linux-kernel - Re: [PATCH v4 12/18] KVM: arm64: Save protected-nVHE (pKVM) hyp stacktrace

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Date:   Mon, 18 Jul 2022 10:36:22 -0700
From:   Kalesh Singh <kaleshsingh@...gle.com>
To:     Fuad Tabba <tabba@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, Mark Rutland <mark.rutland@....com>,
        Mark Brown <broonie@...nel.org>,
        "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>,
        Will Deacon <will@...nel.org>,
        Quentin Perret <qperret@...gle.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        andreyknvl@...il.com, russell.king@...cle.com,
        vincenzo.frascino@....com, Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Marco Elver <elver@...gle.com>, Keir Fraser <keirf@...gle.com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Oliver Upton <oupton@...gle.com>,
        "moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" 
        <linux-arm-kernel@...ts.infradead.org>,
        kvmarm <kvmarm@...ts.cs.columbia.edu>,
        LKML <linux-kernel@...r.kernel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>
Subject: Re: [PATCH v4 12/18] KVM: arm64: Save protected-nVHE (pKVM) hyp stacktrace

On Mon, Jul 18, 2022 at 3:07 AM Fuad Tabba <tabba@...gle.com> wrote:
>
> Hi Kalesh,
>
> On Fri, Jul 15, 2022 at 7:11 AM Kalesh Singh <kaleshsingh@...gle.com> wrote:
> >
> > In protected nVHE mode, the host cannot access private owned hypervisor
> > memory. Also the hypervisor aims to remains simple to reduce the attack
> > surface and does not provide any printk support.
> >
> > For the above reasons, the approach taken to provide hypervisor stacktraces
> > in protected mode is:
> >    1) Unwind and save the hyp stack addresses in EL2 to a shared buffer
> >       with the host (done in this patch).
> >    2) Delegate the dumping and symbolization of the addresses to the
> >       host in EL1 (later patch in the series).
> >
> > Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>
> > ---
> >  arch/arm64/include/asm/stacktrace/nvhe.h | 18 ++++++
> >  arch/arm64/kvm/hyp/nvhe/stacktrace.c     | 70 ++++++++++++++++++++++++
> >  2 files changed, 88 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/stacktrace/nvhe.h b/arch/arm64/include/asm/stacktrace/nvhe.h
> > index 36cf7858ddd8..456a6ae08433 100644
> > --- a/arch/arm64/include/asm/stacktrace/nvhe.h
> > +++ b/arch/arm64/include/asm/stacktrace/nvhe.h
> > @@ -21,6 +21,22 @@
> >
> >  #include <asm/stacktrace/common.h>
> >
> > +/**
> > + * kvm_nvhe_unwind_init - Start an unwind from the given nVHE HYP fp and pc
> > + *
> > + * @fp : frame pointer at which to start the unwinding.
> > + * @pc : program counter at which to start the unwinding.
> > + */
> > +static __always_inline void kvm_nvhe_unwind_init(struct unwind_state *state,
> > +                                                unsigned long fp,
> > +                                                unsigned long pc)
> > +{
> > +       unwind_init_common(state, NULL);
> > +
> > +       state->fp = fp;
> > +       state->pc = pc;
> > +}
> > +
> >  static inline bool on_accessible_stack(const struct task_struct *tsk,
> >                                        unsigned long sp, unsigned long size,
> >                                        struct stack_info *info)
> > @@ -33,6 +49,8 @@ static inline bool on_accessible_stack(const struct task_struct *tsk,
> >   */
> >  #ifdef __KVM_NVHE_HYPERVISOR__
> >
> > +extern void kvm_nvhe_prepare_backtrace(unsigned long fp, unsigned long pc);
> > +
> >  #ifdef CONFIG_PROTECTED_NVHE_STACKTRACE
> >  static inline bool on_overflow_stack(unsigned long sp, unsigned long size,
> >                                      struct stack_info *info)
> > diff --git a/arch/arm64/kvm/hyp/nvhe/stacktrace.c b/arch/arm64/kvm/hyp/nvhe/stacktrace.c
> > index 96c8b93320eb..832a536e440f 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/stacktrace.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/stacktrace.c
> > @@ -11,4 +11,74 @@ DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)], overflow_stack)
> >
> >  #ifdef CONFIG_PROTECTED_NVHE_STACKTRACE
> >  DEFINE_PER_CPU(unsigned long [NVHE_STACKTRACE_SIZE/sizeof(long)], pkvm_stacktrace);
> > +
> > +/**
> > + * pkvm_save_backtrace_entry - Saves a protected nVHE HYP stacktrace entry
> > + *
> > + * @arg    : the position of the entry in the stacktrace buffer
> > + * @where  : the program counter corresponding to the stack frame
> > + *
> > + * Save the return address of a stack frame to the shared stacktrace buffer.
> > + * The host can access this shared buffer from EL1 to dump the backtrace.
> > + */
> > +static bool pkvm_save_backtrace_entry(void *arg, unsigned long where)
> > +{
> > +       unsigned long **stacktrace_pos = (unsigned long **)arg;
> > +       unsigned long stacktrace_start, stacktrace_end;
> > +
> > +       stacktrace_start = (unsigned long)this_cpu_ptr(pkvm_stacktrace);
> > +       stacktrace_end = stacktrace_start + NVHE_STACKTRACE_SIZE - (2 * sizeof(long));
>
> I guess this is related to my comment in patch 9, but why does the end
> happen at 2 * instead of just 1 * before the actual end? I guess
> because it's inclusive. That said, a comment would be helpful.
>

