Re: [PATCH v2 07/10] drivers: perf: Implement perf event mmap support in the SBI backend

Atish Patra
2023-06-16T07:45:10Z

On Thu, Jun 15, 2023 at 6:28 AM Heinrich Schuchardt
<heinrich.schuchardt@canonical.com> wrote:
>
> On 6/15/23 10:41, Atish Patra wrote:
> > On Wed, May 31, 2023 at 8:02 AM Andrew Jones <ajones@ventanamicro.com> wrote:
> >>
> >> On Fri, May 12, 2023 at 10:53:18AM +0200, Alexandre Ghiti wrote:
> >>> We used to unconditionnally expose the cycle and instret csrs to
> >>> userspace, which gives rise to security concerns.
> >>>
> >>> So now we only allow access to hw counters from userspace through the perf
> >>> framework which will handle context switches, per-task events...etc. But
> >>> as we cannot break userspace, we give the user the choice to go back to
> >>> the previous behaviour by setting the sysctl perf_user_access.
> >>>
> >>> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> >>> ---
> >>> arch/riscv/kernel/perf_event.c | 18 ++-
> >>> drivers/perf/riscv_pmu_sbi.c | 194 ++++++++++++++++++++++++++++++++-
> >>> 2 files changed, 205 insertions(+), 7 deletions(-)
> >>>
> >>> diff --git a/arch/riscv/kernel/perf_event.c b/arch/riscv/kernel/perf_event.c
> >>> index 94174a0fc251..3af9ca45b43f 100644
> >>> --- a/arch/riscv/kernel/perf_event.c
> >>> +++ b/arch/riscv/kernel/perf_event.c
> >>> @@ -1,9 +1,13 @@
> >>> // SPDX-License-Identifier: GPL-2.0-only
> >>> +#include <linux/perf/riscv_pmu.h>
> >>> #include <linux/sched_clock.h>
> >>>
> >>> void arch_perf_update_userpage(struct perf_event *event,
> >>> struct perf_event_mmap_page *userpg, u64 now)
> >>> {
> >>> +#ifdef CONFIG_RISCV_PMU_SBI
> >>> + struct riscv_pmu *rvpmu = to_riscv_pmu(event->pmu);
> >>> +#endif
> >>
> >> Can avoid this pair of #ifdef/#endif's by just declaring rvpmu inside the
> >> if block below where it's needed. Or even just using to_riscv_pmu()
> >> directly in place of rvpmu.
> >>
> >>> struct clock_read_data *rd;
> >>> unsigned int seq;
> >>> u64 ns;
> >>> @@ -14,7 +18,19 @@ void arch_perf_update_userpage(struct perf_event *event,
> >>> userpg->cap_user_rdpmc =
> >>> !!(event->hw.flags & PERF_EVENT_FLAG_USER_READ_CNT);
> >>>
> >>> - userpg->pmc_width = 64;
> >>> +#ifdef CONFIG_RISCV_PMU_SBI
> >>> + /*
> >>> + * The counters are 64-bit but the priv spec doesn't mandate all the
> >>> + * bits to be implemented: that's why, counter width can vary based on
> >>> + * the cpu vendor.
> >>> + */
> >>> + if (event->pmu->name &&
> >>> + !strncmp(event->pmu->name,
> >>> + RISCV_PMU_PDEV_NAME, sizeof(RISCV_PMU_PDEV_NAME)))
> >>> + userpg->pmc_width = rvpmu->ctr_get_width(event->hw.idx) + 1;
> >>> + else
> >>> +#endif
> >>> + userpg->pmc_width = 64;
> >>
> >> Can leave the initialization to 64 above the #ifdef CONFIG_RISCV_PMU_SBI
> >> as is and drop the else.
