x86/fpu: Allow setting of XSAVE state

We want to modify the Protection Key rights inside the kernel, so
we need to change PKRU's contents.  But, if we do a plain
'wrpkru', when we return to userspace we might do an XRSTOR and
wipe out the kernel's 'wrpkru'.  So, we need to go after PKRU in
the xsave buffer.

We do this by:

  1. Ensuring that we have the XSAVE registers (fpregs) in the
     kernel FPU buffer (fpstate)
  2. Looking up the location of a given state in the buffer
  3. Filling in the stat
  4. Ensuring that the hardware knows that state is present there
     (basically that the 'init optimization' is not in place).
  5. Copying the newly-modified state back to the registers if
     necessary.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20160212210235.5A3139BF@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 63 insertions, 0 deletions

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 299b58bb975b..dea8e76d60c6 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -354,6 +354,69 @@ void fpu__activate_fpstate_write(struct fpu *fpu)
 }
 
 /*
+ * This function must be called before we write the current
+ * task's fpstate.
+ *
+ * This call gets the current FPU register state and moves
+ * it in to the 'fpstate'.  Preemption is disabled so that
+ * no writes to the 'fpstate' can occur from context
+ * swiches.
+ *
+ * Must be followed by a fpu__current_fpstate_write_end().
+ */
+void fpu__current_fpstate_write_begin(void)
+{
+	struct fpu *fpu = &current->thread.fpu;
+
+	/*
+	 * Ensure that the context-switching code does not write
+	 * over the fpstate while we are doing our update.
+	 */
+	preempt_disable();
+
+	/*
+	 * Move the fpregs in to the fpu's 'fpstate'.
+	 */
+	fpu__activate_fpstate_read(fpu);
+
+	/*
+	 * The caller is about to write to 'fpu'.  Ensure that no
+	 * CPU thinks that its fpregs match the fpstate.  This
+	 * ensures we will not be lazy and skip a XRSTOR in the
+	 * future.
+	 */
+	fpu->last_cpu = -1;
+}
+
+/*
+ * This function must be paired with fpu__current_fpstate_write_begin()
+ *
+ * This will ensure that the modified fpstate gets placed back in
+ * the fpregs if necessary.
+ *
+ * Note: This function may be called whether or not an _actual_
+ * write to the fpstate occurred.
+ */
+void fpu__current_fpstate_write_end(void)
+{
+	struct fpu *fpu = &current->thread.fpu;
+
+	/*
+	 * 'fpu' now has an updated copy of the state, but the
+	 * registers may still be out of date.  Update them with
+	 * an XRSTOR if they are active.
+	 */
+	if (fpregs_active())
+		copy_kernel_to_fpregs(&fpu->state);
+
+	/*
+	 * Our update is done and the fpregs/fpstate are in sync
+	 * if necessary.  Context switches can happen again.
+	 */
+	preempt_enable();
+}
+
+/*
  * 'fpu__restore()' is called to copy FPU registers from
  * the FPU fpstate to the live hw registers and to activate
  * access to the hardware registers, so that FPU instructions