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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 -- Sent from my Android phone with K-9 Mail. Please excuse my brevity. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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