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From: Alexandre Chartre <alexandre.chartre@oracle.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Joerg Roedel <jroedel@suse.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	jan.setjeeilers@oracle.com, Junaid Shahid <junaids@google.com>,
	oweisse@google.com, Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Alexander Graf <graf@amazon.de>,
	mgross@linux.intel.com, kuzuno@gmail.com
Subject: Re: [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of trampoline stack
Date: Mon, 16 Nov 2020 20:37:03 +0100	[thread overview]
Message-ID: <93d1f346-7513-069f-dcd9-24f2ea009145@oracle.com> (raw)
In-Reply-To: <CALCETrUJQJRi6fE=bs3iAySgM8wjmGU1f464FqOuU+PiBwwnQQ@mail.gmail.com>



On 11/16/20 7:34 PM, Andy Lutomirski wrote:
> On Mon, Nov 16, 2020 at 10:10 AM Alexandre Chartre
> <alexandre.chartre@oracle.com> wrote:
>>
>>
>> On 11/16/20 5:57 PM, Andy Lutomirski wrote:
>>> On Mon, Nov 16, 2020 at 6:47 AM Alexandre Chartre
>>> <alexandre.chartre@oracle.com> wrote:
>>>>
>>>> When entering the kernel from userland, use the per-task PTI stack
>>>> instead of the per-cpu trampoline stack. Like the trampoline stack,
>>>> the PTI stack is mapped both in the kernel and in the user page-table.
>>>> Using a per-task stack which is mapped into the kernel and the user
>>>> page-table instead of a per-cpu stack will allow executing more code
>>>> before switching to the kernel stack and to the kernel page-table.
>>>
>>> Why?
>>
>> When executing more code in the kernel, we are likely to reach a point
>> where we need to sleep while we are using the user page-table, so we need
>> to be using a per-thread stack.
>>
>>> I can't immediately evaluate how nasty the page table setup is because
>>> it's not in this patch.
>>
>> The page-table is the regular page-table as introduced by PTI. It is just
>> augmented with a few additional mapping which are in patch 11 (x86/pti:
>> Extend PTI user mappings).
>>
>>>   But AFAICS the only thing that this enables is sleeping with user pagetables.
>>
>> That's precisely the point, it allows to sleep with the user page-table.
>>
>>> Do we really need to do that?
>>
>> Actually, probably not with this particular patchset, because I do the page-table
>> switch at the very beginning and end of the C handler. I had some code where I
>> moved the page-table switch deeper in the kernel handler where you definitively
>> can sleep (for example, if you switch back to the user page-table before
>> exit_to_user_mode_prepare()).
>>
>> So a first step should probably be to not introduce the per-task PTI trampoline stack,
>> and stick with the existing trampoline stack. The per-task PTI trampoline stack can
>> be introduced later when the page-table switch is moved deeper in the C handler and
>> we can effectively sleep while using the user page-table.
> 
> Seems reasonable.
> 
> Where is the code that allocates and frees these stacks hiding?  I
> think I should at least read it.

Stacks are allocated/freed with the task stack, this code is unchanged (see
alloc_thread_stack_node()). The trick is that I have doubled the THREAD_SIZE
(patch 8 "x86/pti: Introduce per-task PTI trampoline stack"). Half the stack
is a used as the kernel stack (mapped only in the kernel page-table), the
other half is used as the PTI stack (mapped in the kernel and user page-table).
The mapping to the user page-table is done in mm_map_task() in fork.c (patch 11
"x86/pti: Extend PTI user mappings").

alex.

  reply	other threads:[~2020-11-16 19:35 UTC|newest]