> >>
> >>>
> >>> do {
> >>> rd = sched_clock_read_begin(&seq);
> >>> diff --git a/drivers/perf/riscv_pmu_sbi.c b/drivers/perf/riscv_pmu_sbi.c
> >>> index 3b0ee2148054..d9bcc5cc6df5 100644
> >>> --- a/drivers/perf/riscv_pmu_sbi.c
> >>> +++ b/drivers/perf/riscv_pmu_sbi.c
> >>> @@ -24,6 +24,14 @@
> >>> #include <asm/sbi.h>
> >>> #include <asm/hwcap.h>
> >>>
> >>> +#define SYSCTL_NO_USER_ACCESS 0
> >>> +#define SYSCTL_USER_ACCESS 1
> >>> +#define SYSCTL_LEGACY 2
> >>> +
> >>> +#define PERF_EVENT_FLAG_NO_USER_ACCESS BIT(SYSCTL_NO_USER_ACCESS)
> >>> +#define PERF_EVENT_FLAG_USER_ACCESS BIT(SYSCTL_USER_ACCESS)
> >>> +#define PERF_EVENT_FLAG_LEGACY BIT(SYSCTL_LEGACY)
> >>> +
> >>> PMU_FORMAT_ATTR(event, "config:0-47");
> >>> PMU_FORMAT_ATTR(firmware, "config:63");
> >>>
> >>> @@ -43,6 +51,9 @@ static const struct attribute_group *riscv_pmu_attr_groups[] = {
> >>> NULL,
> >>> };
> >>>
> >>> +/* Allow legacy access by default */
> >>> +static int sysctl_perf_user_access __read_mostly = SYSCTL_LEGACY;
> >>> +
> >>
> >> I'm still not in favor of this. Hopefully the distro discussions result in
> >> it being changed.
> >>
> >
> > I did not see any feedback from distro guys. I talked to David (fedora
> > maintainer) and he is even okay with
> > SYSCTL_NO_USER_ACCESS :). I would love to hear back from others (cc'd
> > a few distro folks to this thread).
>
> In future we will have to support scenarios for confidential compute.
> Here we have to avoid timing attacks against virtual machines. If access
> to the cycle register is critical, this implies that hiding it behind a
> sysctl setting is not good enough.
>

For virtualization, all counter usages are virtualized by KVM. Thus,
the hypervisor
remains in complete control.

> If we keep this sysctl based access at all, it should be behind a
> Kconfig setting that is disabled by default and were the help text
> clearly indicates the security implications.
>

That would require rebuild/reinstallation of the kernel for users with
root privileges
to enable these. Distros can disable the access by setting
the default to PERF_EVENT_FLAG_NO_USER_ACCESS or PERF_EVENT_FLAG_USER_ACCESS
based on use case. But root privileges users should have a way to opt in.

As per my understanding, that's the intention behind x86/ARM64
implementation as well.

> Best regards
>
> Heinrich
>
> >
> >>> /*
> >>> * RISC-V doesn't have heterogeneous harts yet. This need to be part of
> >>> * per_cpu in case of harts with different pmu counters
> >>> @@ -301,6 +312,11 @@ int riscv_pmu_get_hpm_info(u32 *hw_ctr_width, u32 *num_hw_ctr)
> >>> }
> >>> EXPORT_SYMBOL_GPL(riscv_pmu_get_hpm_info);
> >>>
> >>> +static uint8_t pmu_sbi_csr_index(struct perf_event *event)
> >>> +{
> >>> + return pmu_ctr_list[event->hw.idx].csr - CSR_CYCLE;
> >>> +}
> >>> +
> >>> static unsigned long pmu_sbi_get_filter_flags(struct perf_event *event)
> >>> {
> >>> unsigned long cflags = 0;
> >>> @@ -329,18 +345,34 @@ static int pmu_sbi_ctr_get_idx(struct perf_event *event)
> >>> struct cpu_hw_events *cpuc = this_cpu_ptr(rvpmu->hw_events);
> >>> struct sbiret ret;
> >>> int idx;
> >>> - uint64_t cbase = 0;
> >>> + uint64_t cbase = 0, cmask = rvpmu->cmask;
> >>> unsigned long cflags = 0;
> >>>
> >>> cflags = pmu_sbi_get_filter_flags(event);
> >>> +
> >>> + /*
> >>> + * In legacy mode, we have to force the fixed counters for those events
> >>> + * but not in the user access mode as we want to use the other counters
> >>> + * that support sampling/filtering.
