[PATCH] x86/msr: Block writes to certain MSRs unconditionally

[Borislav Petkov]

From: Borislav Petkov <bp@xxxxxxx>
Date: Sat, 10 Apr 2021 14:08:13 +0200

There are a bunch of MSRs which luserspace has no business poking at,
whatsoever. Add a ban list and put the TSC-related MSRs in there. Issue
a big juicy splat to catch offenders.

Signed-off-by: Borislav Petkov <bp@xxxxxxx>
---
 arch/x86/kernel/msr.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index ed8ac6bcbafb..574bd2c6d4f8 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -78,6 +78,13 @@ static ssize_t msr_read(struct file *file, char __user *buf,
 	return bytes ? bytes : err;
 }
 
+static const u32 msr_ban_list[] = {
+	MSR_IA32_TSC,
+	MSR_TSC_AUX,
+	MSR_IA32_TSC_ADJUST,
+	MSR_IA32_TSC_DEADLINE,
+};
+
 static int filter_write(u32 reg)
 {
 	/*
@@ -89,6 +96,16 @@ static int filter_write(u32 reg)
 	 * avoid saturating the ring buffer.
 	 */
 	static DEFINE_RATELIMIT_STATE(fw_rs, 30 * HZ, 1);
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(msr_ban_list); i++) {
+		if (msr_ban_list[i] != reg)
+			continue;
+
+		WARN_ONCE(1, "Blocked write to MSR 0x%x\n", reg);
+
+		return -EINVAL;
+	}
 
 	switch (allow_writes) {
 	case MSR_WRITES_ON:  return 0;
-- 
2.29.2


-- 
Regards/Gruss,
    Boris.

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