Re: [bug] kernel: bpf: syscall: a possible sleep-in-atomic bug in __bpf_prog_put()
From: Yonghong Song
Date: Tue May 23 2023 - 00:34:38 EST
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>>>
>