linux-kernel - [PATCH v2 18/18] perf: arm_spe: Disallow userspace profiling when arm_kernel_unmapped_at

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Message-Id: <1512059986-21325-19-git-send-email-will.deacon@arm.com>
Date:   Thu, 30 Nov 2017 16:39:46 +0000
From:   Will Deacon <will.deacon@....com>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, catalin.marinas@....com,
        mark.rutland@....com, ard.biesheuvel@...aro.org,
        sboyd@...eaurora.org, dave.hansen@...ux.intel.com,
        keescook@...omium.org, msalter@...hat.com, labbott@...hat.com,
        tglx@...utronix.de, Will Deacon <will.deacon@....com>
Subject: [PATCH v2 18/18] perf: arm_spe: Disallow userspace profiling when arm_kernel_unmapped_at_el0()

When running with the kernel unmapped whilst at EL0, the virtually-addressed
SPE buffer is also unmapped, which can lead to buffer faults if userspace
profiling is enabled.

This patch prohibits SPE profiling of userspace when
arm_kernel_unmapped_at_el0().

Signed-off-by: Will Deacon <will.deacon@....com>
---
 drivers/perf/arm_spe_pmu.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
index 8ce262fc2561..c028db8973a4 100644
--- a/drivers/perf/arm_spe_pmu.c
+++ b/drivers/perf/arm_spe_pmu.c
@@ -675,6 +675,13 @@ static int arm_spe_pmu_event_init(struct perf_event *event)
 		return -EOPNOTSUPP;
 
 	/*
+	 * If kernelspace is unmapped when running at EL0, then the SPE
+	 * buffer will fault and prematurely terminate the AUX session.
+	 */
+	if (arm64_kernel_unmapped_at_el0() && !attr->exclude_user)
+		dev_warn_once(&spe_pmu->pdev->dev, "unable to write to profiling buffer from EL0. Try passing \"kaiser=off\" on the kernel command line");
+
+	/*
 	 * Feedback-directed frequency throttling doesn't work when we
 	 * have a buffer of samples. We'd need to manually count the
 	 * samples in the buffer when it fills up and adjust the event
-- 
2.1.4