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

From: Andrey Konovalov <andreyknvl@google.com>
To: Dave Hansen <dave.hansen@intel.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>,
	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 <linux-arm-kernel@lists.infradead.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	Linux Memory Management List <linux-mm@kvack.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	"open list:KERNEL SELFTEST FRAMEWORK" 
	<linux-kselftest@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	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>
Subject: Re: [PATCH v10 04/12] mm, arm64: untag user pointers passed to memory syscalls
Date: Tue, 26 Feb 2019 15:41:26 +0100	[thread overview]
Message-ID: <CAAeHK+z2qw__Kpeih2mEyhVJ-gbor1vYsOZddT5E71JDiSg7pg@mail.gmail.com> (raw)
In-Reply-To: <81ea4e77-90a4-4fd9-2bc8-135e0da30044@intel.com>

On Sat, Feb 23, 2019 at 12:07 AM Dave Hansen <dave.hansen@intel.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.