> >>> + */
> >>> + if (hwc->flags & PERF_EVENT_FLAG_LEGACY) {
> >>> + if (event->attr.config == PERF_COUNT_HW_CPU_CYCLES) {
> >>> + cflags |= SBI_PMU_CFG_FLAG_SKIP_MATCH;
> >>> + cmask = 1;
> >>> + } else if (event->attr.config == PERF_COUNT_HW_INSTRUCTIONS) {
> >>> + cflags |= SBI_PMU_CFG_FLAG_SKIP_MATCH;
> >>> + cmask = 1UL << (CSR_INSTRET - CSR_CYCLE);
> >>> + }
> >>> + }
> >>> +
> >>> /* retrieve the available counter index */
> >>> #if defined(CONFIG_32BIT)
> >>> ret = sbi_ecall(SBI_EXT_PMU, SBI_EXT_PMU_COUNTER_CFG_MATCH, cbase,
> >>> - rvpmu->cmask, cflags, hwc->event_base, hwc->config,
> >>> + cmask, cflags, hwc->event_base, hwc->config,
> >>> hwc->config >> 32);
> >>> #else
> >>> ret = sbi_ecall(SBI_EXT_PMU, SBI_EXT_PMU_COUNTER_CFG_MATCH, cbase,
> >>> - rvpmu->cmask, cflags, hwc->event_base, hwc->config, 0);
> >>> + cmask, cflags, hwc->event_base, hwc->config, 0);
> >>> #endif
> >>> if (ret.error) {
> >>> pr_debug("Not able to find a counter for event %lx config %llx\n",
> >>> @@ -474,6 +506,14 @@ static u64 pmu_sbi_ctr_read(struct perf_event *event)
> >>> return val;
> >>> }
> >>>
> >>> +static void pmu_sbi_set_scounteren(void *arg)
> >>> +{
> >>> + struct perf_event *event = (struct perf_event *)arg;
> >>> +
> >>> + csr_write(CSR_SCOUNTEREN,
> >>> + csr_read(CSR_SCOUNTEREN) | (1 << pmu_sbi_csr_index(event)));
> >>> +}
> >>> +
> >>> static void pmu_sbi_ctr_start(struct perf_event *event, u64 ival)
> >>> {
> >>> struct sbiret ret;
> >>> @@ -490,6 +530,18 @@ static void pmu_sbi_ctr_start(struct perf_event *event, u64 ival)
> >>> if (ret.error && (ret.error != SBI_ERR_ALREADY_STARTED))
> >>> pr_err("Starting counter idx %d failed with error %d\n",
> >>> hwc->idx, sbi_err_map_linux_errno(ret.error));
> >>> +
> >>> + if (hwc->flags & PERF_EVENT_FLAG_USER_ACCESS &&
> >>> + hwc->flags & PERF_EVENT_FLAG_USER_READ_CNT)
> >>> + pmu_sbi_set_scounteren((void *)event);
> >>> +}
> >>> +
> >>> +static void pmu_sbi_reset_scounteren(void *arg)
> >>> +{
> >>> + struct perf_event *event = (struct perf_event *)arg;
> >>> +
> >>> + csr_write(CSR_SCOUNTEREN,
> >>> + csr_read(CSR_SCOUNTEREN) & ~(1 << pmu_sbi_csr_index(event)));
> >>> }
> >>>
> >>> static void pmu_sbi_ctr_stop(struct perf_event *event, unsigned long flag)
> >>> @@ -497,6 +549,10 @@ static void pmu_sbi_ctr_stop(struct perf_event *event, unsigned long flag)
> >>> struct sbiret ret;
> >>> struct hw_perf_event *hwc = &event->hw;
> >>>
> >>> + if (hwc->flags & PERF_EVENT_FLAG_USER_ACCESS &&
> >>> + hwc->flags & PERF_EVENT_FLAG_USER_READ_CNT)
> >>> + pmu_sbi_reset_scounteren((void *)event);
> >>> +
> >>> ret = sbi_ecall(SBI_EXT_PMU, SBI_EXT_PMU_COUNTER_STOP, hwc->idx, 1, flag, 0, 0, 0);
> >>> if (ret.error && (ret.error != SBI_ERR_ALREADY_STOPPED) &&
> >>> flag != SBI_PMU_STOP_FLAG_RESET)
> >>> @@ -704,10 +760,13 @@ static int pmu_sbi_starting_cpu(unsigned int cpu, struct hlist_node *node)
> >>> struct cpu_hw_events *cpu_hw_evt = this_cpu_ptr(pmu->hw_events);
> >>>
> >>> /*
> >>> - * Enable the access for CYCLE, TIME, and INSTRET CSRs from userspace,
> >>> - * as is necessary to maintain uABI compatibility.
