linux-kernel - Re: Disabling x86 System Management Mode

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Date:	Tue, 17 Apr 2007 18:49:09 +0200
From:	John Sigler <linux.kernel@...e.fr>
To:	Andi Kleen <andi@...stfloor.org>
CC:	linux-kernel@...r.kernel.org, linux.kernel@...e.fr
Subject: Re: Disabling x86 System Management Mode

Andi Kleen wrote:

> Please use a full real name for posting.

OK.

> John Sigler wrote:
> 
>> AFAIU, even a hard real-time OS is "defenseless" against SMIs that
>> kick the CPU into SMM.
> 
> There are usually chipset specific bits that can be set to disable SMMs.
> See the datasheet if you can get them. Unfortunately most chipset vendors
> don't give out data sheets easily.

I've asked the manufacturer to send me the data sheets.
We'll see how they react.

>> .globl foo
>> foo:
>>    push %ebx
>>    push %esi
>>    cpuid
>>    rdtsc
> 
> At least some SMM implementations restore the old TSC value. Sad but true.

Why would they do that?

How would you detect periodic SMM on such a system?

> Besides RDTSC can be speculated around on some CPUs which also adds errors.

I don't understand this sentence. Could you clarify?

I've attached my kernel module (hello.c)

# : >/var/log/kern.log; cat /proc/interrupts; /bin/time insmod houba.ko; 
cat /proc/interrupts; rmmod houba
            CPU0
   0:     519083    XT-PIC-XT        timer
   2:          0    XT-PIC-XT        cascade
   9:          0    XT-PIC-XT        acpi
  10:       9786    XT-PIC-XT        eth0
  11:          5    XT-PIC-XT        eth1
  12:          5    XT-PIC-XT        eth2
  14:      16920    XT-PIC-XT        ide0
NMI:          0
ERR:          0
0.00user 0.00system 5:00.00elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+111minor)pagefaults 0swaps
            CPU0
   0:     549094    XT-PIC-XT        timer
   2:          0    XT-PIC-XT        cascade
   9:          0    XT-PIC-XT        acpi
  10:       9791    XT-PIC-XT        eth0
  11:          5    XT-PIC-XT        eth1
  12:          5    XT-PIC-XT        eth2
  14:      16970    XT-PIC-XT        ide0
NMI:          0
ERR:          0

(HZ=100)
30011 timer interrupts
5 eth0 interrupts
50 ide0 interrupts

# cat /var/log/kern.log
Apr 17 18:22:27 SEND kernel: INIT
Apr 17 18:27:27 SEND kernel: 2350080 29995
Apr 17 18:27:27 SEND kernel: 2369792 1
Apr 17 18:27:27 SEND kernel: 2440192 1
Apr 17 18:27:27 SEND kernel: 2441216 1
Apr 17 18:27:27 SEND kernel: 2583296 1
Apr 17 18:27:27 SEND kernel: 2852096 1
Apr 17 18:27:27 SEND kernel: EXIT

First column is the cycle count clamped to a multiple of 256.
(1266.7 MHz CPU)
Second column is occurence count.

In the second experiment, I added the IRQ disable/enable around foo.

# : >/var/log/kern.log; cat /proc/interrupts; /bin/time insmod houba.ko; 
cat /proc/interrupts; rmmod houba
            CPU0
   0:     583666    XT-PIC-XT        timer
   2:          0    XT-PIC-XT        cascade
   9:          0    XT-PIC-XT        acpi
  10:      10084    XT-PIC-XT        eth0
  11:          5    XT-PIC-XT        eth1
  12:          5    XT-PIC-XT        eth2
  14:      17012    XT-PIC-XT        ide0
NMI:          0
ERR:          0
0.00user 0.01system 5:00.00elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+110minor)pagefaults 0swaps
            CPU0
   0:     613677    XT-PIC-XT        timer
   2:          0    XT-PIC-XT        cascade
   9:          0    XT-PIC-XT        acpi
  10:      10089    XT-PIC-XT        eth0
  11:          5    XT-PIC-XT        eth1
  12:          5    XT-PIC-XT        eth2
  14:      17070    XT-PIC-XT        ide0
NMI:          0
ERR:          0

30011 timer interrupts
5 eth0 interrupts
58 ide0 interrupts

# cat /var/log/kern.log
Apr 17 18:33:12 SEND kernel: INIT
Apr 17 18:38:12 SEND kernel: 2350080 30000
Apr 17 18:38:12 SEND kernel: EXIT

In this experiment, all the calls to foo had a latency between
2350080 and 2350335 cycles (1.88528 to 1.88548 ms).

I don't think I can conclude anything based on this experiment.

I'll let it run all night long.

Regards.

View attachment "hello.c" of type "text/plain" (939 bytes)