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SubjectRe: [PATCH][GIT PULL][v3.3] x86: Test saved %rip in NMI to determine nested NMI
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Vsyscall page, not vdso...

Ingo Molnar <mingo@elte.hu> wrote:

>
>* Steven Rostedt <rostedt@goodmis.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@redhat.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@zytor.com>
>> Signed-off-by: Steven Rostedt <rostedt@goodmis.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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