From: Varad Gautam varad.gautam@suse.com Subject: ipc/mqueue, msg, sem: avoid relying on a stack reference past its expiry

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.

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 ---

ipc/mqueue.c | 6 ++++-- ipc/msg.c | 6 ++++-- ipc/sem.c | 6 ++++-- 3 files changed, 12 insertions(+), 6 deletions(-)

--- a/ipc/mqueue.c~ipc-mqueue-msg-sem-avoid-relying-on-a-stack-reference-past-its-expiry +++ a/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~ipc-mqueue-msg-sem-avoid-relying-on-a-stack-reference-past-its-expiry +++ a/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~ipc-mqueue-msg-sem-avoid-relying-on-a-stack-reference-past-its-expiry +++ a/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 purpose/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) _