linux-kernel - [PATCH 1/2] x86/cpu: Initialize MSR_TSC_AUX if RDTSCP *or* RDPID is supported

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Message-Id: <20210504225632.1532621-2-seanjc@google.com>
Date:   Tue,  4 May 2021 15:56:31 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org
Cc:     "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Reiji Watanabe <reijiw@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>
Subject: [PATCH 1/2] x86/cpu: Initialize MSR_TSC_AUX if RDTSCP *or* RDPID is supported

Initialize MSR_TSC_AUX with CPU node information if RDTSCP or RDPID is
supported.  This fixes a bug where vdso_read_cpunode() will read garbage
via RDPID if RDPID is supported but RDTSCP is not.  While no known CPU
supports RDPID but not RDTSCP, both Intel's SDM and AMD's APM allow for
RDPID to exist without RDTSCP, e.g. it's technically a legal CPU model
for a virtual machine.

Note, technically MSR_TSC_AUX could be initialized if and only if RDPID
is supported since RDTSCP is currently not used to retrieve the CPU node.
But, the cost of the superfluous WRMSR is negigible, whereas leaving
MSR_TSC_AUX uninitialized is just asking for future breakage if someone
decides to utilize RDTSCP.

Fixes: a582c540ac1b ("x86/vdso: Use RDPID in preference to LSL when available")
Cc: stable@...r.kernel.org
Cc: Reiji Watanabe <reijiw@...gle.com>
Cc: Jim Mattson <jmattson@...gle.com>
Cc: Andy Lutomirski <luto@...nel.org>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kernel/cpu/common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index ab640abe26b6..1e576cc831c1 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1850,7 +1850,7 @@ static inline void setup_getcpu(int cpu)
 	unsigned long cpudata = vdso_encode_cpunode(cpu, early_cpu_to_node(cpu));
 	struct desc_struct d = { };

-	if (boot_cpu_has(X86_FEATURE_RDTSCP))
+	if (boot_cpu_has(X86_FEATURE_RDTSCP) || boot_cpu_has(X86_FEATURE_RDPID))
 		write_rdtscp_aux(cpudata);

 	/* Store CPU and node number in limit. */
-- 
2.31.1.527.g47e6f16901-goog