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From: Dave Hansen <dave.hansen@intel.com>
To: Rick Edgecombe <rick.p.edgecombe@intel.com>,
	x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-mm@kvack.org, linux-arch@vger.kernel.org,
	linux-api@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Andy Lutomirski <luto@kernel.org>,
	Balbir Singh <bsingharora@gmail.com>,
	Borislav Petkov <bp@alien8.de>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Eugene Syromiatnikov <esyr@redhat.com>,
	Florian Weimer <fweimer@redhat.com>,
	"H . J . Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	"Ravi V . Shankar" <ravi.v.shankar@intel.com>,
	Dave Martin <Dave.Martin@arm.com>,
	Weijiang Yang <weijiang.yang@intel.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	joao.moreira@intel.com, John Allen <john.allen@amd.com>,
	kcc@google.com, eranian@google.com
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 07/35] x86/mm: Remove _PAGE_DIRTY from kernel RO pages
Date: Mon, 7 Feb 2022 16:13:45 -0800	[thread overview]
Message-ID: <672fa390-c88c-4e2a-aa42-52d171acfd62@intel.com> (raw)
In-Reply-To: <20220130211838.8382-8-rick.p.edgecombe@intel.com>

On 1/30/22 13:18, Rick Edgecombe wrote:
> The x86 family of processors do not directly create read-only and Dirty
> PTEs.  These PTEs are created by software.

That's not strictly correct.

There's nothing in the architecture today to prevent the CPU from
creating Write=0,Dirty=1 PTEs.  In fact, some CPUs do this in weird
situations.  It wouldn't be wrong to say:

	Processors sometimes directly create read-only and Dirty PTEs.

which is the opposite of what is written above.  This is why the CET
spec has the blurb about shadow-stack-supporting CPUs promise not to do
this any more.

> One such case is that kernel
> read-only pages are historically setup as Dirty.

				   ^ set up

> New processors that support Shadow Stack regard read-only and Dirty PTEs as
> shadow stack pages.

This also isn't *quite* correct.  It's not just having a new processor,
it includes enabling shadow stacks.

> This results in ambiguity between shadow stack and kernel read-only
> pages.  To resolve this, removed Dirty from kernel read- only pages.
One thing that's not clear from the spec: does this cause an *actual*
problem?  For instance, does setting:

	IA32_U_CET.SH_STK_EN=1
but
	IA32_S_CET.SH_STK_EN=0

means that shadow stacks are enforced in user *MODE* or on
user-paging-permission (U=0) PTEs?

I think it's modes, but it would be nice to be clear.  *BUT*, if this is
accurate, doesn't it also mean that this patch is not strictly necessary?

Don't get me wrong, the patch is probably still a good idea, but let's
make sure we get the exact reasoning clear.

  reply	other threads:[~2022-02-08  1:07 UTC|newest]

