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Date:   Thu, 26 Aug 2021 19:59:13 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Zhu Lingshan <lingshan.zhu@intel.com>
Cc:     peterz@infradead.org, pbonzini@redhat.com, bp@alien8.de,
        vkuznets@redhat.com, wanpengli@tencent.com, jmattson@google.com,
        joro@8bytes.org, kan.liang@linux.intel.com, ak@linux.intel.com,
        wei.w.wang@intel.com, eranian@google.com, liuxiangdong5@huawei.com,
        linux-kernel@vger.kernel.org, x86@kernel.org, kvm@vger.kernel.org,
        like.xu.linux@gmail.com, boris.ostrvsky@oracle.com,
        Like Xu <like.xu@linux.intel.com>,
        Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
        Guo Ren <guoren@kernel.org>, Nick Hu <nickhu@andestech.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-csky@vger.kernel.org, linux-riscv@lists.infradead.org,
        xen-devel@lists.xenproject.org
Subject: Re: [PATCH V10 01/18] perf/core: Use static_call to optimize
 perf_guest_info_callbacks
Message-ID: <YSfykbECnC6J02Yk@google.com>
References: <20210806133802.3528-1-lingshan.zhu@intel.com>
 <20210806133802.3528-2-lingshan.zhu@intel.com>
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In-Reply-To: <20210806133802.3528-2-lingshan.zhu@intel.com>
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TL;DR: Please don't merge this patch, it's broken and is also built on a shoddy
       foundation that I would like to fix.

On Fri, Aug 06, 2021, Zhu Lingshan wrote:
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 464917096e73..e466fc8176e1 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6489,9 +6489,18 @@ static void perf_pending_event(struct irq_work *entry)
>   */
>  struct perf_guest_info_callbacks *perf_guest_cbs;
>  
> +/* explicitly use __weak to fix duplicate symbol error */
> +void __weak arch_perf_update_guest_cbs(void)
> +{
> +}
> +
>  int perf_register_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
>  {
> +	if (WARN_ON_ONCE(perf_guest_cbs))
> +		return -EBUSY;
> +
>  	perf_guest_cbs = cbs;
> +	arch_perf_update_guest_cbs();

This is horribly broken, it fails to cleanup the static calls when KVM unregisters
the callbacks, which happens when the vendor module, e.g. kvm_intel, is unloaded.
The explosion doesn't happen until 'kvm' is unloaded because the functions are
implemented in 'kvm', i.e. the use-after-free is deferred a bit.

  BUG: unable to handle page fault for address: ffffffffa011bb90
  #PF: supervisor instruction fetch in kernel mode
  #PF: error_code(0x0010) - not-present page
  PGD 6211067 P4D 6211067 PUD 6212063 PMD 102b99067 PTE 0
  Oops: 0010 [#1] PREEMPT SMP
  CPU: 0 PID: 1047 Comm: rmmod Not tainted 5.14.0-rc2+ #460
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:0xffffffffa011bb90
  Code: Unable to access opcode bytes at RIP 0xffffffffa011bb66.
  Call Trace:
   <NMI>
   ? perf_misc_flags+0xe/0x50
   ? perf_prepare_sample+0x53/0x6b0
   ? perf_event_output_forward+0x67/0x160
   ? kvm_clock_read+0x14/0x30
   ? kvm_sched_clock_read+0x5/0x10
   ? sched_clock_cpu+0xd/0xd0
   ? __perf_event_overflow+0x52/0xf0
   ? handle_pmi_common+0x1f2/0x2d0
   ? __flush_tlb_all+0x30/0x30
   ? intel_pmu_handle_irq+0xcf/0x410
   ? nmi_handle+0x5/0x260
   ? perf_event_nmi_handler+0x28/0x50
   ? nmi_handle+0xc7/0x260
   ? lock_release+0x2b0/0x2b0
   ? default_do_nmi+0x6b/0x170
   ? exc_nmi+0x103/0x130
   ? end_repeat_nmi+0x16/0x1f
   ? lock_release+0x2b0/0x2b0
   ? lock_release+0x2b0/0x2b0
   ? lock_release+0x2b0/0x2b0
   </NMI>
  Modules linked in: irqbypass [last unloaded: kvm]

Even more fun, the existing perf_guest_cbs framework is also broken, though it's
much harder to get it to fail, and probably impossible to get it to fail without
some help.  The issue is that perf_guest_cbs is global, which means that it can
be nullified by KVM (during module unload) while the callbacks are being accessed
by a PMI handler on a different CPU.

The bug has escaped notice because all dererfences of perf_guest_cbs follow the
same "perf_guest_cbs && perf_guest_cbs->is_in_guest()" pattern, and AFAICT the
compiler never reload perf_guest_cbs in this sequence.  The compiler does reload
perf_guest_cbs for any future dereferences, but the ->is_in_guest() guard all but
guarantees the PMI handler will win the race, e.g. to nullify perf_guest_cbs,
KVM has to completely exit the guest and teardown down all VMs before it can be
unloaded.

But with a help, e.g. RAED_ONCE(perf_guest_cbs), unloading kvm_intel can trigger
a NULL pointer derference, e.g. this tweak

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 1eb45139fcc6..202e5ad97f82 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2954,7 +2954,7 @@ unsigned long perf_misc_flags(struct pt_regs *regs)
 {
        int misc = 0;

-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
+       if (READ_ONCE(perf_guest_cbs) && READ_ONCE(perf_guest_cbs)->is_in_guest()) {
                if (perf_guest_cbs->is_user_mode())
                        misc |= PERF_RECORD_MISC_GUEST_USER;
                else


while spamming module load/unload leads to:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 0 P4D 0
  Oops: 0000 [#1] PREEMPT SMP
  CPU: 6 PID: 1825 Comm: stress Not tainted 5.14.0-rc2+ #459
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:perf_misc_flags+0x1c/0x70
  Call Trace:
   perf_prepare_sample+0x53/0x6b0
   perf_event_output_forward+0x67/0x160
   __perf_event_overflow+0x52/0xf0
   handle_pmi_common+0x207/0x300
   intel_pmu_handle_irq+0xcf/0x410
   perf_event_nmi_handler+0x28/0x50
   nmi_handle+0xc7/0x260
   default_do_nmi+0x6b/0x170
   exc_nmi+0x103/0x130
   asm_exc_nmi+0x76/0xbf


The good news is that I have a series that should fix both the existing NULL pointer
bug and mostly obviate the need for static calls.  The bad news is that my approach,
making perf_guest_cbs per-CPU, likely complicates turning these into static calls,
though I'm guessing it's still a solvable problem.

Tangentially related, IMO we should make architectures opt-in to getting
perf_guest_cbs and nuke all of the code in the below files.  Except for arm,
which recently lost KVM support, it's all a bunch of useless copy-paste code that
serves no purpose and just complicates cleanups like this.

>  arch/arm/kernel/perf_callchain.c   | 16 +++++++-----
>  arch/csky/kernel/perf_callchain.c  |  4 +--
>  arch/nds32/kernel/perf_event_cpu.c | 16 +++++++-----
>  arch/riscv/kernel/perf_callchain.c |  4 +--