[PATCH v2 2/2] x86/msr: Restrict /dev/cpu/*/msr read to a single MSR per call

[Tim Wiederhake]

Reading /dev/cpu/*/msr with a buffer larger than 8 bytes reads not
successive MSRs, but the same one over and over.  While documented,
this behavior is unintuitive: A user space caller issuing a 16-byte
read might reasonably expect to receive two consecutive MSRs rather
than the same register twice.

Remove the loop and process a single 8-byte MSR value per call to
avoid this ambiguity.

Signed-off-by: Tim Wiederhake <twiederh@xxxxxxxxxx>
---
 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 cc719a6b2ba3..1ea7bd393072 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -60,24 +60,17 @@ 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)
-- 
2.52.0