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

[Yonghong Song <yhs@meta.com>]

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@meta.com 
> <mailto:yhs@meta.com>> wrote:
> 
> 
> 
>     On 5/16/23 4:18 AM, starmiku1207184332@gmail.com
>     <mailto:starmiku1207184332@gmail.com> wrote:
>      > From: Teng Qi <starmiku1207184332@gmail.com
>     <mailto:starmiku1207184332@gmail.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@gmail.com
>     <mailto:starmiku1207184332@gmail.com>>
>