linux-kernel - [PATCH 1/2] x86/msr: Read MSRs individually

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Date:   Tue, 23 May 2023 21:49:48 +0200
From:   Tim Wiederhake <twiederh@...hat.com>
To:     "Borislav Petkov" <bp@...en8.de>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, "Ingo Molnar" <mingo@...hat.com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        "Thomas Gleixner" <tglx@...utronix.de>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Tim Wiederhake <twiederh@...hat.com>
Subject: [PATCH 1/2] x86/msr: Read MSRs individually

Reading from /dev/cpu/*/msr with buffer size > 8 would read the data
of the same msr repeatedly instead of the data for consecutive msrs,
as one might expect.

Solve by restricting MSR reads to one per call.

Signed-off-by: Tim Wiederhake <twiederh@...hat.com>
---
 arch/x86/kernel/msr.c | 21 +++++++--------------
 1 file changed, 7 insertions(+), 14 deletions(-)

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index 7bb17d37db01..058f2b67d0c7 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -58,24 +58,17 @@ static ssize_t msr_read(struct file *file, char __user *buf,
 	u32 reg = *ppos;
 	int cpu = iminor(file_inode(file));
 	int err = 0;
-	ssize_t bytes = 0;
 
-	if (count % 8)
+	if (count < 8)
 		return -EINVAL;	/* Invalid chunk size */
 
-	for (; count; count -= 8) {
-		err = rdmsr_safe_on_cpu(cpu, reg, &data[0], &data[1]);
-		if (err)
-			break;
-		if (copy_to_user(tmp, &data, 8)) {
-			err = -EFAULT;
-			break;
-		}
-		tmp += 2;
-		bytes += 8;
-	}
+	err = rdmsr_safe_on_cpu(cpu, reg, &data[0], &data[1]);
+	if (err)
+		return err;
+	if (copy_to_user(tmp, &data, 8))
+		return -EFAULT;
 
-	return bytes ? bytes : err;
+	return 8;
 }
 
 static int filter_write(u32 reg)
-- 
2.39.2