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Date:   Sat, 10 Apr 2021 17:33:38 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Feng Tang <feng.tang@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org, rui.zhang@intel.com,
        andi.kleen@intel.com, dave.hansen@intel.com, len.brown@intel.com
Subject: [PATCH -v1.1] x86/msr: Block writes to certain MSRs unconditionally
Message-ID: <20210410153338.GD21691@zn.tnic>
References: <20210410121144.GC21691@zn.tnic>
 <437BACC3-A1F7-4C65-80C7-98F6652A14E6@amacapital.net>
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On Sat, Apr 10, 2021 at 07:51:58AM -0700, Andy Lutomirski wrote:
> Can you add STAR, CSTAR, LSTAR, SYSENTER*, SYSCALL*, and EFER please?

Sure.

---

From: Borislav Petkov <bp@suse.de>
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 and the
ring0-entry and features control MSRs in there. Issue a big juicy splat
to catch offenders.

Signed-off-by: Borislav Petkov <bp@suse.de>
---
 arch/x86/kernel/msr.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index ed8ac6bcbafb..2435a619cd9f 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -78,6 +78,21 @@ 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,
+	MSR_EFER,
+	MSR_STAR,
+	MSR_CSTAR,
+	MSR_LSTAR,
+	MSR_SYSCALL_MASK,
+	MSR_IA32_SYSENTER_CS,
+	MSR_IA32_SYSENTER_ESP,
+	MSR_IA32_SYSENTER_EIP,
+};
+
 static int filter_write(u32 reg)
 {
 	/*
@@ -89,6 +104,16 @@ 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.

https://people.kernel.org/tglx/notes-about-netiquette