[v36,03/24] x86/mm: x86/sgx: Signal SIGSEGV with PF_SGX
diff mbox series

Message ID 20200716135303.276442-4-jarkko.sakkinen@linux.intel.com
State New
Headers show
Series
  • Intel SGX foundations
Related show

Commit Message

Jarkko Sakkinen July 16, 2020, 1:52 p.m. UTC
From: Sean Christopherson <sean.j.christopherson@intel.com>

Include SGX bit to the PF error codes and throw SIGSEGV with PF_SGX when
a #PF with SGX set happens.

CPU throws a #PF with the SGX set in the event of Enclave Page Cache Map
(EPCM) conflict. The EPCM is a CPU-internal table, which describes the
properties for a enclave page. Enclaves are measured and signed software
entities, which SGX hosts. [1]

Although the primary purpose of the EPCM conflict checks  is to prevent
malicious accesses to an enclave, an illegit access can happen also for
legit reasons.

All SGX reserved memory, including EPCM is encrypted with a transient key
that does not survive from the power transition. Throwing a SIGSEGV allows
user space software to react when this happens (e.g. recreate the enclave,
which was invalidated).

[1] Intel SDM: 36.5.1 Enclave Page Cache Map (EPCM)

Acked-by: Jethro Beekman <jethro@fortanix.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
---
 arch/x86/include/asm/traps.h | 14 ++++++++------
 arch/x86/mm/fault.c          | 13 +++++++++++++
 2 files changed, 21 insertions(+), 6 deletions(-)

Comments

Darren Kenny Aug. 6, 2020, 1:16 p.m. UTC | #1
On Thursday, 2020-07-16 at 16:52:42 +03, Jarkko Sakkinen wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
>
> Include SGX bit to the PF error codes and throw SIGSEGV with PF_SGX when
> a #PF with SGX set happens.
>
> CPU throws a #PF with the SGX set in the event of Enclave Page Cache Map
> (EPCM) conflict. The EPCM is a CPU-internal table, which describes the
> properties for a enclave page. Enclaves are measured and signed software
> entities, which SGX hosts. [1]
>
> Although the primary purpose of the EPCM conflict checks  is to prevent
> malicious accesses to an enclave, an illegit access can happen also for
> legit reasons.
>
> All SGX reserved memory, including EPCM is encrypted with a transient key
> that does not survive from the power transition. Throwing a SIGSEGV allows
> user space software to react when this happens (e.g. recreate the enclave,
> which was invalidated).
>
> [1] Intel SDM: 36.5.1 Enclave Page Cache Map (EPCM)
>
> Acked-by: Jethro Beekman <jethro@fortanix.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>

Reviewed-by: Darren Kenny <darren.kenny@oracle.com>

> ---
>  arch/x86/include/asm/traps.h | 14 ++++++++------
>  arch/x86/mm/fault.c          | 13 +++++++++++++
>  2 files changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
> index 714b1a30e7b0..4446f95ad997 100644
> --- a/arch/x86/include/asm/traps.h
> +++ b/arch/x86/include/asm/traps.h
> @@ -44,12 +44,13 @@ void __noreturn handle_stack_overflow(const char *message,
>  /*
>   * Page fault error code bits:
>   *
> - *   bit 0 ==	 0: no page found	1: protection fault
> - *   bit 1 ==	 0: read access		1: write access
> - *   bit 2 ==	 0: kernel-mode access	1: user-mode access
> - *   bit 3 ==				1: use of reserved bit detected
> - *   bit 4 ==				1: fault was an instruction fetch
> - *   bit 5 ==				1: protection keys block access
> + *   bit 0  ==	 0: no page found	1: protection fault
> + *   bit 1  ==	 0: read access		1: write access
> + *   bit 2  ==	 0: kernel-mode access	1: user-mode access
> + *   bit 3  ==				1: use of reserved bit detected
> + *   bit 4  ==				1: fault was an instruction fetch
> + *   bit 5  ==				1: protection keys block access
> + *   bit 15 ==				1: inside SGX enclave
>   */
>  enum x86_pf_error_code {
>  	X86_PF_PROT	=		1 << 0,
> @@ -58,5 +59,6 @@ enum x86_pf_error_code {
>  	X86_PF_RSVD	=		1 << 3,
>  	X86_PF_INSTR	=		1 << 4,
>  	X86_PF_PK	=		1 << 5,
> +	X86_PF_SGX	=		1 << 15,
>  };
>  #endif /* _ASM_X86_TRAPS_H */
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 1ead568c0101..1db6fbd7af8e 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1055,6 +1055,19 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
>  	if (error_code & X86_PF_PK)
>  		return 1;
>  
> +	/*
> +	 * Access is blocked by the Enclave Page Cache Map (EPCM), i.e. the
> +	 * access is allowed by the PTE but not the EPCM. This usually happens
> +	 * when the EPCM is yanked out from under us, e.g. by hardware after a
> +	 * suspend/resume cycle. In any case, software, i.e. the kernel, can't
> +	 * fix the source of the fault as the EPCM can't be directly modified by
> +	 * software. Handle the fault as an access error in order to signal
> +	 * userspace so that userspace can rebuild their enclave(s), even though
> +	 * userspace may not have actually violated access permissions.
> +	 */
> +	if (unlikely(error_code & X86_PF_SGX))
> +		return 1;
> +
>  	/*
>  	 * Make sure to check the VMA so that we do not perform
>  	 * faults just to hit a X86_PF_PK as soon as we fill in a
> -- 
> 2.25.1
Borislav Petkov Aug. 20, 2020, 3:31 p.m. UTC | #2
On Thu, Jul 16, 2020 at 04:52:42PM +0300, Jarkko Sakkinen wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
> 
> Include SGX bit to the PF error codes and throw SIGSEGV with PF_SGX when
> a #PF with SGX set happens.
> 
> CPU throws a #PF with the SGX set in the event of Enclave Page Cache Map
> (EPCM) conflict. The EPCM is a CPU-internal table, which describes the
> properties for a enclave page. Enclaves are measured and signed software
> entities, which SGX hosts. [1]
> 
> Although the primary purpose of the EPCM conflict checks  is to prevent
> malicious accesses to an enclave, an illegit access can happen also for
> legit reasons.
> 
> All SGX reserved memory, including EPCM is encrypted with a transient key
> that does not survive from the power transition. Throwing a SIGSEGV allows
> user space software to react when this happens (e.g. recreate the enclave,
> which was invalidated).
> 
> [1] Intel SDM: 36.5.1 Enclave Page Cache Map (EPCM)
> 
> Acked-by: Jethro Beekman <jethro@fortanix.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> ---
>  arch/x86/include/asm/traps.h | 14 ++++++++------
>  arch/x86/mm/fault.c          | 13 +++++++++++++
>  2 files changed, 21 insertions(+), 6 deletions(-)

