linux-kernel - Re: [PATCH] mm: Add new vma flag VM_LOCAL

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Date:   Mon, 14 May 2018 17:41:37 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Boaz Harrosh <boazh@...app.com>
Cc:     Jeff Moyer <jmoyer@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Rik van Riel <riel@...hat.com>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Amit Golander <Amit.Golander@...app.com>
Subject: Re: [PATCH] mm: Add new vma flag VM_LOCAL_CPU

On Mon, May 14, 2018 at 10:37:38PM +0300, Boaz Harrosh wrote:
> On 14/05/18 22:15, Matthew Wilcox wrote:
> > 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.
> > 
> 
> No No this is not an application accessible flag this can only be set
> by the mmap implementor at ->mmap() time (Say same as VM_VM_MIXEDMAP).
> 
> Please see the zuf patches for usage (Again apologise for pushing before
> a user)
> 
> The mmap provider has all the facilities to know that this can not be
> abused, not even by a trusted Server.

I don't think page tables work the way you think they work.

+               err = vm_insert_pfn_prot(zt->vma, zt_addr, pfn, prot);

That doesn't just insert it into the local CPU's page table.  Any CPU
which directly accesses or even prefetches that address will also get
the translation into its cache.