[PATCH v2] x86/msr: Allow unprivileged read access to some MSRs

[Tim Wiederhake <twiederh@redhat.com>]

Software such as qemu and libvirt require the raw content of some MSRs
to calculate host CPU capabilities. This is currently done through
/dev/cpu/*/msr which is locked behind both CAP_SYS_RAWIO and file mode
0600, allowing only root to read and write MSRs.

Expose some non-security sensitive MSRs through sysfs to allow access
for unprivileged processes. This also helps other programs that are
interested in IA32_EFER for x86-64-v1 detection.

Signed-off-by: Tim Wiederhake <twiederh@redhat.com>
---
Changes to v1 (https://lkml.org/lkml/2023/5/23/1230):
* removed patch to limit reads to /dev/cpu/*/msr to 8 bytes per read
* removed CAP_SYS_RAWIO-less access to /dev/cpu/*/msr
* introduced sysfs interface to msrs

With this sysfs-based, unrestricted read access to some select msrs in
place, a later patch could introduce checks for CAP_SYS_RAWIO for every
access to /dev/cpu/*/msr as mentioned in the feedback to v1.
---
 arch/x86/kernel/msr.c | 45 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index 7bb17d37db01..3c8354f3c2bd 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -50,6 +50,31 @@ enum allow_write_msrs {
 
 static enum allow_write_msrs allow_writes = MSR_WRITES_DEFAULT;
 
+struct allow_read_msrs {
+	const char *procname;
+	u32 index;
+	u32 value[2];
+};
+
+static struct allow_read_msrs allow_reads[] = {
+	{
+		.procname = "ia32_core_caps",
+		.index = MSR_IA32_CORE_CAPS,
+	},
+	{
+		.procname = "ia32_arch_capabilities",
+		.index = MSR_IA32_ARCH_CAPABILITIES,
+	},
+	{
+		.procname = "efer",
+		.index = MSR_EFER,
+	},
+};
+
+static struct ctl_table msr_files[ARRAY_SIZE(allow_reads) + 1];
+
+static struct ctl_table_header *msr_files_header;
+
 static ssize_t msr_read(struct file *file, char __user *buf,
 			size_t count, loff_t *ppos)
 {
@@ -258,6 +283,25 @@ static char *msr_devnode(const struct device *dev, umode_t *mode)
 static int __init msr_init(void)
 {
 	int err;
+	int i, j;
+
+	for (i = 0, j = 0; i < ARRAY_SIZE(allow_reads); ++i) {
+		err = rdmsr_safe_on_cpu(0, allow_reads[i].index,
+					&allow_reads[i].value[0],
+					&allow_reads[i].value[1]);
+		if (err)
+			continue;
+		msr_files[j].procname = allow_reads[i].procname;
+		msr_files[j].data = &allow_reads[i].value;
+		msr_files[j].maxlen = 2 * sizeof(u32);
+		msr_files[j].mode = 0444;
+		msr_files[j].proc_handler = proc_doulongvec_minmax;
+		++j;
+	}
+
+	msr_files_header = register_sysctl("vm/msr", msr_files);
+	if (!msr_files_header)
+		return -ENOMEM;
 
 	if (__register_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr", &msr_fops)) {
 		pr_err("unable to get major %d for msr\n", MSR_MAJOR);
@@ -287,6 +331,7 @@ module_init(msr_init);
 
 static void __exit msr_exit(void)
 {
+	unregister_sysctl_table(msr_files_header);
 	cpuhp_remove_state(cpuhp_msr_state);
 	class_destroy(msr_class);
 	__unregister_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr");
-- 
2.39.2