linux-kernel - Re: [PATCH v4 0/7] arm64: untag user pointers passed to the kernel

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Message-ID: <20180628104610.czsnq4w3lfhxrn53@ltop.local>
Date:   Thu, 28 Jun 2018 12:46:11 +0200
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Mark Rutland <Mark.Rutland@....com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        Will Deacon <Will.Deacon@....com>,
        Kostya Serebryany <kcc@...gle.com>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Shuah Khan <shuah@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Lee Smith <Lee.Smith@....com>,
        Al Viro <viro@...iv.linux.org.uk>, nd <nd@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Robin Murphy <Robin.Murphy@....com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v4 0/7] arm64: untag user pointers passed to the kernel

On Thu, Jun 28, 2018 at 11:27:42AM +0100, Catalin Marinas wrote:
> 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.

OK, I'll add such an option this evening.
 
> 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.

Mmmm yes.
I tend to favor a sort of opposite approach. When we have an address
that must not be dereferenced as-such (and sometimes when the address
can be from both __user & __kernel space) I prefer to use a ulong
which will force the use of the required operation before being
able to do any sort of dereferencing and this won't need horrible
casts with __force (it, of course, all depends on the full context).

-- Luc