linux-kernel - Re: [PATCH][GIT PULL][v3.3] x86: Test saved %rip in NMI to determine nested NMI

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Date:	Sun, 19 Feb 2012 05:46:13 -0800
From:	"hpanvin@...il.com" <hpa@...or.com>
To:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>
CC:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH][GIT PULL][v3.3] x86: Test saved %rip in NMI to determine nested NMI

Vsyscall page, not vdso...

Ingo Molnar <mingo@...e.hu> wrote:

>
>* Steven Rostedt <rostedt@...dmis.org> wrote:
>
>> Ingo,
>> 
>> I found that it is possible for userspace to prevent an NMI 
>> from triggering while it is running by setting its stack 
>> pointer to that of the NMI stack. This tricks the NMI nested 
>> algorithm in thinking that the NMI is nested. The easy 
>> solution to this is to test the %rip to make sure that the NMI 
>> happened in kernel mode before testing for nesting.
>
>Ouch...
>
>> I've tested a program that exhibits the missing NMIs and this 
>> patch corrects that behavior.
>
>Does it need a -stable tag?
>
>> Please pull the latest tip/perf/urgent tree, which can be 
>> found at:
>> 
>>  
>git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git
>> tip/perf/urgent
>> 
>> Head SHA1: b80ddc7b1636474297815d47fbfed7552f9b8f2c
>> 
>> 
>> Steven Rostedt (1):
>>       x86: Test saved %rip in NMI to determine nested NMI
>> 
>> ----
>>  arch/x86/kernel/entry_64.S |    8 ++++++++
>>  1 files changed, 8 insertions(+), 0 deletions(-)
>> ---------------------------
>> commit b80ddc7b1636474297815d47fbfed7552f9b8f2c
>> Author: Steven Rostedt <srostedt@...hat.com>
>> Date:   Sat Feb 18 20:26:52 2012 -0500
>> 
>>     x86: Test saved %rip in NMI to determine nested NMI
>>     
>>     Currently, the NMI handler tests if it is nested by checking the
>>     special variable saved no the stack (set during NMI handling) and
>>     whether the saved stack is the NMI stack as well (to prevent the
>race
>>     when the variable is set to zero). But userspace may set their
>%rsp
>>     to any value as long as the do not derefence it, and it may make
>it
>>     point to the NMI stack, which will prevent NMIs from triggering
>while
>>     the userspace app is running. (I tested this, and it is indeed
>the case)
>>     
>>     Add another check to determine nested NMIs by looking at the
>saved
>>     %rip and making sure that it is a kernel pointer (negative).
>>     
>>     Cc: H. Peter Anvin <hpa@...or.com>
>>     Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
>> 
>> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
>> index 3fe8239..7c35a7a 100644
>> --- a/arch/x86/kernel/entry_64.S
>> +++ b/arch/x86/kernel/entry_64.S
>> @@ -1532,6 +1532,14 @@ ENTRY(nmi)
>>  	pushq_cfi %rdx
>>  
>>  	/*
>> +	 * If the RIP is not negative then we are in userspace where this
>is not
>> +	 * a nested NMI. 
>> +	 */
>> +	movq 8(%rsp), %rdx
>> +	testq %rdx, %rdx
>> +	jns first_nmi
>
>Does this do the right thing for the vDSO as well? It is in 
>negative addresses:
>
>ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                
> [vsyscall]
>
>Thanks,
>
>	Ingo

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