Message-ID: <113dc8c1-0840-9ee3-2840-28246731604c@meta.com>
Date:   Wed, 24 May 2023 12:34:31 -0700
From:   Yonghong Song <yhs@...a.com>
To:     Teng Qi <starmiku1207184332@...il.com>
Cc:     ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com,
        andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org,
        yhs@...com, kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com,
        jolsa@...nel.org, davem@...emloft.net, kuba@...nel.org,
        hawk@...nel.org, bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [bug] kernel: bpf: syscall: a possible sleep-in-atomic bug in
 __bpf_prog_put()



On 5/24/23 5:42 AM, Teng Qi wrote:
> Thank you.
> 
>> We cannot use rcu_read_lock_held() in the 'if' statement. The return
>> value rcu_read_lock_held() could be 1 for some configurations regardless
>> whether rcu_read_lock() is really held or not. In most cases,
>> rcu_read_lock_held() is used in issuing potential warnings.
>> Maybe there are other ways to record whether rcu_read_lock() is held or not?
> 
> Sorry. I was not aware of the dependency of configurations of
> rcu_read_lock_held().
> 
>> If we cannot resolve rcu_read_lock() presence issue, maybe the condition
>> can be !in_interrupt(), so any process-context will go to a workqueue.
> 
> I agree that using !in_interrupt() as a condition is an acceptable solution.

This should work although it could be conservative.

> 
>> Alternatively, we could have another solution. We could add another
>> function e.g., bpf_prog_put_rcu(), which indicates that bpf_prog_put()
>> will be done in rcu context.
> 
> Implementing a new function like bpf_prog_put_rcu() is a solution that involves
> more significant changes.

Maybe we can change signature of bpf_prog_put instead? Like
    void bpf_prog_put(struct bpf_prog *prog, bool in_rcu)
and inside bpf_prog_put we can add
    WARN_ON_ONCE(in_rcu && !bpf_rcu_lock_held());

> 
>> So if in_interrupt(), do kvfree, otherwise,
>> put into a workqueue.
> 
> Shall we proceed with submitting a patch following this approach?

You could choose either of the above although I think with newer 
bpf_prog_put() is better.

BTW, please do create a test case, e.g, sockmap test case which
can show the problem with existing code base.