Thread overview: 69+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-16 14:47 [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 01/21] x86/syscall: Add wrapper for invoking syscall function Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 02/21] x86/entry: Update asm_call_on_stack to support more function arguments Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 03/21] x86/entry: Consolidate IST entry from userspace Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 04/21] x86/sev-es: Define a setup stack function for the VC idtentry Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 05/21] x86/entry: Implement ret_from_fork body with C code Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 06/21] x86/pti: Provide C variants of PTI switch CR3 macros Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 07/21] x86/entry: Fill ESPFIX stack using C code Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 08/21] x86/pti: Introduce per-task PTI trampoline stack Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 09/21] x86/pti: Function to clone page-table entries from a specified mm Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 10/21] x86/pti: Function to map per-cpu page-table entry Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 11/21] x86/pti: Extend PTI user mappings Alexandre Chartre
2020-11-16 19:48   ` Andy Lutomirski
2020-11-16 20:21     ` Alexandre Chartre
2020-11-16 23:06       ` Andy Lutomirski
2020-11-17  8:42         ` Alexandre Chartre
2020-11-17 15:49           ` Andy Lutomirski
2020-11-19 19:15           ` Thomas Gleixner
2020-11-16 14:47 ` [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of trampoline stack Alexandre Chartre
2020-11-16 16:57   ` Andy Lutomirski
2020-11-16 18:10     ` Alexandre Chartre
2020-11-16 18:34       ` Andy Lutomirski
2020-11-16 19:37         ` Alexandre Chartre [this message]
2020-11-17 15:09         ` Alexandre Chartre
2020-11-17 15:52           ` Andy Lutomirski
2020-11-17 17:01             ` Alexandre Chartre
2020-11-19  1:49               ` Andy Lutomirski
2020-11-19  8:05                 ` Alexandre Chartre
2020-11-19 12:06                   ` Alexandre Chartre
2020-11-19 16:06                     ` Andy Lutomirski
2020-11-19 17:02                       ` Alexandre Chartre
2020-11-16 21:24       ` David Laight
2020-11-17  8:27         ` Alexandre Chartre
2020-11-19 19:10       ` Thomas Gleixner
2020-11-19 19:55         ` Alexandre Chartre
2020-11-19 21:20           ` Thomas Gleixner
2020-11-24  7:20   ` [x86/pti] 5da9e742d1: PANIC:double_fault kernel test robot
2020-11-16 14:47 ` [RFC][PATCH v2 13/21] x86/pti: Execute syscall functions on the kernel stack Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 14/21] x86/pti: Execute IDT handlers " Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 15/21] x86/pti: Execute IDT handlers with error code " Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 16/21] x86/pti: Execute system vector handlers " Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 17/21] x86/pti: Execute page fault handler " Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 18/21] x86/pti: Execute NMI " Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 19/21] x86/pti: Defer CR3 switch to C code for IST entries Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 20/21] x86/pti: Defer CR3 switch to C code for non-IST and syscall entries Alexandre Chartre
2020-11-16 14:47 ` [RFC][PATCH v2 21/21] x86/pti: Use a different stack canary with the user and kernel page-table Alexandre Chartre
2020-11-16 16:56   ` Andy Lutomirski
2020-11-16 18:34     ` Alexandre Chartre
2020-11-16 20:17 ` [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code Borislav Petkov
2020-11-17  7:56   ` Alexandre Chartre
2020-11-17 16:55     ` Borislav Petkov
2020-11-17 18:12       ` Alexandre Chartre
2020-11-17 18:28         ` Borislav Petkov
2020-11-17 19:02           ` Alexandre Chartre
2020-11-17 21:23             ` Borislav Petkov
2020-11-18  7:08               ` Alexandre Chartre
2020-11-17 21:26         ` Borislav Petkov
2020-11-18  7:41           ` Alexandre Chartre
2020-11-18  9:30             ` David Laight
2020-11-18 10:29               ` Alexandre Chartre
2020-11-18 13:22                 ` David Laight
2020-11-18 17:15                   ` Alexandre Chartre
2020-11-18 11:29             ` Borislav Petkov
2020-11-18 19:37               ` Alexandre Chartre
2020-11-16 20:24 ` Borislav Petkov
2020-11-17  8:19   ` Alexandre Chartre
2020-11-17 17:07     ` Borislav Petkov
2020-11-17 18:24       ` Alexandre Chartre
2020-11-19 19:32     ` Thomas Gleixner

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