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Date: Wed, 13 Dec 2017 16:54:27 +0100
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [patch 05/16] mm: Allow special mappings with user access cleared
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To: Dave Hansen <dave.hansen@intel.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>, Andy Lutomirski <luto@kernel.org>, Thomas Gleixner <tglx@linutronix.de>, LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>, Linus Torvalds <torvalds@linux-foundation.org>, Borislav Petkov <bpetkov@suse.de>, Greg KH <gregkh@linuxfoundation.org>, Kees Cook <keescook@google.com>, Hugh Dickins <hughd@google.com>, Brian Gerst <brgerst@gmail.com>, Josh Poimboeuf <jpoimboe@redhat.com>, Denys Vlasenko <dvlasenk@redhat.com>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, Juergen Gross <jgross@suse.com>, David Laight <David.Laight@aculab.com>, Eduardo Valentin <eduval@amazon.com>, aliguori@amazon.com, Will Deacon <will.deacon@arm.com>, "linux-mm@kvack.org" <linux-mm@kvack.org>, kirill.shutemov@linux.intel.com, aneesh.kumar@linux.vnet.ibm.com

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?

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