> 
> I would like to mention something unrelated to the possible bug. At this
> moment, things seem to be more puzzling. vfree() is safe under in_interrupt()
> but not safe under other atomic contexts.
> This disorder challenges our conventional belief, a monotonic incrementation
> of limitations of the hierarchical atomic contexts, that programer needs
> to be more and more careful to write code under rcu read lock, spin lock,
> bh disable, interrupt...
> This disorder can lead to unexpected consequences, such as code being safe
> under interrupts but not safe under spin locks.
> The disorder makes kernel programming more complex and may result in more bugs.
> Even though we find a way to resolve the possible bug about the bpf_prog_put(),
> I feel sad for undermining of kernel`s maintainability and disorder of
> hierarchy of atomic contexts.
> 
> -- Teng Qi
> 
> On Tue, May 23, 2023 at 12:33 PM Yonghong Song <yhs@...a.com> wrote:
>>
>>
>>
>> On 5/21/23 6:39 AM, Teng Qi wrote:
>>> Thank you.
>>>
>>>   > Your above analysis makes sense if indeed that kvfree cannot appear
>>>   > inside a spin lock region or RCU read lock region. But is it true?
>>>   > I checked a few code paths in kvfree/kfree. It is either guarded
>>>   > with local_irq_save/restore or by
>>>   > spin_lock_irqsave/spin_unlock_
>>>   > irqrestore, etc. Did I miss
>>>   > anything? Are you talking about RT kernel here?
>>>
>>> To see the sleepable possibility of kvfree, it is important to analyze the
>>> following calling stack:
>>> mm/util.c: 645 kvfree()
>>> mm/vmalloc.c: 2763 vfree()
>>>
>>> In kvfree(), to call vfree, if the pointer addr points to memory
>>> allocated by
>>> vmalloc(), it calls vfree().
>>> void kvfree(const void *addr)
>>> {
>>>           if (is_vmalloc_addr(addr))
>>>                   vfree(addr);
>>>           else
>>>                   kfree(addr);
>>> }
>>>
>>> In vfree(), in_interrupt() and might_sleep() need to be considered.
>>> void vfree(const void *addr)
>>> {
>>>           // ...
>>>           if (unlikely(in_interrupt()))
>>>           {
>>>                   vfree_atomic(addr);
>>>                   return;
>>>           }
>>>           // ...
>>>           might_sleep();
>>>           // ...
>>> }
>>
>> Sorry. I didn't check vfree path. So it does look like that
>> we need to pay special attention to non interrupt part.
>>
>>>
>>> The vfree() may sleep if in_interrupt() == false. The RCU read lock region
>>> could have in_interrupt() == false and spin lock region which only disables
>>> preemption also has in_interrupt() == false. So the kvfree() cannot appear
>>> inside a spin lock region or RCU read lock region if the pointer addr points
>>> to memory allocated by vmalloc().
>>>
>>>   > > Therefore, we propose modifying the condition to include
>>>   > > in_atomic(). Could we
>>>   > > update the condition as follows: "in_irq() || irqs_disabled() ||
>>>   > > in_atomic()"?
>>>   > Thank you! We look forward to your feedback.
>>>
>>> We now think that ‘irqs_disabled() || in_atomic() ||
>>> rcu_read_lock_held()’ is
>>> more proper. irqs_disabled() is for irq flag reg, in_atomic() is for
>>> preempt count and rcu_read_lock_held() is for RCU read lock region.
>>
>> We cannot use rcu_read_lock_held() in the 'if' statement. The return
>> value rcu_read_lock_held() could be 1 for some configuraitons regardless
>> whether rcu_read_lock() is really held or not. In most cases,
>> rcu_read_lock_held() is used in issuing potential warnings.
>> Maybe there are other ways to record whether rcu_read_lock() is held or not?
>>
>> I agree with your that 'irqs_disabled() || in_atomic()' makes sense
>> since it covers process context local_irq_save() and spin_lock() cases.
>>
>> If we cannot resolve rcu_read_lock() presence issue, maybe the condition
>> can be !in_interrupt(), so any process-context will go to a workqueue.
>>
>> Alternatively, we could have another solution. We could add another
>> function e.g., bpf_prog_put_rcu(), which indicates that bpf_prog_put()
