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Subject: Re: [RFC][PATCH v2 11/21] x86/pti: Extend PTI user mappings
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
References: <20201116144757.1920077-1-alexandre.chartre@oracle.com>
 <20201116144757.1920077-12-alexandre.chartre@oracle.com>
 <CALCETrXoykRjRPYPfZr6gBKoMnHuRYiJTDOcFYMq8GLef00j1A@mail.gmail.com>
 <820278dc-5f8e-6224-71b4-7c61819f68d1@oracle.com>
 <CALCETrXYP13pPcRfDDkwetLgzA3quYOBg7OTo5nbpLpPfSqaLw@mail.gmail.com>
From:   Alexandre Chartre <alexandre.chartre@oracle.com>
Message-ID: <d4be0149-1a28-24e8-7821-e8c96f98a7ac@oracle.com>
Date:   Tue, 17 Nov 2020 09:42:41 +0100
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On 11/17/20 12:06 AM, Andy Lutomirski wrote:
> On Mon, Nov 16, 2020 at 12:18 PM Alexandre Chartre
> <alexandre.chartre@oracle.com> wrote:
>>
>>
>> On 11/16/20 8:48 PM, Andy Lutomirski wrote:
>>> On Mon, Nov 16, 2020 at 6:49 AM Alexandre Chartre
>>> <alexandre.chartre@oracle.com> wrote:
>>>>
>>>> Extend PTI user mappings so that more kernel entry code can be executed
>>>> with the user page-table. To do so, we need to map syscall and interrupt
>>>> entry code, per cpu offsets (__per_cpu_offset, which is used some in
>>>> entry code), the stack canary, and the PTI stack (which is defined per
>>>> task).
>>>
>>> Does anything unmap the PTI stack?  Mapping is easy, and unmapping
>>> could be a pretty big mess.
>>>
>>
>> No, there's no unmap. The mapping exists as long as the task page-table
>> does (i.e. as long as the task mm exits). I assume that the task stack
>> and mm are freed at the same time but that's not something I have checked.
>>
> 
> Nope.  A multi-threaded mm will free task stacks when the task exits,
> but the mm may outlive the individual tasks.  Additionally, if you
> allocate page tables as part of mapping PTI stacks, you need to make
> sure the pagetables are freed.

So I think I just need to unmap the PTI stack from the user page-table
when the task exits. Everything else is handled because the kernel and
PTI stack are allocated in a single chunk (referenced by task->stack).


>  Finally, you need to make sure that
> the PTI stacks have appropriate guard pages -- just doubling the
> allocation is not safe enough.

The PTI stack does have guard pages because it maps only a part of the task
stack into the user page-table, so pages around the PTI stack are not mapped
into the user-pagetable (the page below is the task stack guard, and the page
above is part of the kernel-only stack so it's never mapped into the user
page-table).

+ *   +-------------+
+ *   |             | ^                       ^
+ *   | kernel-only | | KERNEL_STACK_SIZE     |
+ *   |    stack    | |                       |
+ *   |             | V                       |
+ *   +-------------+ <- top of kernel stack  | THREAD_SIZE
+ *   |             | ^                       |
+ *   | kernel and  | | KERNEL_STACK_SIZE     |
+ *   | PTI stack   | |                       |
+ *   |             | V                       v
+ *   +-------------+ <- top of stack

> My intuition is that this is going to be far more complexity than is justified.

Sounds like only the PTI stack unmap is missing, which is hopefully not
that bad. I will check that.

alex.