> >>> + * We keep enabling userspace access to CYCLE, TIME and INSRET via the
> >>> + * legacy option but that will be removed in the future.
> >>> */
> >>> - csr_write(CSR_SCOUNTEREN, 0x7);
> >>> + if (sysctl_perf_user_access == SYSCTL_LEGACY)
> >>> + csr_write(CSR_SCOUNTEREN, 0x7);
> >>> + else
> >>> + csr_write(CSR_SCOUNTEREN, 0x2);
> >>>
> >>> /* Stop all the counters so that they can be enabled from perf */
> >>> pmu_sbi_stop_all(pmu);
> >>> @@ -851,6 +910,123 @@ static void riscv_pmu_destroy(struct riscv_pmu *pmu)
> >>> cpuhp_state_remove_instance(CPUHP_AP_PERF_RISCV_STARTING, &pmu->node);
> >>> }
> >>>
> >>> +static void pmu_sbi_event_init(struct perf_event *event)
> >>> +{
> >>> + /*
> >>> + * The permissions are set at event_init so that we do not depend
> >>> + * on the sysctl value that can change.
> >>> + */
> >>> + if (sysctl_perf_user_access == SYSCTL_NO_USER_ACCESS)
> >>> + event->hw.flags |= PERF_EVENT_FLAG_NO_USER_ACCESS;
> >>> + else if (sysctl_perf_user_access == SYSCTL_USER_ACCESS)
> >>> + event->hw.flags |= PERF_EVENT_FLAG_USER_ACCESS;
> >>> + else
> >>> + event->hw.flags |= PERF_EVENT_FLAG_LEGACY;
> >>> +}
> >>> +
> >>> +static void pmu_sbi_event_mapped(struct perf_event *event, struct mm_struct *mm)
> >>> +{
> >>> + if (event->hw.flags & PERF_EVENT_FLAG_NO_USER_ACCESS)
> >>> + return;
> >>> +
> >>> + /* In legacy mode, the first 3 CSRs are available. */
> >>
> >> first and third
> >>
> >>> + if (event->hw.flags & PERF_EVENT_FLAG_LEGACY) {
> >>> + if (event->attr.config != PERF_COUNT_HW_CPU_CYCLES &&
> >>> + event->attr.config != PERF_COUNT_HW_INSTRUCTIONS) {
> >>> + return;
> >>> + }
> >>> + }
> >>> +
> >>> + /*
> >>> + * The user mmapped the event to directly access it: this is where
> >>> + * we determine based on sysctl_perf_user_access if we grant userspace
> >>> + * the direct access to this event. That means that within the same
> >>> + * task, some events may be directly accessible and some other may not,
> >>> + * if the user changes the value of sysctl_perf_user_accesss in the
> >>> + * meantime.
> >>> + */
> >>> +
> >>> + event->hw.flags |= PERF_EVENT_FLAG_USER_READ_CNT;
> >>> +
> >>> + /*
> >>> + * We must enable userspace access *before* advertising in the user page
> >>> + * that it is possible to do so to avoid any race.
> >>> + * And we must notify all cpus here because threads that currently run
> >>> + * on other cpus will try to directly access the counter too without
> >>> + * calling pmu_sbi_ctr_start.
> >>> + */
> >>> + if (event->hw.flags & PERF_EVENT_FLAG_USER_ACCESS)
> >>> + on_each_cpu_mask(mm_cpumask(mm),
> >>> + pmu_sbi_set_scounteren, (void *)event, 1);
> >>> +}
> >>> +
> >>> +static void pmu_sbi_event_unmapped(struct perf_event *event, struct mm_struct *mm)
> >>> +{
> >>> + if (event->hw.flags & PERF_EVENT_FLAG_NO_USER_ACCESS)
> >>> + return;
> >>> +
> >>> + /* In legacy mode, the first 3 CSRs are available. */
> >>
> >> first and third
> >>
> >>> + if (event->hw.flags & PERF_EVENT_FLAG_LEGACY) {
> >>> + if (event->attr.config != PERF_COUNT_HW_CPU_CYCLES &&
> >>> + event->attr.config != PERF_COUNT_HW_INSTRUCTIONS) {
> >>> + return;
> >>> + }
> >>> + }
> >>> +
> >>> + /*
> >>> + * Here we can directly remove user access since the user does not have
> >>> + * access to the user page anymore so we avoid the racy window where the
> >>> + * user could have read cap_user_rdpmc to true right before we disable
> >>> + * it.