>> will be done in rcu context. So if in_interrupt(), do kvfree, otherwise,
>> put into a workqueue.
>>
>>
>>>
>>> -- Teng Qi
>>>
>>> On Sun, May 21, 2023 at 11:45 AM Yonghong Song <yhs@...a.com
>>> <mailto:yhs@...a.com>> wrote:
>>>
>>>
>>>
>>>      On 5/19/23 7:18 AM, Teng Qi wrote:
>>>       > Thank you for your response.
>>>       >  > Looks like you only have suspicion here. Could you find a real
>>>      violation
>>>       >  > here where __bpf_prog_put() is called with !in_irq() &&
>>>       >  > !irqs_disabled(), but inside spin_lock or rcu read lock? I
>>>      have not seen
>>>       >  > things like that.
>>>       >
>>>       > For the complex conditions to call bpf_prog_put() with 1 refcnt,
>>>      we have
>>>       > been
>>>       > unable to really trigger this atomic violation after trying to
>>>      construct
>>>       > test cases manually. But we found that it is possible to show
>>>      cases with
>>>       > !in_irq() && !irqs_disabled(), but inside spin_lock or rcu read lock.
>>>       > For example, even a failed case, one of selftest cases of bpf,
>>>      netns_cookie,
>>>       > calls bpf_sock_map_update() and may indirectly call bpf_prog_put()
>>>       > only inside rcu read lock: The possible call stack is:
>>>       > net/core/sock_map.c: 615 bpf_sock_map_update()
>>>       > net/core/sock_map.c: 468 sock_map_update_common()
>>>       > net/core/sock_map.c:  217 sock_map_link()
>>>       > kernel/bpf/syscall.c: 2111 bpf_prog_put()
>>>       >
>>>       > The files about netns_cookie include
>>>       > tools/testing/selftests/bpf/progs/netns_cookie_prog.c and
>>>       > tools/testing/selftests/bpf/prog_tests/netns_cookie.c. We
>>>      inserted the
>>>       > following code in
>>>       > ‘net/core/sock_map.c: 468 sock_map_update_common()’:
>>>       > static int sock_map_update_common(..)
>>>       > {
>>>       >          int inIrq = in_irq();
>>>       >          int irqsDisabled = irqs_disabled();
>>>       >          int preemptBits = preempt_count();
>>>       >          int inAtomic = in_atomic();
>>>       >          int rcuHeld = rcu_read_lock_held();
>>>       >          printk("in_irq() %d, irqs_disabled() %d, preempt_count() %d,
>>>       >            in_atomic() %d, rcu_read_lock_held() %d", inIrq,
>>>      irqsDisabled,
>>>       >            preemptBits, inAtomic, rcuHeld);
>>>       > }
>>>       >
>>>       > The output message is as follows:
>>>       > root@(none):/root/bpf# ./test_progs -t netns_cookie
>>>       > [  137.639188] in_irq() 0, irqs_disabled() 0, preempt_count() 0,
>>>       > in_atomic() 0,
>>>       >          rcu_read_lock_held() 1
>>>       > #113     netns_cookie:OK
>>>       > Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED
>>>       >
>>>       > We notice that there are numerous callers in kernel/, net/ and
>>>      drivers/,
>>>       > so we
>>>       > highly suggest modifying __bpf_prog_put() to address this gap.
>>>      The gap
>>>       > exists
>>>       > because __bpf_prog_put() is only safe under in_irq() ||
>>>      irqs_disabled()
>>>       > but not in_atomic() || rcu_read_lock_held(). The following code
>>>      snippet may
>>>       > mislead developers into thinking that bpf_prog_put() is safe in all
>>>       > contexts.
>>>       > if (in_irq() || irqs_disabled()) {
>>>       >          INIT_WORK(&aux->work, bpf_prog_put_deferred);
>>>       >          schedule_work(&aux->work);
>>>       > } else {
>>>       >          bpf_prog_put_deferred(&aux->work);
>>>       > }
>>>       >
>>>       > Implicit dependency may lead to issues.
>>>       >
>>>       >  > Any problem here?
>>>       > We mentioned it to demonstrate the possibility of kvfree() being
>>>       > called by __bpf_prog_put_noref().
>>>       >
>>>       > Thanks.
>>>       > -- Teng Qi
>>>       >
>>>       > On Wed, May 17, 2023 at 1:08 AM Yonghong Song <yhs@...a.com
>>>      <mailto:yhs@...a.com>
>>>       > <mailto:yhs@...a.com <mailto:yhs@...a.com>>> wrote:
>>>       >
>>>       >