Thread overview: 155+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-30 21:18 [PATCH 00/35] Shadow stacks for userspace Rick Edgecombe
2022-01-30 21:18 ` [PATCH 01/35] Documentation/x86: Add CET description Rick Edgecombe
2022-01-30 21:18 ` [PATCH 02/35] x86/cet/shstk: Add Kconfig option for Shadow Stack Rick Edgecombe
2022-02-07 22:39   ` Dave Hansen
2022-02-08  8:41     ` Thomas Gleixner
2022-02-08 20:20       ` Edgecombe, Rick P
2022-02-08  8:39   ` Thomas Gleixner
2022-01-30 21:18 ` [PATCH 03/35] x86/cpufeatures: Add CET CPU feature flags for Control-flow Enforcement Technology (CET) Rick Edgecombe
2022-02-07 22:45   ` Dave Hansen
2022-02-08 20:23     ` Edgecombe, Rick P
2022-02-09  1:10   ` Kees Cook
2022-01-30 21:18 ` [PATCH 04/35] x86/cpufeatures: Introduce CPU setup and option parsing for CET Rick Edgecombe
2022-02-07 22:49   ` Dave Hansen
2022-02-08 20:29     ` Edgecombe, Rick P
2022-01-30 21:18 ` [PATCH 05/35] x86/fpu/xstate: Introduce CET MSR and XSAVES supervisor states Rick Edgecombe
2022-02-07 23:28   ` Dave Hansen
2022-02-08 21:36     ` Edgecombe, Rick P
2022-01-30 21:18 ` [PATCH 06/35] x86/cet: Add control-protection fault handler Rick Edgecombe
2022-02-07 23:56   ` Dave Hansen
2022-02-08 22:23     ` Edgecombe, Rick P
2022-01-30 21:18 ` [PATCH 07/35] x86/mm: Remove _PAGE_DIRTY from kernel RO pages Rick Edgecombe
2022-02-08  0:13   ` Dave Hansen [this message]
2022-02-08 22:52     ` Edgecombe, Rick P
2022-01-30 21:18 ` [PATCH 08/35] x86/mm: Move pmd_write(), pud_write() up in the file Rick Edgecombe
2022-01-30 21:18 ` [PATCH 09/35] x86/mm: Introduce _PAGE_COW Rick Edgecombe
2022-02-08  1:05   ` Dave Hansen
2022-01-30 21:18 ` [PATCH 10/35] drm/i915/gvt: Change _PAGE_DIRTY to _PAGE_DIRTY_BITS Rick Edgecombe
2022-02-09 16:58   ` Dave Hansen
2022-02-11  1:39     ` Edgecombe, Rick P
2022-02-11  7:13       ` Wang, Zhi A
2022-02-12  1:45         ` Edgecombe, Rick P
2022-01-30 21:18 ` [PATCH 11/35] x86/mm: Update pte_modify for _PAGE_COW Rick Edgecombe
2022-02-09 18:00   ` Dave Hansen
2022-01-30 21:18 ` [PATCH 12/35] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW Rick Edgecombe
2022-02-09 18:30   ` Dave Hansen
2022-01-30 21:18 ` [PATCH 13/35] mm: Move VM_UFFD_MINOR_BIT from 37 to 38 Rick Edgecombe
2022-01-30 21:18 ` [PATCH 14/35] mm: Introduce VM_SHADOW_STACK for shadow stack memory Rick Edgecombe
2022-02-09 21:55   ` Dave Hansen
2022-01-30 21:18 ` [PATCH 15/35] x86/mm: Check Shadow Stack page fault errors Rick Edgecombe
2022-02-09 19:06   ` Dave Hansen
2022-01-30 21:18 ` [PATCH 16/35] x86/mm: Update maybe_mkwrite() for shadow stack Rick Edgecombe
2022-02-09 21:16   ` Dave Hansen
2022-01-30 21:18 ` [PATCH 17/35] mm: Fixup places that call pte_mkwrite() directly Rick Edgecombe
2022-02-09 21:51   ` Dave Hansen
2022-01-30 21:18 ` [PATCH 18/35] mm: Add guard pages around a shadow stack Rick Edgecombe
2022-02-09 22:23   ` Dave Hansen
2022-02-10 22:38     ` David Laight
2022-02-10 23:42       ` Edgecombe, Rick P
2022-02-11  9:08         ` David Laight
2022-02-10 22:43   ` Dave Hansen
2022-02-10 23:07     ` Andy Lutomirski
2022-02-10 23:40       ` Edgecombe, Rick P
2022-02-11 17:54         ` Andy Lutomirski
2022-02-12  0:10           ` Edgecombe, Rick P
2022-01-30 21:18 ` [PATCH 19/35] mm/mmap: Add shadow stack pages to memory accounting Rick Edgecombe
2022-02-09 22:27   ` Dave Hansen
2022-01-30 21:18 ` [PATCH 20/35] mm: Update can_follow_write_pte() for shadow stack Rick Edgecombe
2022-02-09 22:50   ` Dave Hansen
2022-02-09 22:52   ` Dave Hansen
2022-02-10 22:45     ` David Laight
2022-01-30 21:18 ` [PATCH 21/35] mm/mprotect: Exclude shadow stack from preserve_write Rick Edgecombe
2022-02-10 19:27   ` Dave Hansen
2022-01-30 21:18 ` [PATCH 22/35] x86/mm: Prevent VM_WRITE shadow stacks Rick Edgecombe
2022-02-11 22:19   ` Dave Hansen
2022-02-12  1:44     ` Edgecombe, Rick P
2022-01-30 21:18 ` [PATCH 23/35] x86/fpu: Add helpers for modifying supervisor xstate Rick Edgecombe
2022-02-08  8:51   ` Thomas Gleixner
2022-02-09 19:55     ` Edgecombe, Rick P
2022-02-12  0:27   ` Dave Hansen
2022-02-12  2:31     ` Edgecombe, Rick P
2022-01-30 21:18 ` [PATCH 24/35] mm: Re-introduce vm_flags to do_mmap() Rick Edgecombe
2022-01-30 21:18 ` [PATCH 25/35] x86/cet/shstk: Add user-mode shadow stack support Rick Edgecombe
2022-02-11 23:37   ` Dave Hansen
2022-02-12  0:07     ` Andy Lutomirski
2022-02-12  0:11       ` Dave Hansen
2022-02-12  0:12     ` Edgecombe, Rick P
