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Date: Mon, 14 May 2018 12:15:51 -0700
From: Matthew Wilcox <willy@infradead.org>
To: Boaz Harrosh <boazh@netapp.com>
Cc: Jeff Moyer <jmoyer@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Rik van Riel <riel@redhat.com>, Jan Kara <jack@suse.cz>,
        Matthew Wilcox <mawilcox@microsoft.com>,
        Amit Golander <Amit.Golander@netapp.com>
Subject: Re: [PATCH] mm: Add new vma flag VM_LOCAL_CPU
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On Mon, May 14, 2018 at 08:28:01PM +0300, Boaz Harrosh wrote:
> On a call to mmap an mmap provider (like an FS) can put
> this flag on vma->vm_flags.
> 
> The VM_LOCAL_CPU flag tells the Kernel that the vma will be used
> from a single-core only, and therefore invalidation (flush_tlb) of
> PTE(s) need not be a wide CPU scheduling.

I still don't get this.  You're opening the kernel up to being exploited
by any application which can persuade it to set this flag on a VMA.

> NOTE: This vma (VM_LOCAL_CPU) is never used during a page_fault. It is
> always used in a synchronous way from a thread pinned to a single core.

It's not a question of how your app is going to use this flag.  It's a
question about how another app can abuse this flag (or how your app is
going to be exploited to abuse this flag) to break into the kernel.