From mboxrd@z Thu Jan 1 00:00:00 1970 From: Masami Hiramatsu Subject: Re: [RFC PATCH 2/6] x86/mm: temporary mm struct Date: Wed, 29 Aug 2018 18:49:25 +0900 Message-ID: <20180829184925.64caee4dadf705080373f84f@kernel.org> References: <20180829081147.184610-1-namit@vmware.com> <20180829081147.184610-3-namit@vmware.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20180829081147.184610-3-namit@vmware.com> Sender: linux-kernel-owner@vger.kernel.org To: Nadav Amit Cc: Thomas Gleixner , linux-kernel@vger.kernel.org, Ingo Molnar , x86@kernel.org, Arnd Bergmann , linux-arch@vger.kernel.org, Andy Lutomirski , Masami Hiramatsu , Kees Cook , Peter Zijlstra List-Id: linux-arch.vger.kernel.org On Wed, 29 Aug 2018 01:11:43 -0700 Nadav Amit wrote: > From: Andy Lutomirski > > Sometimes we want to set a temporary page-table entries (PTEs) in one of > the cores, without allowing other cores to use - even speculatively - > these mappings. There are two benefits for doing so: > > (1) Security: if sensitive PTEs are set, temporary mm prevents their use > in other cores. This hardens the security as it prevents exploding a > dangling pointer to overwrite sensitive data using the sensitive PTE. > > (2) Avoiding TLB shootdowns: the PTEs do not need to be flushed in > remote page-tables. > > To do so a temporary mm_struct can be used. Mappings which are private > for this mm can be set in the userspace part of the address-space. > During the whole time in which the temporary mm is loaded, interrupts > must be disabled. > > The first use-case for temporary PTEs, which will follow, is for poking > the kernel text. > > [ Commit message was written by Nadav ] > > Cc: Andy Lutomirski > Cc: Masami Hiramatsu > Cc: Kees Cook > Cc: Peter Zijlstra > Signed-off-by: Nadav Amit > --- > arch/x86/include/asm/mmu_context.h | 20 ++++++++++++++++++++ > 1 file changed, 20 insertions(+) > > diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h > index eeeb9289c764..96afc8c0cf15 100644 > --- a/arch/x86/include/asm/mmu_context.h > +++ b/arch/x86/include/asm/mmu_context.h > @@ -338,4 +338,24 @@ static inline unsigned long __get_current_cr3_fast(void) > return cr3; > } > > +typedef struct { > + struct mm_struct *prev; > +} temporary_mm_state_t; > + > +static inline temporary_mm_state_t use_temporary_mm(struct mm_struct *mm) > +{ > + temporary_mm_state_t state; > + > + lockdep_assert_irqs_disabled(); > + state.prev = this_cpu_read(cpu_tlbstate.loaded_mm); > + switch_mm_irqs_off(NULL, mm, current); > + return state; > +} Hmm, why don't we return mm_struct *prev directly? Thank you, > + > +static inline void unuse_temporary_mm(temporary_mm_state_t prev) > +{ > + lockdep_assert_irqs_disabled(); > + switch_mm_irqs_off(NULL, prev.prev, current); > +} > + > #endif /* _ASM_X86_MMU_CONTEXT_H */ > -- > 2.17.1 > -- Masami Hiramatsu From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org ([198.145.29.99]:47142 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727099AbeH2Npe (ORCPT ); Wed, 29 Aug 2018 09:45:34 -0400 Date: Wed, 29 Aug 2018 18:49:25 +0900 From: Masami Hiramatsu Subject: Re: [RFC PATCH 2/6] x86/mm: temporary mm struct Message-ID: <20180829184925.64caee4dadf705080373f84f@kernel.org> In-Reply-To: <20180829081147.184610-3-namit@vmware.com> References: <20180829081147.184610-1-namit@vmware.com> <20180829081147.184610-3-namit@vmware.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: Nadav Amit Cc: Thomas Gleixner , linux-kernel@vger.kernel.org, Ingo Molnar , x86@kernel.org, Arnd Bergmann , linux-arch@vger.kernel.org, Andy Lutomirski , Masami Hiramatsu , Kees Cook , Peter Zijlstra Message-ID: <20180829094925.1qoqhJLwwPurn1vf9pEvqoEKQlxNdV-oxhJGTg8RUec@z> On Wed, 29 Aug 2018 01:11:43 -0700 Nadav Amit wrote: > From: Andy Lutomirski > > Sometimes we want to set a temporary page-table entries (PTEs) in one of > the cores, without allowing other cores to use - even speculatively - > these mappings. There are two benefits for doing so: > > (1) Security: if sensitive PTEs are set, temporary mm prevents their use > in other cores. This hardens the security as it prevents exploding a > dangling pointer to overwrite sensitive data using the sensitive PTE. > > (2) Avoiding TLB shootdowns: the PTEs do not need to be flushed in > remote page-tables. > > To do so a temporary mm_struct can be used. Mappings which are private > for this mm can be set in the userspace part of the address-space. > During the whole time in which the temporary mm is loaded, interrupts > must be disabled. > > The first use-case for temporary PTEs, which will follow, is for poking > the kernel text. > > [ Commit message was written by Nadav ] > > Cc: Andy Lutomirski > Cc: Masami Hiramatsu > Cc: Kees Cook > Cc: Peter Zijlstra > Signed-off-by: Nadav Amit > --- > arch/x86/include/asm/mmu_context.h | 20 ++++++++++++++++++++ > 1 file changed, 20 insertions(+) > > diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h > index eeeb9289c764..96afc8c0cf15 100644 > --- a/arch/x86/include/asm/mmu_context.h > +++ b/arch/x86/include/asm/mmu_context.h > @@ -338,4 +338,24 @@ static inline unsigned long __get_current_cr3_fast(void) > return cr3; > } > > +typedef struct { > + struct mm_struct *prev; > +} temporary_mm_state_t; > + > +static inline temporary_mm_state_t use_temporary_mm(struct mm_struct *mm) > +{ > + temporary_mm_state_t state; > + > + lockdep_assert_irqs_disabled(); > + state.prev = this_cpu_read(cpu_tlbstate.loaded_mm); > + switch_mm_irqs_off(NULL, mm, current); > + return state; > +} Hmm, why don't we return mm_struct *prev directly? Thank you, > + > +static inline void unuse_temporary_mm(temporary_mm_state_t prev) > +{ > + lockdep_assert_irqs_disabled(); > + switch_mm_irqs_off(NULL, prev.prev, current); > +} > + > #endif /* _ASM_X86_MMU_CONTEXT_H */ > -- > 2.17.1 > -- Masami Hiramatsu