linux-kernel - [tip: x86/sgx] x86/mm: Signal SIGSEGV with PF

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Date:   Wed, 18 Nov 2020 17:18:27 -0000
From:   "tip-bot2 for Sean Christopherson" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        Jethro Beekman <jethro@...tanix.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/sgx] x86/mm: Signal SIGSEGV with PF_SGX

The following commit has been merged into the x86/sgx branch of tip:

Commit-ID:     74faeee06db81a06add0def6a394210c8fef0ab7
Gitweb:        https://git.kernel.org/tip/74faeee06db81a06add0def6a394210c8fef0ab7
Author:        Sean Christopherson <sean.j.christopherson@...el.com>
AuthorDate:    Fri, 13 Nov 2020 00:01:17 +02:00
Committer:     Borislav Petkov <bp@...e.de>
CommitterDate: Tue, 17 Nov 2020 14:36:13 +01:00

x86/mm: Signal SIGSEGV with PF_SGX

The x86 architecture has a set of page fault error codes.  These indicate
things like whether the fault occurred from a write, or whether it
originated in userspace.

The SGX hardware architecture has its own per-page memory management
metadata (EPCM) [*] and hardware which is separate from the normal x86 MMU.
The architecture has a new page fault error code: PF_SGX.  This new error
code bit is set whenever a page fault occurs as the result of the SGX MMU.

These faults occur for a variety of reasons.  For instance, an access
attempt to enclave memory from outside the enclave causes a PF_SGX fault.
PF_SGX would also be set for permission conflicts, such as if a write to an
enclave page occurs and the page is marked read-write in the x86 page
tables but is read-only in the EPCM.

These faults do not always indicate errors, though.  SGX pages are
encrypted with a key that is destroyed at hardware reset, including
suspend. Throwing a SIGSEGV allows user space software to react and recover
when these events occur.

Include PF_SGX in the PF error codes list and throw SIGSEGV when it is
encountered.

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

 [ bp: Add bit 15 to the comment above enum x86_pf_error_code too. ]

Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
Signed-off-by: Borislav Petkov <bp@...e.de>
Acked-by: Jethro Beekman <jethro@...tanix.com>
Link: https://lkml.kernel.org/r/20201112220135.165028-7-jarkko@kernel.org
---
 arch/x86/include/asm/trap_pf.h |  2 ++
 arch/x86/mm/fault.c            | 12 ++++++++++++
 2 files changed, 14 insertions(+)

diff --git a/arch/x86/include/asm/trap_pf.h b/arch/x86/include/asm/trap_pf.h
index 305bc12..10b1de5 100644
--- a/arch/x86/include/asm/trap_pf.h
+++ b/arch/x86/include/asm/trap_pf.h
@@ -11,6 +11,7 @@
  *   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: SGX MMU page-fault
  */
 enum x86_pf_error_code {
 	X86_PF_PROT	=		1 << 0,
@@ -19,6 +20,7 @@ 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_TRAP_PF_H */
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 82bf37a..9339fee 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1102,6 +1102,18 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
 		return 1;

 	/*
+	 * SGX hardware blocked the access.  This usually happens
+	 * when the enclave memory contents have been destroyed, like
+	 * after a suspend/resume cycle. In any case, the kernel can't
+	 * fix the cause of the fault.  Handle the fault as an access
+	 * error even in cases where no actual access violation
+	 * occurred.  This allows userspace to rebuild the enclave in
+	 * response to the signal.
+	 */
+	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
 	 * page.