Reading /dev/cpu/*/msr from userspace: operation not permitted

Question

I am trying to write a simple application that can read msr registers, and am running this application from userspace.

I have loaded the msr module and given read permissions for everyone to /dev/cpu/*/msr. But still the user is not able to access these files but the root can.

The permissions look like this:

crw-r--r-- 1 root root 202, 0 sep  6 17:55 /dev/cpu/0/msr

crw-r--r-- 1 root root 202, 1 sep  6 17:55 /dev/cpu/1/msr

crw-r--r-- 1 root root 202, 2 sep  6 17:55 /dev/cpu/2/msr

crw-r--r-- 1 root root 202, 3 sep  6 17:55 /dev/cpu/3/msr

I keep getting "Operation not permitted" error message when I try to read these files from userspace but works fine when root tries to access them. What am I doing wrong? I am on Ubuntu 13.04 with kernel version 3.11.0.

Answer 1

Changes in the mainline Linux kernel since around 3.7 now require an executable to have capability CAP_SYS_RAWIO to open the MSR device file [2]. Besides loading the MSR kernel module and setting the appropriate file permissions on the msr device file, one must grant the CAP_SYS_RAWIO capability to any user executable that needs access to the MSR driver, using the command below:

sudo setcap cap_sys_rawio=ep <user_executable>

Answer 2

You can see vfs_read:

ssize_t vfs_read(struct file *file, char __user *buf, size_t count, loff_t *pos)
{
    ret = rw_verify_area(READ, file, pos, count);
    if (ret >= 0) {
        ...
        if (file->f_op->read)  // your driver read .
            ret = file->f_op->read(file, buf, count, pos);
        else
            ret = do_sync_read(file, buf, count, pos);
        ....

    }
    // here,  if the ret is  13.  your error will be occur.
    return ret;
}