UPSTREAM: x86/msr: Add capabilities check

UPSTREAM: 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>
BUG=chromium-os:38633
TEST=link build, reading MSR correctly fails as uid 0 without caps:
 (can't use iotools with minijail because iotools is statically linked)
 `minijail0 -c 0 /usr/local/bin/python -c 'from struct import *; \
    m = open("/dev/cpu/0/msr"); m.seek(0x3a); \
    print "0x%016x" % (unpack("Q", m.read(8)))'`

(cherry picked from upstream commit c903f0456bc69176912dee6dd25c6a66ee1aed00)
Change-Id: I9d636ff7baac98a4243392428b48c0730600c7bc
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/42684
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>