Reviewed-by: Borislav Petkov <bp@suse.de>
Jarkko Sakkinen Aug. 21, 2020, 5:35 p.m. UTC | #3
On Thu, Aug 20, 2020 at 05:31:14PM +0200, Borislav Petkov wrote:
> On Thu, Jul 16, 2020 at 04:52:42PM +0300, Jarkko Sakkinen wrote:
> > From: Sean Christopherson <sean.j.christopherson@intel.com>
> > 
> > Include SGX bit to the PF error codes and throw SIGSEGV with PF_SGX when
> > a #PF with SGX set happens.
> > 
> > CPU throws a #PF with the SGX set in the event of Enclave Page Cache Map
> > (EPCM) conflict. The EPCM is a CPU-internal table, which describes the
> > properties for a enclave page. Enclaves are measured and signed software
> > entities, which SGX hosts. [1]
> > 
> > Although the primary purpose of the EPCM conflict checks  is to prevent
> > malicious accesses to an enclave, an illegit access can happen also for
> > legit reasons.
> > 
> > All SGX reserved memory, including EPCM is encrypted with a transient key
> > that does not survive from the power transition. Throwing a SIGSEGV allows
> > user space software to react when this happens (e.g. recreate the enclave,
> > which was invalidated).
> > 
> > [1] Intel SDM: 36.5.1 Enclave Page Cache Map (EPCM)
> > 
> > Acked-by: Jethro Beekman <jethro@fortanix.com>
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> > Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> > ---
> >  arch/x86/include/asm/traps.h | 14 ++++++++------
> >  arch/x86/mm/fault.c          | 13 +++++++++++++
> >  2 files changed, 21 insertions(+), 6 deletions(-)
> 
> Reviewed-by: Borislav Petkov <bp@suse.de>

Thank you. Appended to the commit.

> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette

/Jarkko

Patch
diff mbox series

diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
index 714b1a30e7b0..4446f95ad997 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -44,12 +44,13 @@  void __noreturn handle_stack_overflow(const char *message,
 /*
  * Page fault error code bits:
  *
- *   bit 0 ==	 0: no page found	1: protection fault
- *   bit 1 ==	 0: read access		1: write access
- *   bit 2 ==	 0: kernel-mode access	1: user-mode access
- *   bit 3 ==				1: use of reserved bit detected
- *   bit 4 ==				1: fault was an instruction fetch
- *   bit 5 ==				1: protection keys block access
+ *   bit 0  ==	 0: no page found	1: protection fault
+ *   bit 1  ==	 0: read access		1: write access
+ *   bit 2  ==	 0: kernel-mode access	1: user-mode access
+ *   bit 3  ==				1: use of reserved bit detected
+ *   bit 4  ==				1: fault was an instruction fetch
+ *   bit 5  ==				1: protection keys block access
+ *   bit 15 ==				1: inside SGX enclave
  */
 enum x86_pf_error_code {
 	X86_PF_PROT	=		1 << 0,
@@ -58,5 +59,6 @@  enum x86_pf_error_code {
 	X86_PF_RSVD	=		1 << 3,
 	X86_PF_INSTR	=		1 << 4,
 	X86_PF_PK	=		1 << 5,
+	X86_PF_SGX	=		1 << 15,
 };
 #endif /* _ASM_X86_TRAPS_H */
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 1ead568c0101..1db6fbd7af8e 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1055,6 +1055,19 @@  access_error(unsigned long error_code, struct vm_area_struct *vma)
 	if (error_code & X86_PF_PK)
 		return 1;
 
+	/*
+	 * Access is blocked by the Enclave Page Cache Map (EPCM), i.e. the
+	 * access is allowed by the PTE but not the EPCM. This usually happens
+	 * when the EPCM is yanked out from under us, e.g. by hardware after a
+	 * suspend/resume cycle. In any case, software, i.e. the kernel, can't
+	 * fix the source of the fault as the EPCM can't be directly modified by
+	 * software. Handle the fault as an access error in order to signal
+	 * userspace so that userspace can rebuild their enclave(s), even though
+	 * userspace may not have actually violated access permissions.
+	 */
+	if (unlikely(error_code & X86_PF_SGX))
+		return 1;
+
 	/*
 	 * Make sure to check the VMA so that we do not perform
 	 * faults just to hit a X86_PF_PK as soon as we fill in a