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@intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Jethro Beekman <jethro@fortanix.com>
Link: https://lkml.kernel.org/r/20201112220135.165028-7-jarkko@kernel.org

diff --git a/arch/x86/include/asm/trap_pf.h b/arch/x86/include/asm/trap_pf.h
index 305bc12..10b1de5 100644 (file)
--- 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 (file)
--- 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.