The intention is to check that we can fit 2 entries (the stacktrace
entry and null entry). I think the "end" naming is a bit misleading.
Let me try to clarify it better in the next version.

Thanks,
Kalesh

> Thanks,
> /fuad
>
> > +
> > +       if ((unsigned long) *stacktrace_pos > stacktrace_end)
> > +               return false;
> > +
> > +       /* Save the entry to the current pos in stacktrace buffer */
> > +       **stacktrace_pos = where;
> > +
> > +       /* A zero entry delimits the end of the stacktrace. */
> > +       *(*stacktrace_pos + 1) = 0UL;
> > +
> > +       /* Increment the current pos */
> > +       ++*stacktrace_pos;
> > +
> > +       return true;
> > +}
> > +
> > +/**
> > + * pkvm_save_backtrace - Saves the protected nVHE HYP stacktrace
> > + *
> > + * @fp : frame pointer at which to start the unwinding.
> > + * @pc : program counter at which to start the unwinding.
> > + *
> > + * Save the unwinded stack addresses to the shared stacktrace buffer.
> > + * The host can access this shared buffer from EL1 to dump the backtrace.
> > + */
> > +static void pkvm_save_backtrace(unsigned long fp, unsigned long pc)
> > +{
> > +       void *stacktrace_start = (void *)this_cpu_ptr(pkvm_stacktrace);
> > +       struct unwind_state state;
> > +
> > +       kvm_nvhe_unwind_init(&state, fp, pc);
> > +
> > +       unwind(&state, pkvm_save_backtrace_entry, &stacktrace_start);
> > +}
> > +#else /* !CONFIG_PROTECTED_NVHE_STACKTRACE */
> > +static void pkvm_save_backtrace(unsigned long fp, unsigned long pc)
> > +{
> > +}
> >  #endif /* CONFIG_PROTECTED_NVHE_STACKTRACE */
> > +
> > +/**
> > + * kvm_nvhe_prepare_backtrace - prepare to dump the nVHE backtrace
> > + *
> > + * @fp : frame pointer at which to start the unwinding.
> > + * @pc : program counter at which to start the unwinding.
> > + *
> > + * Saves the information needed by the host to dump the nVHE hypervisor
> > + * backtrace.
> > + */
> > +void kvm_nvhe_prepare_backtrace(unsigned long fp, unsigned long pc)
> > +{
> > +       if (is_protected_kvm_enabled())
> > +               pkvm_save_backtrace(fp, pc);
> > +}
> > --
> > 2.37.0.170.g444d1eabd0-goog
> >