preempt notifier emulation host crash fix

[Andrea Arcangeli <andrea-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Hello,

there's a small glitch in the preempt notifier external module
emulation. The overloaded debug handler will not detect when a debug
exception has been generated by ptrace and it'll crash the host by
calling the preempt emulator like if this was a KVM preempt emulated
exception, instead of notifying userland.

To detect when the preempt emulation is needed and when the exception
should be bypassed to the original handler, I decided to use bit 10 of
db7 that can't be set to 1 by ptrace, the mask against the userland
passed db7 value is 0xfc00, so bit 10 is forbidden to be on unless it
was KVM setting it manually with 0x701 (kvm really only needs 0x301 to
get exact exception, dunno what 0x400 means, it's defined reserved,
but it doesn't matter what it means as long as ptrace can't set it ;).

So this fixes the host crash for me:

Signed-off-by: Andrea Arcangeli <andrea-atKUWr5tajBWk0Htik3J/w@public.gmane.org>

diff --git a/kernel/preempt.c b/kernel/preempt.c
index ed5d1c1..0ae69d7 100644
--- a/kernel/preempt.c
+++ b/kernel/preempt.c
@@ -143,10 +143,10 @@ unsigned long orig_int1_handler;
 
 asm ("pn_int1_handler:  \n\t"
      "push "  TMP " \n\t"
-     "mov %db6, " TMP " \n\t"
-     "test $1, " TMP " \n\t"
+     "mov %db7, " TMP " \n\t"
+     "cmp $0x701, " TMP " \n\t"
      "pop "  TMP " \n\t"
-     "jz .Lnotme \n\t"
+     "jnz .Lnotme \n\t"
      SAVE_REGS "\n\t"
 #ifdef CONFIG_X86_64
      "leaq 120(%rsp),%rdi\n\t"


Testing is very easy, after loading kvm:

andrea@svm ~ $ cat main.c
main() {}
andrea@svm ~ $ gcc main.c -g
andrea@svm ~ $ gdb a.out
GNU gdb 6.7.1
Copyright (C) 2007 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
<http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show
copying"
and "show warranty" for details.
This GDB was configured as "x86_64-pc-linux-gnu"...
Using host libthread_db library "/lib/libthread_db.so.1".
(gdb) hbreak main
Hardware assisted breakpoint 1 at 0x4004bc: file main.c, line 1.
(gdb) r
Starting program: /home/andrea/a.out

Breakpoint 1, main () at main.c:1
1       main() {}


Whenever the external module was loaded host would reboot instantly
after "r". To test it further I added the WARN_ON back to vcpu_put
handler to verify the vcpu->cpu matches smp_processor_id the whole
time (so preempt emulation is working ok, with SVM that would be
visible only with rdtsc not being monotone from the point of view of
each vcpu in smp host w/o taskset binding the vcpu to a single
host-cpu, only vmx would crash the host if preempt notifiers don't
fire).

I suppose the bug existed way before I rewritten the sched_in
emulation, because I didn't touch or pay attention to the ptrace
bypass.

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