x86/sched/64: Don't save flags on context switch (reinstated)

This reinstates the following commit:

  2c7577a7 ("sched/x86_64: Don't save flags on context switch")

which was reverted in:

  512255a2 ("Revert 'sched/x86_64: Don't save flags on context switch'")

Historically, Linux has always saved and restored EFLAGS across
context switches.  As far as I know, the only reason to do this
is because of the NT flag.  In particular, if something calls
switch_to() with the NT flag set, then we don't want to leak the
NT flag into a different task that might try to IRET and fail
because NT is set.

Before this commit:

  8c7aa698

 ("x86_64, entry: Filter RFLAGS.NT on entry from userspace")

we could run system call bodies with NT set.  This would be a DoS or possibly
privilege escalation hole if scheduling in such a system call would leak
NT into a different task.

Importantly, we don't need to worry about NT being set while
preemptible or across page faults.  The only way we can schedule
due to preemption or a page fault is in an interrupt entry that
nests inside the SYSENTER prologue.  The CPU will clear NT when
entering through an interrupt gate, so we won't schedule with NT
set.

The only other interesting flags are IOPL and AC.  Allowing
switch_to() to change IOPL has no effect, as the value loaded
during kernel execution doesn't matter at all except between a
SYSENTER entry and the subsequent PUSHF, and anythign that
interrupts in that window will restore IOPL on return.

If we call __switch_to() with AC set, we have bigger problems.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/d4440fdc2a89247bffb7c003d2a9a2952bd46827.1441146105.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>