linux-kernel - Re: [Xen-devel] [RFC v3 2/2] x86/xen: allow privcmd hypercalls to be preempted

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Date:	Thu, 22 Jan 2015 13:21:56 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"Luis R. Rodriguez" <mcgrof@...e.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	kvm list <kvm@...r.kernel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Jan Beulich <JBeulich@...e.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...e.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [Xen-devel] [RFC v3 2/2] x86/xen: allow privcmd hypercalls to be preempted

On Thu, Jan 22, 2015 at 1:16 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Thu, 22 Jan 2015 12:58:00 -0800
> Andy Lutomirski <luto@...capital.net> wrote:
>
>> On Thu, Jan 22, 2015 at 12:37 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>> > On Thu, 22 Jan 2015 12:24:47 -0800
>> > Andy Lutomirski <luto@...capital.net> wrote:
>>
>> >>
>> >> The other issue, above and beyond RCU, is that we can't let kprobes
>> >> run on the int3 stack.  If Xen upcalls can happen when interrupts are
>> >> off, then we may need this protection to prevent that type of
>> >> recursion.  (This will be much less scary in 3.20, because userspace
>> >> int3 instructions will no longer execute on the int3 stack.)
>> >
>> > Does this execute between the start of the int3 interrupt handler and
>> > the call of do_int3()?
>>
>> I doubt it.
>>
>> The thing I worry about is that, if do_int3 nests inside itself by any
>> means (e.g. int3 sends a signal, scheduling for whatever reason
>> (really shouldn't happen, but I haven't looked that hard)), then we're
>> completely hosed -- the inner int3 will overwrite the outer int3's
>> stack frame.  Since I have no idea what Xen upcalls do, I don't know
>> whether they can fire inside do_int3.
>
> I thought there's logic in the do_int3 handler (in the assembly code)
> that can handle nested int3s.

Nope :(

In 3.20, there's likely to be logic that can handle a single level of
nesting as long as the outer one came from user space.

--Andy
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