE_NONE);
+ * schedule_hrtimeout()
+ * ->state = STATE_READY
+ * <timeout returns>
While this comment is fine, for completeness we should document and
expand
the scope of such races, because it's not only timeouts, but can also
happen
upon a signal or spurious wakeup. Perhaps replacing (in a separate
patch):
<timeout returns>
with
<returns: timeout/signal/spurious wakeup>
Thanks,
Davidlohr
