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

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