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Date:   Tue, 5 Apr 2022 10:14:27 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Bharata B Rao <bharata@amd.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-mm@kvack.org, the arch/x86 maintainers <x86@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, shuah@kernel.org,
        Oleg Nesterov <oleg@redhat.com>, ananth.narayan@amd.com
Subject: Re: [RFC PATCH v0 0/6] x86/AMD: Userspace address tagging
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On Wed, Mar 23, 2022 at 01:18:41PM +0530, Bharata B Rao wrote:
> On 3/22/2022 3:59 AM, Andy Lutomirski wrote:

> > I hate to be a pain, but I'm really not convinced that this feature
> > is suitable for Linux.  There are a few reasons:
> > 
> > Right now, the concept that the high bit of an address determines
> > whether it's a user or a kernel address is fairly fundamental to the
> > x86_64 (and x86_32!) code.  It may not be strictly necessary to
> > preserve this, but violating it would require substantial thought.
> > With UAI enabled, kernel and user addresses are, functionally,
> > interleaved.  This makes things like access_ok checks, and more
> > generally anything that operates on a range of addresses, behave
> > potentially quite differently.  A lot of auditing of existing code
> > would be needed to make it safe.
> 
> Ok got that. However can you point to me a few instances in the current
> kernel code where such assumption of high bit being user/kernel address
> differentiator exists so that I get some idea of what it takes to
> audit all such cases?

The fact that you have to ask and can't readily find them should be a
big honking clue on its own, no?

Anyway, see here:

arch/x86/events/perf_event.h:static inline bool kernel_ip(unsigned long ip)
arch/x86/events/perf_event.h:{
arch/x86/events/perf_event.h:#ifdef CONFIG_X86_32
arch/x86/events/perf_event.h:   return ip > PAGE_OFFSET;
arch/x86/events/perf_event.h:#else
arch/x86/events/perf_event.h:   return (long)ip < 0;
arch/x86/events/perf_event.h:#endif
arch/x86/events/perf_event.h:}