linux-kernel - Re: [PATCH PTI v2 6/6] x86/pti: Put the LDT in its own PGD if PTI is on

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Date:   Mon, 11 Dec 2017 11:47:00 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH PTI v2 6/6] x86/pti: Put the LDT in its own PGD if PTI is
 on

On 12/11/2017 11:39 AM, Andy Lutomirski wrote:
>> I thought there would be a "fast path" where we just use the normal
>> clear_LDT() LDT from the cpu_entry_area and don't have to do any of
>> this, but I'm missing where that happens.  Do we need a check in
>> (un)map_ldt_struct() for !mm->context.ldt?
> I'm confused.
> 
> if (unlikely(ldt)) {
>   do something slowish;
> } else {
>   clear_LD();
> }

I was looking at the map/unmap paths.  It looks to me like the cases
where there is map/unmap overhead, we *are* doing checking against
mm->context.ldt.  It just wasn't visible from the patch context.

In any case, it would be really nice to call that out if you revise
these in the patch description: none of these LDT acrobatics are used in
the common case.  Virtually every process uses the !ldt paths which
don't do any of this.