x86/msr: Add capabilities check

At the moment the MSR driver only relies upon file system
checks. This means that anything as root with any capability set
can write to MSRs. Historically that wasn't very interesting but
on modern processors the MSRs are such that writing to them
provides several ways to execute arbitary code in kernel space.
Sample code and documentation on doing this is circulating and
MSR attacks are used on Windows 64bit rootkits already.

In the Linux case you still need to be able to open the device
file so the impact is fairly limited and reduces the security of
some capability and security model based systems down towards
that of a generic "root owns the box" setup.

Therefore they should require CAP_SYS_RAWIO to prevent an
elevation of capabilities. The impact of this is fairly minimal
on most setups because they don't have heavy use of
capabilities. Those using SELinux, SMACK or AppArmor rules might
want to consider if their rulesets on the MSR driver could be
tighter.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Horses <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index a7c5661f84962a2f6eb80509c1791392352a6fc8..4929502c1372db979d7e1b176b22a9981e8ec96e 100644 (file)
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -174,6 +174,9 @@ static int msr_open(struct inode *inode, struct file *file)
        unsigned int cpu;
        struct cpuinfo_x86 *c;
 
+       if (!capable(CAP_SYS_RAWIO))
+               return -EPERM;
+
        cpu = iminor(file->f_path.dentry->d_inode);
        if (cpu >= nr_cpu_ids || !cpu_online(cpu))
                return -ENXIO;  /* No such CPU */