linux-kernel - Re: [PATCH 12/27] x86/msr: Restrict MSR access when the kernel is locked down

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Date:   Mon, 23 Oct 2017 15:49:44 +0100
From:   David Howells <dhowells@...hat.com>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>, jlee@...e.com
Cc:     dhowells@...hat.com, linux-security-module@...r.kernel.org,
        linux-efi@...r.kernel.org, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, jforbes@...hat.com
Subject: Re: [PATCH 12/27] x86/msr: Restrict MSR access when the kernel is locked down

Alan Cox <gnomes@...rguk.ukuu.org.uk> wrote:

> There are a load of standard tools that use this so I think you are going
> to need a whitelist. Can you at least log *which* MSR in the failing case
> so a whitelist can be built over time ?

Will the attached change work for you?

David
---
diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index a05a97863286..f18cadbc31ce 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -84,8 +84,10 @@ static ssize_t msr_write(struct file *file, const char __user *buf,
 	int err = 0;
 	ssize_t bytes = 0;
 
-	if (kernel_is_locked_down("Direct MSR access"))
+	if (kernel_is_locked_down("Direct MSR access")) {
+		pr_info("Direct access to MSR %x\n", reg);
 		return -EPERM;
+	}
 
 	if (count % 8)
 		return -EINVAL;	/* Invalid chunk size */
@@ -135,6 +137,7 @@ static long msr_ioctl(struct file *file, unsigned int ioc, unsigned long arg)
 			break;
 		}
 		if (kernel_is_locked_down("Direct MSR access")) {
+			pr_info("Direct access to MSR %x\n", reg[1]); /* Display %ecx */
 			err = -EPERM;
 			break;
 		}