From: Alexandre Chartre <alexandre.chartre@oracle.com>
To: Thomas Gleixner <tglx@linutronix.de>, Andy Lutomirski <luto@kernel.org>
Cc: 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: Thu, 19 Nov 2020 20:55:33 +0100 [thread overview]
Message-ID: <a9aa4887-02ee-0e9a-a37e-63e9cb0ff27f@oracle.com> (raw)
In-Reply-To: <87ft55p3gp.fsf@nanos.tec.linutronix.de>
On 11/19/20 8:10 PM, Thomas Gleixner wrote:
> On Mon, Nov 16 2020 at 19:10, Alexandre Chartre 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 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.
>
> Coming late, but this does not make any sense to me.
>
> Unless you map most of the kernel into the user page-table sleeping with
> the user page-table _cannot_ work. And if you do that you broke KPTI.
>
> You can neither pick arbitrary points in the C code of an exception
> handler to switch to the kernel mapping unless you mapped everything
> which might be touched before that into user space.
>
> How is that supposed to work?
>
Sorry I mixed up a few thing; I got confused with my own code which is not a
good sign...
It's not sleeping with the user page-table which, as you mentioned, doesn't
make sense, it's sleeping with the kernel page-table but with the PTI stack.
Basically, it is:
- entering C code with (user page-table, PTI stack);
- then it switches to the kernel page-table so we have (kernel page-table, PTI stack);
- and then it switches to the kernel stack so we have (kernel page-table, kernel stack).
As this is all C code, some of which is executed with the PTI stack, we need the PTI stack
to be per-task so that the stack is preserved, in case that C code does a sleep/schedule
(no matter if this happens when using the PTI stack or the kernel stack).
alex.
next prev parent reply other threads:[~2020-11-19 19:56 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
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 [this message]
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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