linux-kernel - Re: [RESEND RFC PATCH 1/3] Introduce per thread user-kernel shared structure

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Date:   Thu, 9 Sep 2021 17:30:45 +0000
From:   Prakash Sangappa <prakash.sangappa@...cle.com>
To:     Jann Horn <jannh@...gle.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>
Subject: Re: [RESEND RFC PATCH 1/3] Introduce per thread user-kernel shared
 structure



> On Sep 8, 2021, at 6:50 PM, Jann Horn <jannh@...gle.com> wrote:
> 
> On Thu, Sep 9, 2021 at 2:16 AM Prakash Sangappa
> <prakash.sangappa@...cle.com> wrote:
>> A structure per thread is allocated from a page that is shared mapped
>> between user space and kernel as means for faster communication. This will
>> facilitate sharing information, Ex: per thread stats shared between kernel
>> and user space, that can be read by applications without the need for
>> making frequent system calls in latency sensitive code path.
> [...]
>> +               /* Try to map as high as possible, this is only a hint. */
>> +               pg->vaddr = get_unmapped_area(NULL, TASK_SIZE - PAGE_SIZE,
>> +                                       PAGE_SIZE, 0, 0);
> 
> I don't have an opinion on the rest of this patch, but from a security
> perspective, please don't map things at fixed addresses; let the mmap
> code pick some random address that it wants to use. If an attacker
> manages to overwrite some userspace pointer with chosen data, we don't
> want them to know an address where they can be sure something is
> mapped.

Sure. I will address this. 
Thanks,
-Prakash