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Subject: Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor
To:     Andy Lutomirski <luto@kernel.org>
Cc:     David Laight <David.Laight@aculab.com>,
        "kernel-hardening@lists.openwall.com" 
        <kernel-hardening@lists.openwall.com>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-security-module@vger.kernel.org" 
        <linux-security-module@vger.kernel.org>,
        "oleg@redhat.com" <oleg@redhat.com>,
        "x86@kernel.org" <x86@kernel.org>
References: <20200728131050.24443-1-madvenka@linux.microsoft.com>
 <c23de6ec47614f489943e1a89a21dfa3@AcuMS.aculab.com>
 <f5cfd11b-04fe-9db7-9d67-7ee898636edb@linux.microsoft.com>
 <CALCETrUta5-0TLJ9-jfdehpTAp2Efmukk2npYadFzz9ozOrG2w@mail.gmail.com>
From:   "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
Message-ID: <59246260-e535-a9f1-d89e-4e953288b977@linux.microsoft.com>
Date:   Tue, 28 Jul 2020 13:52:01 -0500
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On 7/28/20 12:16 PM, Andy Lutomirski wrote:
> On Tue, Jul 28, 2020 at 9:32 AM Madhavan T. Venkataraman
> <madvenka@linux.microsoft.com> wrote:
>> Thanks. See inline..
>>
>> On 7/28/20 10:13 AM, David Laight wrote:
>>> From:  madvenka@linux.microsoft.com
>>>> Sent: 28 July 2020 14:11
>>> ...
>>>> The kernel creates the trampoline mapping without any permissions. When
>>>> the trampoline is executed by user code, a page fault happens and the
>>>> kernel gets control. The kernel recognizes that this is a trampoline
>>>> invocation. It sets up the user registers based on the specified
>>>> register context, and/or pushes values on the user stack based on the
>>>> specified stack context, and sets the user PC to the requested target
>>>> PC. When the kernel returns, execution continues at the target PC.
>>>> So, the kernel does the work of the trampoline on behalf of the
>>>> application.
>>> Isn't the performance of this going to be horrid?
>> It takes about the same amount of time as getpid(). So, it is
>> one quick trip into the kernel. I expect that applications will
>> typically not care about this extra overhead as long as
>> they are able to run.
> What did you test this on?  A page fault on any modern x86_64 system
> is much, much, much, much slower than a syscall.

I sent a response to this. But the mail was returned to me.
I am resending.

I tested it in on a KVM guest running Ubuntu. So, when you say that a
page fault is much slower, do you mean a regular page fault that is handled
through the VM layer? Here is the relevant code in do_user_addr_fault():

        if (unlikely(access_error(hw_error_code, vma))) {
                /*                 
                 * If it is a user execute fault, it could be a trampoline
                 * invocation.
                 */
                if ((hw_error_code & tflags) == tflags &&
                     trampfd_fault(vma, regs)) {
                         up_read(&mm->mmap_sem);
                         return;
                 }
                 bad_area_access_error(regs, hw_error_code, address, vma);
                 return;
         }
         ...
         fault = handle_mm_fault(vma, address, flags);

trampfd faults are instruction faults that go through a different code path than
the one that calls handle_mm_fault(). Perhaps, it is the handle_mm_fault() that
is time consuming. Could you clarify?

Thanks.

Madhavan