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Date: Thu, 21 Jul 2022 10:58:23 +0100
From: Fuad Tabba <tabba@...gle.com>
To: Kalesh Singh <kaleshsingh@...gle.com>
Cc: maz@...nel.org, mark.rutland@....com, broonie@...nel.org,
madvenka@...ux.microsoft.com, will@...nel.org, qperret@...gle.com,
james.morse@....com, alexandru.elisei@....com,
suzuki.poulose@....com, catalin.marinas@....com,
andreyknvl@...il.com, vincenzo.frascino@....com,
mhiramat@...nel.org, ast@...nel.org, wangkefeng.wang@...wei.com,
elver@...gle.com, keirf@...gle.com, yuzenghui@...wei.com,
ardb@...nel.org, oupton@...gle.com,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v5 12/17] KVM: arm64: Save protected-nVHE (pKVM) hyp stacktrace
Hi Kalesh,
On Thu, Jul 21, 2022 at 6:58 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).
>
> On hyp_panic(), the hypervisor prepares the stacktrace before returning to
> the host.
>
> Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>
> ---
Reviewed-by: Fuad Tabba <tabba@...gle.com>
Cheers,
/fuad
>
> Changes in v5:
> - Comment/clarify pkvm_save_backtrace_entry(), per Fuad
> - kvm_nvhe_unwind_init(), doesn't need to be always inline, make it
> inline instead to avoid linking issues, per Marc
> - Use regular comments instead of doc comments, per Fuad
>
> arch/arm64/include/asm/stacktrace/nvhe.h | 17 ++++++
> arch/arm64/kvm/hyp/nvhe/stacktrace.c | 78 ++++++++++++++++++++++++
> arch/arm64/kvm/hyp/nvhe/switch.c | 6 ++
> 3 files changed, 101 insertions(+)
>
> diff --git a/arch/arm64/include/asm/stacktrace/nvhe.h b/arch/arm64/include/asm/stacktrace/nvhe.h
> index 3078501f8e22..05d7e03e0a8c 100644
> --- a/arch/arm64/include/asm/stacktrace/nvhe.h
> +++ b/arch/arm64/include/asm/stacktrace/nvhe.h
> @@ -21,6 +21,23 @@
>
> #include <asm/stacktrace/common.h>
>
> +/*
> + * kvm_nvhe_unwind_init - Start an unwind from the given nVHE HYP fp and pc
> + *
> + * @state : unwind_state to initialize
> + * @fp : frame pointer at which to start the unwinding.
> + * @pc : program counter at which to start the unwinding.
> + */
> +static 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)
> diff --git a/arch/arm64/kvm/hyp/nvhe/stacktrace.c b/arch/arm64/kvm/hyp/nvhe/stacktrace.c
> index 96c8b93320eb..60461c033a04 100644
> --- a/arch/arm64/kvm/hyp/nvhe/stacktrace.c
> +++ b/arch/arm64/kvm/hyp/nvhe/stacktrace.c
> @@ -11,4 +11,82 @@ 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_entry = (unsigned long **)arg;
> + int nr_entries = NVHE_STACKTRACE_SIZE / sizeof(long);
> + unsigned long *stacktrace_start, *stacktrace_end;
> +
> + stacktrace_start = (unsigned long *)this_cpu_ptr(pkvm_stacktrace);
> + stacktrace_end = stacktrace_start + nr_entries;
> +
> + /*
> + * Need 2 free slots: 1 for current entry and 1 for the
> + * trailing zero entry delimiter.
> + */
> + if (*stacktrace_entry > stacktrace_end - 2)
> + return false;
> +
> + /* Save the current entry */
> + **stacktrace_entry = where;
> +
> + /* Add trailing zero entry delimiter */
> + *(*stacktrace_entry + 1) = 0UL;
> +
> + /*
> + * Increment the current entry position. The zero entry
> + * will be overwritten by the next backtrace entry (if any)
> + */
> + ++*stacktrace_entry;
> +
> + 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_entry = (void *)this_cpu_ptr(pkvm_stacktrace);
> + struct unwind_state state;
> +
> + kvm_nvhe_unwind_init(&state, fp, pc);
> +
> + unwind(&state, pkvm_save_backtrace_entry, &stacktrace_entry);
> +}
> +#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);
> +}
> diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
> index 6db801db8f27..64e13445d0d9 100644
> --- a/arch/arm64/kvm/hyp/nvhe/switch.c
> +++ b/arch/arm64/kvm/hyp/nvhe/switch.c
> @@ -34,6 +34,8 @@ DEFINE_PER_CPU(struct kvm_host_data, kvm_host_data);
> DEFINE_PER_CPU(struct kvm_cpu_context, kvm_hyp_ctxt);
> DEFINE_PER_CPU(unsigned long, kvm_hyp_vector);
>
> +extern void kvm_nvhe_prepare_backtrace(unsigned long fp, unsigned long pc);
> +
> static void __activate_traps(struct kvm_vcpu *vcpu)
> {
> u64 val;
> @@ -375,6 +377,10 @@ asmlinkage void __noreturn hyp_panic(void)
> __sysreg_restore_state_nvhe(host_ctxt);
> }
>
> + /* Prepare to dump kvm nvhe hyp stacktrace */
> + kvm_nvhe_prepare_backtrace((unsigned long)__builtin_frame_address(0),
> + _THIS_IP_);
> +
> __hyp_do_panic(host_ctxt, spsr, elr, par);
> unreachable();
> }
> --
> 2.37.0.170.g444d1eabd0-goog
>
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