Re: [PATCH 08/10] x86, pkeys: default to a restrictive init PKRU

On Fri, Jul 29, 2016 at 9:30 AM, Dave Hansen [off-list ref] wrote:

From: Dave Hansen <dave.hansen@linux.intel.com>

PKRU is the register that lets you disallow writes or all access
to a given protection key.

The XSAVE hardware defines an "init state" of 0 for PKRU: its
most permissive state, allowing access/writes to everything.
Since we start off all new processes with the init state, we
start all processes off with the most permissive possible PKRU.

This is unfortunate.  If a thread is clone()'d [1] before a
program has time to set PKRU to a restrictive value, that thread
will be able to write to all data, no matter what pkey is set on
it.  This weakens any integrity guarantees that we want pkeys to
provide.

To fix this, we define a very restrictive PKRU to override the
XSAVE-provided value when we create a new FPU context.  We choose
a value that only allows access to pkey 0, which is as
restrictive as we can practically make it.

This does not cause any practical problems with applications
using protection keys because we require them to specify initial
permissions for each key when it is allocated, which override the
restrictive default.

In the end, this ensures that threads which do not know how to
manage their own pkey rights can not do damage to data which is
pkey-protected.

I think you missed the fpu__clear() caller in kernel/fpu/signal.c.

ISTM it might be more comprehensible to change fpu__clear in general
and then special case things you want to behave differently.