Right after SYSENTER, we can get a #DB or NMI.  On x86_32, there's no IST,
so the exception handler is invoked on the temporary SYSENTER stack.

Because the SYSENTER stack is very small, we have a fixup to switch
off the stack quickly when this happens.  The old fixup had several issues:

 1. It checked the interrupt frame's CS and EIP.  This wasn't
    obviously correct on Xen or if vm86 mode was in use [1].

 2. In the NMI handler, it did some frightening digging into the
    stack frame.  I'm not convinced this digging was correct.

 3. The fixup didn't switch stacks and then switch back.  Instead, it
    synthesized a brand new stack frame that would redirect the IRET
    back to the SYSENTER code.  That frame was highly questionable.
    For one thing, if NMI nested inside #DB, we would effectively
    abort the #DB prologue, which was probably safe but was
    frightening.  For another, the code used PUSHFL to write the
    FLAGS portion of the frame, which was simply bogus -- by the time
    PUSHFL was called, at least TF, NT, VM, and all of the arithmetic
    flags were clobbered.

Simplify this considerably.  Instead of looking at the saved frame
to see where we came from, check the hardware ESP register against
the SYSENTER stack directly.  Malicious user code cannot spoof the
kernel ESP register, and by moving the check after SAVE_ALL, we can
use normal PER_CPU accesses to find all the relevant addresses.

With this patch applied, the improved syscall_nt_32 test finally
passes on 32-bit kernels.

[1] It isn't obviously correct, but it is nonetheless safe from vm86
    shenanigans as far as I can tell.  A user can't point EIP at
    entry_SYSENTER_32 while in vm86 mode because entry_SYSENTER_32,
    like all kernel addresses, is greater than 0xffff and would thus
    violate the CS segment limit.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
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: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/b2cdbc037031c07ecf2c40a96069318aec0e7971.1457578375.git.luto@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>