Re: [bug] kernel: bpf: syscall: a possible sleep-in-atomic bug in __bpf_prog_put()

[Teng Qi]

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

I add a printk in bpf_prog_put_deferred():
static void bpf_prog_put_deferred(struct work_struct *work)
{
        // . . .
        int inIrq = in_irq();
        int irqsDisabled = irqs_disabled();
        int preemptBits = preempt_count();
        int inAtomic = in_atomic();
        int rcuHeld = rcu_read_lock_held();
        printk("bpf_prog_put: in_irq() %d, irqs_disabled() %d, preempt_count()
         %d, in_atomic() %d, rcu_read_lock_held() %d",
        inIrq, irqsDisabled, preemptBits, inAtomic, rcuHeld);
        // . . .
}

When running the selftest, I see the following output:
[255340.388339] bpf_prog_put: in_irq() 0, irqs_disabled() 0,
        preempt_count() 256, in_atomic() 1, rcu_read_lock_held() 1
[255393.237632] bpf_prog_put: in_irq() 0, irqs_disabled() 0,
        preempt_count() 0, in_atomic() 0, rcu_read_lock_held() 1

Based on this output, I believe it is sufficient to construct a self-test case
for bpf_prog_put_deferred() called under preempt disabled or rcu read lock
region. However, I'm a bit confused about what I should do to build the
self-test case. Are we looking to create a checker that verifies the
context of bpf_prog_put_deferred() is valid? Or do we need a test case that
can trigger this bug?

Could you show me more ideas to construct a self test case? I am not familiar
with it and have no idea.

-- Teng Qi

On Thu, May 25, 2023 at 3:34 AM Yonghong Song <yhs@xxxxxxxx> wrote:
>
>
>
> 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@xxxxxxxx> 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@xxxxxxxx
> >>> <mailto:yhs@xxxxxxxx>> 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@xxxxxxxx
> >>>      <mailto:yhs@xxxxxxxx>
> >>>       > <mailto:yhs@xxxxxxxx <mailto:yhs@xxxxxxxx>>> wrote:
> >>>       >
> >>>       >
> >>>       >
> >>>       >     On 5/16/23 4:18 AM, starmiku1207184332@xxxxxxxxx
> >>>      <mailto:starmiku1207184332@xxxxxxxxx>
> >>>       >     <mailto:starmiku1207184332@xxxxxxxxx
> >>>      <mailto:starmiku1207184332@xxxxxxxxx>> wrote:
> >>>       >      > From: Teng Qi <starmiku1207184332@xxxxxxxxx
> >>>      <mailto:starmiku1207184332@xxxxxxxxx>
> >>>       >     <mailto:starmiku1207184332@xxxxxxxxx
> >>>      <mailto:starmiku1207184332@xxxxxxxxx>>>
> >>>       >      >
> >>>       >      > 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@xxxxxxxxx
> >>>      <mailto:starmiku1207184332@xxxxxxxxx>
> >>>       >     <mailto:starmiku1207184332@xxxxxxxxx
> >>>      <mailto:starmiku1207184332@xxxxxxxxx>>>
> >>>       >
> >>>