linux-kernel - Re: [RFC v2] x86-64: Allow emulated vsyscalls from user addresses

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Message-ID: <CAObL_7Et-CcamO+X4D5tvH3oWh9=Zog0t=MzOnyqNPzmGqmm2A@mail.gmail.com>
Date:	Sun, 7 Aug 2011 08:58:56 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...e.hu>,
	Andy Lutomirski <luto@....edu>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, lueckintel@...oo.com,
	kimwooyoung@...il.com, Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: [RFC v2] x86-64: Allow emulated vsyscalls from user addresses

On Sun, Aug 7, 2011 at 8:19 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Sat, Aug 06, 2011 at 08:45:52AM +0200, Ingo Molnar wrote:
>> > diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
>> > index 9682ec5..6ae5e3a 100644
>> > --- a/arch/x86/kernel/traps.c
>> > +++ b/arch/x86/kernel/traps.c
>> > @@ -873,9 +873,17 @@ void __init trap_init(void)
>> >  #endif
>> >
>> >  #ifdef CONFIG_X86_64
>> > -   BUG_ON(test_bit(VSYSCALL_EMU_VECTOR, used_vectors));
>> > -   set_system_intr_gate(VSYSCALL_EMU_VECTOR, &emulate_vsyscall);
>> > -   set_bit(VSYSCALL_EMU_VECTOR, used_vectors);
>> > +   BUG_ON(test_bit(VSYSCALL0_EMU_VECTOR, used_vectors));
>> > +   set_system_intr_gate(VSYSCALL0_EMU_VECTOR, &emulate_vsyscall0);
>> > +   set_bit(VSYSCALL0_EMU_VECTOR, used_vectors);
>> > +
>> > +   BUG_ON(test_bit(VSYSCALL1_EMU_VECTOR, used_vectors));
>> > +   set_system_intr_gate(VSYSCALL1_EMU_VECTOR, &emulate_vsyscall1);
>> > +   set_bit(VSYSCALL1_EMU_VECTOR, used_vectors);
>> > +
>> > +   BUG_ON(test_bit(VSYSCALL2_EMU_VECTOR, used_vectors));
>> > +   set_system_intr_gate(VSYSCALL2_EMU_VECTOR, &emulate_vsyscall2);
>> > +   set_bit(VSYSCALL2_EMU_VECTOR, used_vectors);
>> >  #endif
>
> ..
>
>> Surprisingly, this looks a bit cleaner to me than the original code,
>> as the emulated syscalls separate out so nicely.
>>
>> The flip side is using up more of our vector space - but realistically
>> we could put all this code behind a default-off LEGACY_VSYSCALL switch
>> a year or two down the line, when distros have ugpraded glibc.
>
> This probably needs an entry in <Documentation/feature-removal-schedule.txt>.

Will do.  Maybe that will encourage glibc to stop using them in static binaries.

I'll wire up sys_getcpu on x86_64 at the same time for good measure.
It's not currently available as a real syscall.

>
> Also, what do we do with userspace which decides to hardcode "int 0x4[012]"
> somewhere in the meantime?

Break it?  Any code that does that will get an unconditional warning
with this patch.

--Andy
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