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Subject: Re: [PATCH v10 04/12] mm, arm64: untag user pointers passed to memory
 syscalls
To:     Andrey Konovalov <andreyknvl@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Kate Stewart <kstewart@linuxfoundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Shuah Khan <shuah@kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
        linux-mm@kvack.org, linux-arch@vger.kernel.org,
        linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Dmitry Vyukov <dvyukov@google.com>,
        Kostya Serebryany <kcc@google.com>,
        Evgeniy Stepanov <eugenis@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>,
        Chintan Pandya <cpandya@codeaurora.org>,
        Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Kevin Brodsky <kevin.brodsky@arm.com>,
        Szabolcs Nagy <Szabolcs.Nagy@arm.com>
References: <cover.1550839937.git.andreyknvl@google.com>
 <3875fa863b755d8cb43afa7bb0fe543e5fd05a5d.1550839937.git.andreyknvl@google.com>
From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <81ea4e77-90a4-4fd9-2bc8-135e0da30044@intel.com>
Date:   Fri, 22 Feb 2019 15:07:45 -0800
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Sender: linux-kernel-owner@vger.kernel.org
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On 2/22/19 4:53 AM, Andrey Konovalov wrote:
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -578,6 +578,7 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
>  SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
>  		unsigned long, prot)
>  {
> +	start = untagged_addr(start);
>  	return do_mprotect_pkey(start, len, prot, -1);
>  }
>  
> @@ -586,6 +587,7 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
>  SYSCALL_DEFINE4(pkey_mprotect, unsigned long, start, size_t, len,
>  		unsigned long, prot, int, pkey)
>  {
> +	start = untagged_addr(start);
>  	return do_mprotect_pkey(start, len, prot, pkey);
>  }

This seems to have taken the approach of going as close as possible to
the syscall boundary and untagging the pointer there.  I guess that's
OK, but it does lead to more churn than necessary.  For instance, why
not just do the untagging in do_mprotect_pkey()?

I think that's an overall design question.  I kinda asked the same thing
about patching call sites vs. VMA lookup functions.