Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
In-Reply-To: <CAAeHK+xF0X-SQuPVmZF--VnmvYrbpDkznSwYTh_3LHq1H2V=9Q@mail.gmail.com>
References: <cover.1520600533.git.andreyknvl@google.com> <beea8ac394bfae3c7c949645fb887ceacc3f3bb3.1520600533.git.andreyknvl@google.com>
 <20180309155315.4x44sbp3darractt@armageddon.cambridge.arm.com> <CAAeHK+xF0X-SQuPVmZF--VnmvYrbpDkznSwYTh_3LHq1H2V=9Q@mail.gmail.com>
From:   Evgenii Stepanov <eugenis@google.com>
Date:   Fri, 9 Mar 2018 09:42:29 -0800
Message-ID: <CAFKCwrgPEhBEbXn95ugxVuxHw7UTiQWuo3+HJ_t7UBG69fbUcw@mail.gmail.com>
Subject: Re: [RFC PATCH 3/6] mm, arm64: untag user addresses in memory syscalls
To:     Andrey Konovalov <andreyknvl@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>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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:
>> On Fri, Mar 09, 2018 at 03:02:01PM +0100, Andrey Konovalov wrote:
>>> Memory subsystem syscalls accept user addresses as arguments, but don't use
>>> copy_from_user and other similar functions, so we need to handle this case
>>> separately.
>>>
>>> Untag user pointers passed to madvise, mbind, get_mempolicy, mincore,
>>> mlock, mlock2, brk, mmap_pgoff, old_mmap, munmap, remap_file_pages,
>>> mprotect, pkey_mprotect, mremap and msync.
>>>
>>> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
>>
>> Please keep the cc list small (maybe linux-arch, linux-arm-kernel) as
>> I'm sure some lists would consider this spam.
>
> OK.
>
>>
>>>  mm/madvise.c   | 2 ++
>>>  mm/mempolicy.c | 6 ++++++
>>>  mm/mincore.c   | 2 ++
>>>  mm/mlock.c     | 5 +++++
>>>  mm/mmap.c      | 9 +++++++++
>>>  mm/mprotect.c  | 2 ++
>>>  mm/mremap.c    | 2 ++
>>>  mm/msync.c     | 3 +++
>>
>> 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.