> >>> + */
> >>> + event->hw.flags &= ~PERF_EVENT_FLAG_USER_READ_CNT;
> >>> +
> >>> + if (event->hw.flags & PERF_EVENT_FLAG_USER_ACCESS)
> >>> + on_each_cpu_mask(mm_cpumask(mm),
> >>> + pmu_sbi_reset_scounteren, (void *)event, 1);
> >>> +}
> >>> +
> >>> +static void riscv_pmu_update_counter_access(void *info)
> >>> +{
> >>> + if (sysctl_perf_user_access == SYSCTL_LEGACY)
> >>> + csr_write(CSR_SCOUNTEREN, 0x7);
> >>> + else
> >>> + csr_write(CSR_SCOUNTEREN, 0x2);
> >>> +}
> >>> +
> >>> +static int riscv_pmu_proc_user_access_handler(struct ctl_table *table,
> >>> + int write, void *buffer,
> >>> + size_t *lenp, loff_t *ppos)
> >>> +{
> >>> + int prev = sysctl_perf_user_access;
> >>> + int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> >>> +
> >>> + /*
> >>> + * Test against the previous value since we clear SCOUNTEREN when
> >>> + * sysctl_perf_user_access is set to SYSCTL_USER_ACCESS, but we should
> >>> + * not do that if that was already the case.
> >>> + */
> >>> + if (ret || !write || prev == sysctl_perf_user_access)
> >>> + return ret;
> >>> +
> >>> + on_each_cpu(riscv_pmu_update_counter_access, (void *)&prev, 1);
> >>
> >> Instead of passing prev shouldn't we pass NULL, as it's not used?
> >>
> >>> +
> >>> + return 0;
> >>> +}
> >>> +
> >>> +static struct ctl_table sbi_pmu_sysctl_table[] = {
> >>> + {
> >>> + .procname = "perf_user_access",
> >>> + .data = &sysctl_perf_user_access,
> >>> + .maxlen = sizeof(unsigned int),
> >>> + .mode = 0644,
> >>> + .proc_handler = riscv_pmu_proc_user_access_handler,
> >>> + .extra1 = SYSCTL_ZERO,
> >>> + .extra2 = SYSCTL_TWO,
> >>> + },
> >>> + { }
> >>> +};
> >>> +
> >>> static int pmu_sbi_device_probe(struct platform_device *pdev)
> >>> {
> >>> struct riscv_pmu *pmu = NULL;
> >>> @@ -888,6 +1064,10 @@ static int pmu_sbi_device_probe(struct platform_device *pdev)
> >>> pmu->ctr_get_width = pmu_sbi_ctr_get_width;
> >>> pmu->ctr_clear_idx = pmu_sbi_ctr_clear_idx;
> >>> pmu->ctr_read = pmu_sbi_ctr_read;
> >>> + pmu->event_init = pmu_sbi_event_init;
> >>> + pmu->event_mapped = pmu_sbi_event_mapped;
> >>> + pmu->event_unmapped = pmu_sbi_event_unmapped;
> >>> + pmu->csr_index = pmu_sbi_csr_index;
> >>>
> >>> ret = cpuhp_state_add_instance(CPUHP_AP_PERF_RISCV_STARTING, &pmu->node);
> >>> if (ret)
> >>> @@ -901,6 +1081,8 @@ static int pmu_sbi_device_probe(struct platform_device *pdev)
> >>> if (ret)
> >>> goto out_unregister;
> >>>
> >>> + register_sysctl("kernel", sbi_pmu_sysctl_table);
> >>> +
> >>> return 0;
> >>>
> >>> out_unregister:
> >>> --
> >>> 2.37.2
> >>>
> >>
> >> Thanks,
> >> drew
> >
> >
> >
>

--
Regards,
Atish