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

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Date:   Mon, 14 May 2018 22:37:38 +0300
From:   Boaz Harrosh <boazh@...app.com>
To:     Matthew Wilcox <willy@...radead.org>
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 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.

>> 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.
> 

If you look at the zuf user you will see that the faults all return
SIG_BUS. These can never fault. The server has access to this mapping
from a single thread pinned to a core.

Again it is not an app visible flag in anyway

Thanks for looking
Boaz