linux-kernel - [PATCH 3/3] arm64: compat: Fix __arch_get_hw

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Date:   Tue, 25 Jun 2019 17:18:04 +0100
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Cc:     catalin.marinas@....com, will.deacon@....com, arnd@...db.de,
        linux@...linux.org.uk, ralf@...ux-mips.org, paul.burton@...s.com,
        daniel.lezcano@...aro.org, tglx@...utronix.de, salyzyn@...roid.com,
        pcc@...gle.com, shuah@...nel.org, 0x7f454c46@...il.com,
        linux@...musvillemoes.dk, huw@...eweavers.com,
        sthotton@...vell.com, andre.przywara@....com
Subject: [PATCH 3/3] arm64: compat: Fix __arch_get_hw_counter() implementation

Provide the following fixes for the __arch_get_hw_counter()
implementation on arm64:
- Fallback on syscall when an unstable counter is detected.
- Introduce isb()s before and after the counter read to avoid
speculation of the counter value and of the seq lock
respectively.
The second isb() is a temporary solution that will be revisited
in 5.3-rc1.

These fixes restore the semantics that __arch_counter_get_cntvct()
had on arm64.

Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Will Deacon <will.deacon@....com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
---
 .../include/asm/vdso/compat_gettimeofday.h     | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/arm64/include/asm/vdso/compat_gettimeofday.h b/arch/arm64/include/asm/vdso/compat_gettimeofday.h
index 93dbd935b66d..f4812777f5c5 100644
--- a/arch/arm64/include/asm/vdso/compat_gettimeofday.h
+++ b/arch/arm64/include/asm/vdso/compat_gettimeofday.h
@@ -12,6 +12,8 @@
 
 #include <asm/vdso/compat_barrier.h>
 
+#define __VDSO_USE_SYSCALL		ULLONG_MAX
+
 #define VDSO_HAS_CLOCK_GETRES		1
 
 static __always_inline
@@ -74,8 +76,24 @@ static __always_inline u64 __arch_get_hw_counter(s32 clock_mode)
 {
 	u64 res;
 
+	/*
+	 * clock_mode == 0 implies that vDSO are enabled otherwise
+	 * fallback on syscall.
+	 */
+	if (clock_mode)
+		return __VDSO_USE_SYSCALL;
+
+	/*
+	 * This isb() is required to prevent that the counter value
+	 * is speculated.
+	 */
 	isb();
 	asm volatile("mrrc p15, 1, %Q0, %R0, c14" : "=r" (res));
+	/*
+	 * This isb() is required to prevent that the seq lock is
+	 * speculated.
+	 */
+	isb();
 
 	return res;
 }
-- 
2.22.0