From: Yu-cheng Yu <yu-cheng.yu@intel.com>
To: Peter Zijlstra <peterz@infradead.org>,
Dave Hansen <dave.hansen@linux.intel.com>
Cc: Jann Horn <jannh@google.com>,
the arch/x86 maintainers <x86@kernel.org>,
"H . Peter Anvin" <hpa@zytor.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
kernel list <linux-kernel@vger.kernel.org>,
linux-doc@vger.kernel.org, Linux-MM <linux-mm@kvack.org>,
linux-arch <linux-arch@vger.kernel.org>,
Linux API <linux-api@vger.kernel.org>,
Arnd Bergmann <arnd@arndb.de>,
Andy Lutomirski <luto@amacapital.net>,
Balbir Singh <bsingharora@gmail.com>,
Cyrill Gorcunov <gorcunov@gmail.com>,
Florian Weimer <fweimer@redhat.com>,
hjl.tools@gmail.com, Jonathan Corbet <corbet@lwn.net>,
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>,
ravi.v.shankar@intel.com, vedvyas.shanbhogue@intel.com
Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW
Date: Fri, 14 Sep 2018 13:39:03 -0700 [thread overview]
Message-ID: <1536957543.12990.9.camel@intel.com> (raw)
In-Reply-To: <20180831162920.GQ24124@hirez.programming.kicks-ass.net>
On Fri, 2018-08-31 at 18:29 +0200, Peter Zijlstra wrote:
> On Fri, Aug 31, 2018 at 08:58:39AM -0700, Dave Hansen wrote:
> >
> > On 08/31/2018 08:48 AM, Yu-cheng Yu wrote:
> > >
> > > To trigger a race in ptep_set_wrprotect(), we need to fork from one of
> > > three pthread siblings.
> > >
> > > Or do we measure only how much this affects fork?
> > > If there is no racing, the effect should be minimal.
> > We don't need a race.
> >
> > I think the cmpxchg will be slower, even without a race, than the code
> > that was there before. The cmpxchg is a simple, straightforward
> > solution, but we're putting it in place of a plain memory write, which
> > is suboptimal.
> Note quite, the clear_bit() is LOCK prefixed.
With the updated ptep_set_wrprotect() below, I did MADV_WILLNEED to a shadow
stack of 8 MB, then 10,000 fork()'s, but could not prove it is more or less
efficient than the other. So can we say this is probably fine in terms of
efficiency?
Yu-cheng
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1203,7 +1203,36 @@ static inline pte_t ptep_get_and_clear_full(struct
mm_struct *mm,
static inline void ptep_set_wrprotect(struct mm_struct *mm,
unsigned long addr, pte_t *ptep)
{
+#ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
+ pte_t new_pte, pte = READ_ONCE(*ptep);
+
+ /*
+ * Some processors can start a write, but end up
+ * seeing a read-only PTE by the time they get
+ * to the Dirty bit. In this case, they will
+ * set the Dirty bit, leaving a read-only, Dirty
+ * PTE which looks like a Shadow Stack PTE.
+ *
+ * However, this behavior has been improved and
+ * will not occur on processors supporting
+ * Shadow Stacks. Without this guarantee, a
+ * transition to a non-present PTE and flush the
+ * TLB would be needed.
+ *
+ * When changing a writable PTE to read-only and
+ * if the PTE has _PAGE_DIRTY_HW set, we move
+ * that bit to _PAGE_DIRTY_SW so that the PTE is
+ * not a valid Shadow Stack PTE.
