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From: Kees Cook <keescook@chromium.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>,
	dave.hansen@intel.com, luto@kernel.org, linux-mm@kvack.org,
	x86@kernel.org, akpm@linux-foundation.org,
	linux-hardening@vger.kernel.org,
	kernel-hardening@lists.openwall.com, ira.weiny@intel.com,
	rppt@kernel.org, dan.j.williams@intel.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC 0/9] PKS write protected page tables
Date: Wed, 5 May 2021 11:38:39 -0700	[thread overview]
Message-ID: <202105051132.7958C3B@keescook> (raw)
In-Reply-To: <YJJZSdVoP6yBbIjN@hirez.programming.kicks-ass.net>

On Wed, May 05, 2021 at 10:37:29AM +0200, Peter Zijlstra wrote:
> On Tue, May 04, 2021 at 11:25:31PM -0700, Kees Cook wrote:
> 
> > It looks like PKS-protected page tables would be much like the
> > RO-protected text pages in the sense that there is already code in
> > the kernel to do things to make it writable, change text, and set it
> > read-only again (alternatives, ftrace, etc).
> 
> We don't actually modify text by changing the mapping at all. We modify
> through a writable (but not executable) temporary alias on the page (on
> x86).
> 
> Once a mapping is RX it will *never* be writable again (until we tear it
> all down).

Yes, quite true. I was trying to answer the concern about "is it okay
that there is a routine in the kernel that can write to page tables
(via temporary disabling of PKS)?" by saying "yes, this is fine -- we
already have similar routines in the kernel that bypass memory
protections, and that's okay because the defense is primarily about
blocking flaws that allow attacker-controlled writes to be used to
leverage greater control over kernel state, of which the page tables are
pretty central. :)

-- 
Kees Cook

  reply	other threads:[~2021-05-05 18:38 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-05  0:30 [PATCH RFC 0/9] PKS write protected page tables Rick Edgecombe
2021-05-05  0:30 ` [PATCH RFC 1/9] list: Support getting most recent element in list_lru Rick Edgecombe
2021-05-05  0:30 ` [PATCH RFC 2/9] list: Support list head not in object for list_lru Rick Edgecombe
2021-05-05  0:30 ` [PATCH RFC 3/9] x86/mm/cpa: Add grouped page allocations Rick Edgecombe
2021-05-05 12:08   ` Mike Rapoport
2021-05-05 13:09     ` Peter Zijlstra
2021-05-05 18:45       ` Mike Rapoport
2021-05-05 21:57         ` Edgecombe, Rick P
2021-05-05 21:57           ` Edgecombe, Rick P
2021-05-09  9:39           ` Mike Rapoport
2021-05-09  9:39             ` Mike Rapoport
2021-05-10 19:38             ` Edgecombe, Rick P
2021-05-10 19:38               ` Edgecombe, Rick P
2021-05-05  0:30 ` [PATCH RFC 4/9] mm: Explicitly zero page table lock ptr Rick Edgecombe
2021-05-05  0:30 ` [PATCH RFC 5/9] x86, mm: Use cache of page tables Rick Edgecombe
2021-05-05  8:51   ` Peter Zijlstra
2021-05-05 12:09     ` Mike Rapoport
2021-05-05 13:19       ` Peter Zijlstra
2021-05-05 21:54         ` Edgecombe, Rick P
2021-05-05 21:54           ` Edgecombe, Rick P
2021-05-06 17:59       ` Matthew Wilcox
2021-05-06 18:24   ` Shakeel Butt
2021-05-06 18:24     ` Shakeel Butt
2021-05-07 16:27     ` Edgecombe, Rick P
2021-05-07 16:27       ` Edgecombe, Rick P
2021-05-05  0:30 ` [PATCH RFC 6/9] x86/mm/cpa: Add set_memory_pks() Rick Edgecombe
2021-05-05  0:30 ` [PATCH RFC 7/9] x86/mm/cpa: Add perm callbacks to grouped pages Rick Edgecombe
2021-05-05  0:30 ` [PATCH RFC 8/9] x86, mm: Protect page tables with PKS Rick Edgecombe
2021-05-05  0:30 ` [PATCH RFC 9/9] x86, cpa: PKS protect direct map page tables Rick Edgecombe
2021-05-05  2:03 ` [PATCH RFC 0/9] PKS write protected " Ira Weiny
2021-05-05  6:25 ` Kees Cook
2021-05-05  8:37   ` Peter Zijlstra
2021-05-05 18:38     ` Kees Cook [this message]
2021-05-05 19:51   ` Edgecombe, Rick P
2021-05-05 19:51     ` Edgecombe, Rick P
2021-05-06  0:00   ` Ira Weiny
2021-05-05 11:08 ` Vlastimil Babka
2021-05-05 11:56   ` Peter Zijlstra
2021-05-05 19:46     ` Edgecombe, Rick P
2021-05-05 19:46       ` Edgecombe, Rick P

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