linux-kernel - Re: [patch 05/16] mm: Allow special mappings with user access cleared

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Date:   Wed, 13 Dec 2017 16:54:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>, aliguori@...zon.com,
        Will Deacon <will.deacon@....com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        kirill.shutemov@...ux.intel.com, aneesh.kumar@...ux.vnet.ibm.com
Subject: Re: [patch 05/16] mm: Allow special mappings with user access cleared

On Wed, Dec 13, 2017 at 07:47:46AM -0800, Dave Hansen wrote:
> On 12/13/2017 07:32 AM, Peter Zijlstra wrote:
> >> This will fault writing a byte to 'addr':
> >>
> >> 	char *addr = malloc(PAGE_SIZE);
> >> 	pkey_mprotect(addr, PAGE_SIZE, 13);
> >> 	pkey_deny_access(13);
> >> 	*addr[0] = 'f';
> >>
> >> But this will write one byte to addr successfully (if it uses the kernel
> >> mapping of the physical page backing 'addr'):
> >>
> >> 	char *addr = malloc(PAGE_SIZE);
> >> 	pkey_mprotect(addr, PAGE_SIZE, 13);
> >> 	pkey_deny_access(13);
> >> 	read(fd, addr, 1);
> >>
> > This seems confused to me; why are these two cases different?
> 
> Protection keys doesn't work in the kernel direct map, so if the read()
> was implemented by writing to the direct map alias of 'addr' then this
> would bypass protection keys.

Which is why get_user_pages() _should_ enforce this.

What use are protection keys if you can trivially circumvent them?