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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Wed, 14 Mar 2018 16:45:20 +0100
Message-ID: <CAAeHK+y3hJ4jAByvfGDYyNb_RZ3_As4r-FCyZL9u69Vv_HS2AA@mail.gmail.com>
Subject: Re: [RFC PATCH 3/6] mm, arm64: untag user addresses in memory syscalls
To:     Evgenii Stepanov <eugenis@google.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, linux-arch@vger.kernel.org,
        Dmitry Vyukov <dvyukov@google.com>,
        Kostya Serebryany <kcc@google.com>,
        Lee Smith <Lee.Smith@arm.com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@arm.com>,
        Jacob Bramley <Jacob.Bramley@arm.com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@arm.com>
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On Fri, Mar 9, 2018 at 6:42 PM, Evgenii Stepanov <eugenis@google.com> wrote:
> On Fri, Mar 9, 2018 at 9:31 AM, Andrey Konovalov <andreyknvl@google.com> wrote:
>> On Fri, Mar 9, 2018 at 4:53 PM, Catalin Marinas <catalin.marinas@arm.com> wrote:
>>> I'm not yet convinced these functions need to allow tagged pointers.
>>> They are not doing memory accesses but rather dealing with the memory
>>> range, hence an untagged pointer is better suited. There is probably a
>>> reason why the "start" argument is "unsigned long" vs "void __user *"
>>> (in the kernel, not the man page).
>>
>> So that would make the user to untag pointers before passing to these syscalls.
>>
>> Evgeniy, would that be possible to untag pointers in HWASan before
>> using memory subsystem syscalls? Is there a reason for untagging them
>> in the kernel?
>
> Generally, no. It's possible to intercept a libc call using symbol
> interposition, but I don't know how to rewrite arguments of a raw
> system call other than through ptrace, and that creates more problems
> than it solves.
>
> AFAIU, it's valid for a program to pass an address obtained from
> malloc or, better, posix_memalign to an mm syscall like mprotect().
> These arguments are pointers from the userspace point of view.

Catalin, do you think this is a good reason to have the untagging done
in the kernel?