2022-01-30 21:18 ` [PATCH 26/35] x86/process: Change copy_thread() argument 'arg' to 'stack_size' Rick Edgecombe
2022-02-08  8:38   ` Thomas Gleixner
2022-02-11  2:09     ` Edgecombe, Rick P
2022-02-14 12:33   ` Jann Horn
2022-02-15  1:22     ` Edgecombe, Rick P
2022-02-15  8:49       ` Christian Brauner
2022-01-30 21:18 ` [PATCH 27/35] x86/fpu: Add unsafe xsave buffer helpers Rick Edgecombe
2022-01-30 21:18 ` [PATCH 28/35] x86/cet/shstk: Handle thread shadow stack Rick Edgecombe
2022-01-30 21:18 ` [PATCH 29/35] x86/cet/shstk: Introduce shadow stack token setup/verify routines Rick Edgecombe
2022-01-30 21:18 ` [PATCH 30/35] x86/cet/shstk: Handle signals for shadow stack Rick Edgecombe
2022-01-30 21:18 ` [PATCH 31/35] x86/cet/shstk: Add arch_prctl elf feature functions Rick Edgecombe
2022-01-30 21:18 ` [PATCH 32/35] x86/cet/shstk: Introduce map_shadow_stack syscall Rick Edgecombe
2022-01-30 21:18 ` [PATCH 33/35] selftests/x86: Add map_shadow_stack syscall test Rick Edgecombe
2022-01-30 21:18   ` Rick Edgecombe
2022-02-03 22:42   ` Dave Hansen
2022-02-04  1:22     ` Edgecombe, Rick P
2022-01-30 21:18 ` [PATCH 34/35] x86/cet/shstk: Support wrss for userspace Rick Edgecombe
2022-01-31  7:56   ` Florian Weimer
2022-01-31 18:26     ` H.J. Lu
2022-01-31 18:45       ` Florian Weimer
2022-01-30 21:18 ` [PATCH 35/35] x86/cpufeatures: Limit shadow stack to Intel CPUs Rick Edgecombe
2022-02-03 21:58   ` John Allen
2022-02-03 22:23     ` H.J. Lu
2022-02-04 22:21       ` John Allen
2022-02-03 21:07 ` [PATCH 00/35] Shadow stacks for userspace Thomas Gleixner
2022-02-04  1:08   ` Edgecombe, Rick P
2022-02-04  5:20     ` Andy Lutomirski
2022-02-04 20:23       ` Edgecombe, Rick P
2022-02-05 13:26     ` David Laight
2022-02-05 13:29       ` H.J. Lu
2022-02-05 20:15         ` Edgecombe, Rick P
2022-02-05 20:21           ` H.J. Lu
2022-02-06 13:19             ` Peter Zijlstra
2022-02-06 13:42           ` David Laight
2022-02-06 13:55             ` H.J. Lu
2022-02-07 10:22             ` Florian Weimer
2022-02-08  1:46             ` Edgecombe, Rick P
2022-02-08  1:31           ` Andy Lutomirski
2022-02-08  9:31             ` Thomas Gleixner
2022-02-08 16:15               ` Andy Lutomirski
2022-02-06 13:06     ` Peter Zijlstra
2022-02-06 18:42 ` Mike Rapoport
2022-02-07  7:20   ` Adrian Reber
2022-02-07 16:30     ` Dave Hansen
2022-02-08  9:16       ` Mike Rapoport
2022-02-08  9:29         ` Cyrill Gorcunov
2022-02-08 16:21           ` Andy Lutomirski
2022-02-08 17:02             ` Cyrill Gorcunov
2022-02-08 21:54               ` Dmitry Safonov
2022-02-09  6:37                 ` Cyrill Gorcunov
2022-02-09  2:18               ` Edgecombe, Rick P
2022-02-09  6:43                 ` Cyrill Gorcunov
2022-02-09 10:53                 ` Mike Rapoport
2022-02-10  2:37                 ` Andy Lutomirski
2022-02-10  2:53                   ` H.J. Lu
2022-02-10 13:52                     ` Willgerodt, Felix
2022-02-11  7:41                   ` avagin
2022-02-11  8:04                     ` Mike Rapoport
2022-02-28 20:27                   ` Mike Rapoport
2022-02-28 20:30                     ` Andy Lutomirski
2022-02-28 21:30                       ` Mike Rapoport
2022-02-28 22:55                         ` Andy Lutomirski
2022-03-03 19:40                           ` Mike Rapoport
2022-03-03 23:00                             ` Andy Lutomirski
2022-03-04  1:30                               ` Edgecombe, Rick P
2022-03-04 19:13                                 ` Andy Lutomirski
2022-03-07 18:56                                   ` Mike Rapoport
2022-03-07 19:07                                     ` H.J. Lu
2022-05-31 11:59                                       ` Mike Rapoport
2022-05-31 16:25                                         ` Edgecombe, Rick P
2022-05-31 16:36                                           ` Mike Rapoport
2022-05-31 17:34                                             ` Edgecombe, Rick P
2022-05-31 18:00                                               ` H.J. Lu
2022-06-01 17:27                                                 ` Edgecombe, Rick P
2022-06-01 19:27                                                   ` H.J. Lu
2022-06-01  8:06                                               ` Mike Rapoport
2022-06-01 17:24                                                 ` Edgecombe, Rick P
2022-06-09 18:04                                                   ` Mike Rapoport
2022-03-07 22:21                                     ` David Laight

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