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Date: Thu, 28 Jun 2018 11:27:42 +0100
From: Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v4 0/7] arm64: untag user pointers passed to the kernel
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To: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Cc: Mark Rutland <Mark.Rutland@arm.com>, Kate Stewart <kstewart@linuxfoundation.org>, "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>, Will Deacon <Will.Deacon@arm.com>, Kostya Serebryany <kcc@google.com>, "linux-kselftest@vger.kernel.org" <linux-kselftest@vger.kernel.org>, Chintan Pandya <cpandya@codeaurora.org>, Shuah Khan <shuah@kernel.org>, Ingo Molnar <mingo@kernel.org>, "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>, Jacob Bramley <Jacob.Bramley@arm.com>, Dmitry Vyukov <dvyukov@google.com>, Evgeniy Stepanov <eugenis@google.com>, Kees Cook <keescook@chromium.org>, Ruben Ayrapetyan <Ruben.Ayrapetyan@arm.com>, Andrey Konovalov <andreyknvl@google.com>, Lee Smith <Lee.Smith@arm.com>, Al Viro <viro@zeniv.linux.org.uk>nd <nd@arm.com>, Linux ARM <linux-arm-kernel@lists.infradead.org>, Linux Memory Management List <linux-mm@kvack.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, LKML <linux-kernel@vger.kernel.org>, Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>, Andrew Morton <akpm@linux-foundation.org>, Robin Murphy <Robin.Murphy@arm.com>, "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>

On Thu, Jun 28, 2018 at 08:17:59AM +0200, Luc Van Oostenryck wrote:
> On Wed, Jun 27, 2018 at 06:17:58PM +0100, Catalin Marinas wrote:
> > sparse is indeed an option. The current implementation doesn't warn on
> > an explicit cast from (void __user *) to (unsigned long) since that's a
> > valid thing in the kernel. I couldn't figure out if there's any other
> > __attribute__ that could be used to warn of such conversion.
> 
> sparse doesn't have such attribute but would an new option that would warn
> on such cast be a solution for your case?

I can't tell for sure whether such sparse option would be the full
solution but detecting explicit __user pointer casts to long is a good
starting point. So far this patchset pretty much relies on detecting
a syscall failure and trying to figure out why, patching the kernel. It
doesn't really scale.

As a side note, we have cases in the user-kernel ABI where the user
address type is "unsigned long": mmap() and friends. My feedback on an
early version of this patchset was to always require untagged pointers
coming from user space on such syscalls, so no need for explicit
untagging.

-- 
Catalin