[kernel-hardening] [PATCH] x86/fpu: Simplify fpu__activate_fpstate_read()

[Ingo Molnar <mingo@kernel.org>]

* Eric Biggers <ebiggers3@gmail.com> wrote:

> On Fri, Sep 22, 2017 at 07:33:14AM +0200, Ingo Molnar wrote:
> > 
> > * Eric Biggers <ebiggers3@gmail.com> wrote:
> > 
> > > From: Eric Biggers <ebiggers@google.com>
> > > 
> > > This series fixes the bug found by syzkaller where the ptrace syscall
> > > can be used to set invalid bits in a task's FPU state.  I also found
> > > that an equivalent bug was reachable using the sigreturn syscall, so the
> > > first patch fixes the bug in both cases.
> > > 
> > > The other two patches start validating the other parts of the
> > > xstate_header and make it so that invalid FPU states can no longer be
> > > abused to leak the FPU registers of other processes.
> > > 
> > > Changes since v2:
> > >     - Use an exception handler to handle invalid FPU states
> > >       (suggested by Andy Lutomirski)
> > >     - Check the size of xstate_header.reserved at build time
> > >       (suggested by Dave Hansen)
> > > 
> > > Eric Biggers (3):
> > >   x86/fpu: don't let userspace set bogus xcomp_bv
> > >   x86/fpu: tighten validation of user-supplied xstate_header
> > >   x86/fpu: reinitialize FPU registers if restoring FPU state fails
> > > 
> > >  arch/x86/include/asm/fpu/internal.h | 51 +++++++++++--------------------------
> > >  arch/x86/include/asm/fpu/xstate.h   | 25 ++++++++++++++++++
> > >  arch/x86/kernel/fpu/regset.c        | 20 +++++++--------
> > >  arch/x86/kernel/fpu/signal.c        | 15 ++++++++---
> > >  arch/x86/kernel/fpu/xstate.c        | 27 ++++++++------------
> > >  arch/x86/mm/extable.c               | 24 +++++++++++++++++
> > >  6 files changed, 94 insertions(+), 68 deletions(-)
> > 
> > Ok - could you please rebase these to to tip:master that is at:
> > 
> > 	git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git master
> > 
> > In particular this has a WIP.x86/fpu branch with FPU fixes+changes queued up but 
> > not merged upstream (yet), which conflict with these changes. I'd like to merge 
> > them all together.
> > 
> 
> Working on it, but there is a problem with current tip.  PTRACE_GETREGSET is
> causing the following warning:

Yes, the warning should be harmless, and I fixed it locally earlier today - does 
the patch below solve it for you as well?

Thanks,

	Ingo

============================>
Subject: x86/fpu: Simplify fpu__activate_fpstate_read()
From: Ingo Molnar <mingo@kernel.org>
Date: Sat Sep 23 11:41:23 CEST 2017

fpu__activate_fpstate_read() can only ever be called for a non-current,
non-executing (stopped) task - so make sure this is checked via a warning
and remove the current-task logic.

This also fixes an incorrect (but harmless) warning triggered by of the earlier 
patches.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
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: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/kernel/fpu/core.c |   24 ++++++++----------------
 1 file changed, 8 insertions(+), 16 deletions(-)

Index: linux/arch/x86/kernel/fpu/core.c
===================================================================
--- linux.orig/arch/x86/kernel/fpu/core.c
+++ linux/arch/x86/kernel/fpu/core.c
@@ -256,26 +256,18 @@ EXPORT_SYMBOL_GPL(fpu__activate_curr);
  *
  * If the task has not used the FPU before then initialize its
  * fpstate.
- *
- * If the task has used the FPU before then save it.
  */
 void fpu__activate_fpstate_read(struct fpu *fpu)
 {
-	/*
-	 * If fpregs are active (in the current CPU), then
-	 * copy them to the fpstate:
-	 */
-	if (fpu->fpstate_active) {
-		fpu__save(fpu);
-	} else {
-		if (!fpu->fpstate_active) {
-			fpstate_init(&fpu->state);
-			trace_x86_fpu_init_state(fpu);
+	WARN_ON_FPU(fpu == &current->thread.fpu);
+
+	if (!fpu->fpstate_active) {
+		fpstate_init(&fpu->state);
+		trace_x86_fpu_init_state(fpu);
 
-			trace_x86_fpu_activate_state(fpu);
-			/* Safe to do for current and for stopped child tasks: */
-			fpu->fpstate_active = 1;
-		}
+		trace_x86_fpu_activate_state(fpu);
+		/* Safe to do for current and for stopped child tasks: */
+		fpu->fpstate_active = 1;
 	}
 }