+ */
+ do {
+ new_pte = pte_wrprotect(pte);
+ new_pte.pte |= (new_pte.pte & _PAGE_DIRTY_HW) >>
+ _PAGE_BIT_DIRTY_HW << _PAGE_BIT_DIRTY_SW;
+ new_pte.pte &= ~_PAGE_DIRTY_HW;
+ } while (!try_cmpxchg(ptep, &pte, new_pte));
+#else
clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
+#endif
}
next prev parent reply other threads:[~2018-09-14 20:43 UTC|newest]
Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-30 14:38 [RFC PATCH v3 00/24] Control Flow Enforcement: Shadow Stack Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 01/24] x86/cpufeatures: Add CPUIDs for Control-flow Enforcement Technology (CET) Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 02/24] x86/fpu/xstate: Change some names to separate XSAVES system and user states Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 03/24] x86/fpu/xstate: Enable XSAVES system states Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 04/24] x86/fpu/xstate: Add XSAVES system states for shadow stack Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 05/24] Documentation/x86: Add CET description Yu-cheng Yu
2018-08-30 20:39 ` Pavel Machek
2018-08-30 22:49 ` Yu-cheng Yu
2018-09-14 21:17 ` Yu-cheng Yu
2018-09-03 2:56 ` Randy Dunlap
2018-08-30 14:38 ` [RFC PATCH v3 06/24] x86/cet: Control protection exception handler Yu-cheng Yu
2018-08-31 15:01 ` Jann Horn
2018-08-31 16:20 ` Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 07/24] x86/cet/shstk: Add Kconfig option for user-mode shadow stack Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 08/24] mm: Introduce VM_SHSTK for shadow stack memory Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 09/24] x86/mm: Change _PAGE_DIRTY to _PAGE_DIRTY_HW Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 10/24] x86/mm: Introduce _PAGE_DIRTY_SW Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 11/24] drm/i915/gvt: Update _PAGE_DIRTY to _PAGE_DIRTY_BITS Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW Yu-cheng Yu
2018-08-30 15:49 ` Jann Horn
2018-08-30 16:02 ` Yu-cheng Yu
2018-08-30 16:08 ` Dave Hansen
2018-08-30 16:23 ` Jann Horn
2018-08-30 17:19 ` Dave Hansen
2018-08-30 17:26 ` Yu-cheng Yu
2018-08-30 17:33 ` Dave Hansen
2018-08-30 17:54 ` Yu-cheng Yu
2018-08-30 17:59 ` Jann Horn
2018-08-30 20:21 ` Yu-cheng Yu
2018-08-30 20:44 ` Jann Horn
2018-08-30 20:52 ` Yu-cheng Yu
2018-08-30 21:01 ` Jann Horn
2018-08-30 21:47 ` Jann Horn
2018-08-31 9:53 ` Peter Zijlstra
2018-08-31 14:33 ` Yu-cheng Yu
2018-08-31 14:47 ` Dave Hansen
2018-08-31 15:48 ` Yu-cheng Yu
2018-08-31 15:58 ` Dave Hansen
2018-08-31 16:29 ` Peter Zijlstra
2018-09-14 20:39 ` Yu-cheng Yu [this message]
2018-09-14 20:46 ` Dave Hansen
2018-09-14 21:08 ` Yu-cheng Yu
2018-09-14 21:33 ` Dave Hansen
2018-08-31 1:23 ` Andy Lutomirski
2018-08-30 17:34 ` Andy Lutomirski
2018-08-30 18:55 ` Dave Hansen
2018-08-31 17:46 ` Andy Lutomirski
2018-08-31 17:52 ` Dave Hansen
2018-08-30 19:59 ` Randy Dunlap
2018-08-30 20:23 ` Yu-cheng Yu
2018-08-31 16:29 ` Dave Hansen
2018-08-30 14:38 ` [RFC PATCH v3 13/24] x86/mm: Shadow stack page fault error checking Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 14/24] mm: Handle shadow stack page fault Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 15/24] mm: Handle THP/HugeTLB " Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 16/24] mm: Update can_follow_write_pte/pmd for shadow stack Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 17/24] mm: Introduce do_mmap_locked() Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 18/24] x86/cet/shstk: User-mode shadow stack support Yu-cheng Yu
2018-08-30 16:10 ` Jann Horn
2018-08-30 16:20 ` Yu-cheng Yu
2018-08-30 14:38 ` [RFC PATCH v3 19/24] x86/cet/shstk: Introduce WRUSS instruction Yu-cheng Yu
2018-08-30 15:39 ` Jann Horn
2018-08-30 15:55 ` Andy Lutomirski
2018-08-30 16:22 ` Yu-cheng Yu
2018-08-31 21:49 ` Yu-cheng Yu
2018-08-31 22:16 ` Andy Lutomirski
2018-09-14 20:46 ` Yu-cheng Yu
2018-08-30 14:39 ` [RFC PATCH v3 20/24] x86/cet/shstk: Signal handling for shadow stack Yu-cheng Yu
2018-08-30 14:39 ` [RFC PATCH v3 21/24] x86/cet/shstk: ELF header parsing of Shadow Stack Yu-cheng Yu
2018-08-30 14:39 ` [RFC PATCH v3 22/24] x86/cet/shstk: Handle thread shadow stack Yu-cheng Yu
2018-08-30 14:39 ` [RFC PATCH v3 23/24] x86/cet/shstk: Add arch_prctl functions for Shadow Stack Yu-cheng Yu
2018-08-30 14:39 ` [RFC PATCH v3 24/24] x86/cet/shstk: Add Shadow Stack instructions to opcode map Yu-cheng Yu
2018-09-02 8:13 ` [RFC PATCH v3 00/24] Control Flow Enforcement: Shadow Stack Balbir Singh
2018-09-04 14:47 ` Yu-cheng Yu
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