linux-kernel - Re: [PATCH v10 04/12] mm, arm64: untag user pointers passed to memory syscalls

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Date:   Tue, 26 Feb 2019 15:41:26 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Robin Murphy <robin.murphy@....com>,
        Kees Cook <keescook@...omium.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Shuah Khan <shuah@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Lee Smith <Lee.Smith@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Dave Martin <Dave.Martin@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>
Subject: Re: [PATCH v10 04/12] mm, arm64: untag user pointers passed to memory syscalls

On Sat, Feb 23, 2019 at 12:07 AM Dave Hansen <dave.hansen@...el.com> wrote:
>
> 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 makes more sense, will do in the next version, thanks!

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

Replied in the other thread.