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

Andrew Jones
2023-05-31T15:03:05Z

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.

> /*
> * 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