[PATCH 5.12 090/127] ipc/mqueue, msg, sem: avoid relying on a stack reference past its expiry

Greg Kroah-Hartman
2021-05-24T16:06:03Z

From: Varad Gautam <varad.gautam@suse.com>

commit a11ddb37bf367e6b5239b95ca759e5389bb46048 upstream.

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:

RIP: 0010:wake_q_add_safe+0x13/0x60
Call Trace:
__x64_sys_mq_timedsend+0x2a9/0x490
do_syscall_64+0x80/0x680
entry_SYSCALL_64_after_hwframe+0x44/0xa9
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.

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_safe on the receiver's
task_struct returned by get_task_struct, instead of dereferencing `this`
which sits on the receiver's stack.

As Manfred pointed out, the race potentially also exists in
ipc/msg.c::expunge_all and ipc/sem.c::wake_up_sem_queue_prepare. Fix
those in the same way.

Link: https://lkml.kernel.org/r/20210510102950.12551-1-varad.gautam@suse.com
Fixes: c5b2cbdbdac563 ("ipc/mqueue.c: update/document memory barriers")
Fixes: 8116b54e7e23ef ("ipc/sem.c: document and update memory barriers")
Fixes: 0d97a82ba830d8 ("ipc/msg.c: update and document memory barriers")
Signed-off-by: Varad Gautam <varad.gautam@suse.com>
Reported-by: Matthias von Faber <matthias.vonfaber@aox-tech.de>
Acked-by: Davidlohr Bueso <dbueso@suse.de>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
ipc/mqueue.c | 6 ++++--
ipc/msg.c | 6 ++++--
ipc/sem.c | 6 ++++--
3 files changed, 12 insertions(+), 6 deletions(-)

--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -1004,12 +1004,14 @@ static inline void __pipelined_op(struct
struct mqueue_inode_info *info,
struct ext_wait_queue *this)
{
+ struct task_struct *task;
+
list_del(&this->list);
- get_task_struct(this->task);
+ task = get_task_struct(this->task);

/* see MQ_BARRIER for purpose/pairing */
smp_store_release(&this->state, STATE_READY);
- wake_q_add_safe(wake_q, this->task);
+ wake_q_add_safe(wake_q, task);
}

/* pipelined_send() - send a message directly to the task waiting in
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -251,11 +251,13 @@ static void expunge_all(struct msg_queue
struct msg_receiver *msr, *t;

list_for_each_entry_safe(msr, t, &msq->q_receivers, r_list) {
- get_task_struct(msr->r_tsk);
+ struct task_struct *r_tsk;
+
+ r_tsk = get_task_struct(msr->r_tsk);

/* see MSG_BARRIER for purpose/pairing */
smp_store_release(&msr->r_msg, ERR_PTR(res));
- wake_q_add_safe(wake_q, msr->r_tsk);
+ wake_q_add_safe(wake_q, r_tsk);
}
}

--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -784,12 +784,14 @@ would_block:
static inline void wake_up_sem_queue_prepare(struct sem_queue *q, int error,
struct wake_q_head *wake_q)
{
- get_task_struct(q->sleeper);
+ struct task_struct *sleeper;
+
+ sleeper = get_task_struct(q->sleeper);

/* see SEM_BARRIER_2 for purpuse/pairing */
smp_store_release(&q->status, error);

- wake_q_add_safe(wake_q, q->sleeper);
+ wake_q_add_safe(wake_q, sleeper);
}

static void unlink_queue(struct sem_array *sma, struct sem_queue *q)