>>>       >
>>>       >     On 5/16/23 4:18 AM, starmiku1207184332@...il.com
>>>      <mailto:starmiku1207184332@...il.com>
>>>       >     <mailto:starmiku1207184332@...il.com
>>>      <mailto:starmiku1207184332@...il.com>> wrote:
>>>       >      > From: Teng Qi <starmiku1207184332@...il.com
>>>      <mailto:starmiku1207184332@...il.com>
>>>       >     <mailto:starmiku1207184332@...il.com
>>>      <mailto:starmiku1207184332@...il.com>>>
>>>       >      >
>>>       >      > Hi, bpf developers,
>>>       >      >
>>>       >      > We are developing a static tool to check the matching between
>>>       >     helpers and the
>>>       >      > context of hooks. During our analysis, we have discovered some
>>>       >     important
>>>       >      > findings that we would like to report.
>>>       >      >
>>>       >      > ‘kernel/bpf/syscall.c: 2097 __bpf_prog_put()’ shows that
>>>      function
>>>       >      > bpf_prog_put_deferred() won`t be called in the condition of
>>>       >      > ‘in_irq() || irqs_disabled()’.
>>>       >      > if (in_irq() || irqs_disabled()) {
>>>       >      >      INIT_WORK(&aux->work, bpf_prog_put_deferred);
>>>       >      >      schedule_work(&aux->work);
>>>       >      > } else {
>>>       >      >
>>>       >      >      bpf_prog_put_deferred(&aux->work);
>>>       >      > }
>>>       >      >
>>>       >      > We suspect this condition exists because there might be
>>>      sleepable
>>>       >     operations
>>>       >      > in the callees of the bpf_prog_put_deferred() function:
>>>       >      > kernel/bpf/syscall.c: 2097 __bpf_prog_put()
>>>       >      > kernel/bpf/syscall.c: 2084 bpf_prog_put_deferred()
>>>       >      > kernel/bpf/syscall.c: 2063 __bpf_prog_put_noref()
>>>       >      > kvfree(prog->aux->jited_linfo);
>>>       >      > kvfree(prog->aux->linfo);
>>>       >
>>>       >     Looks like you only have suspicion here. Could you find a real
>>>       >     violation
>>>       >     here where __bpf_prog_put() is called with !in_irq() &&
>>>       >     !irqs_disabled(), but inside spin_lock or rcu read lock? I
>>>      have not seen
>>>       >     things like that.
>>>       >
>>>       >      >
>>>       >      > Additionally, we found that array prog->aux->jited_linfo is
>>>       >     initialized in
>>>       >      > ‘kernel/bpf/core.c: 157 bpf_prog_alloc_jited_linfo()’:
>>>       >      > prog->aux->jited_linfo = kvcalloc(prog->aux->nr_linfo,
>>>       >      >    sizeof(*prog->aux->jited_linfo),
>>>      bpf_memcg_flags(GFP_KERNEL |
>>>       >     __GFP_NOWARN));
>>>       >
>>>       >     Any problem here?
>>>       >
>>>       >      >
>>>       >      > Our question is whether the condition 'in_irq() ||
>>>       >     irqs_disabled() == false' is
>>>       >      > sufficient for calling 'kvfree'. We are aware that calling
>>>       >     'kvfree' within the
>>>       >      > context of a spin lock or an RCU lock is unsafe.
>>>
>>>      Your above analysis makes sense if indeed that kvfree cannot appear
>>>      inside a spin lock region or RCU read lock region. But is it true?
>>>      I checked a few code paths in kvfree/kfree. It is either guarded
>>>      with local_irq_save/restore or by
>>>      spin_lock_irqsave/spin_unlock_irqrestore, etc. Did I miss
>>>      anything? Are you talking about RT kernel here?
>>>
>>>
>>>       >      >
>>>       >      > Therefore, we propose modifying the condition to include
>>>       >     in_atomic(). Could we
>>>       >      > update the condition as follows: "in_irq() ||
>>>      irqs_disabled() ||
>>>       >     in_atomic()"?
>>>       >      >
>>>       >      > Thank you! We look forward to your feedback.
>>>       >      >
>>>       >      > Signed-off-by: Teng Qi <starmiku1207184332@...il.com
>>>      <mailto:starmiku1207184332@...il.com>
>>>       >     <mailto:starmiku1207184332@...il.com
>>>      <mailto:starmiku1207184332@...il.com>>>
>>>       >
>>>

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