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