On Tue, Sep 25, 2018 at 04:06:45PM +0300, Jarkko Sakkinen wrote: > From: Sean Christopherson <[email protected]> > > The SGX Enclave Page Cache Map (EPCM) is a hardware-managed table > that enforces accesses to an enclave's EPC page in addition to the > software-managed kernel page tables, i.e. the effective permissions > for an EPC page are a logical AND of the kernel's page tables and > the corresponding EPCM entry. The primary purpose of the EPCM is > to prevent a malcious or compromised kernel from attacking an enclave > by modifying the enclave's page tables. The EPCM entires for an > enclave are populated when the enclave is built and verified, using > metadata provided by the enclave that is included in the measurement > used to verify the enclave. > > In normal operation of a properly functioning, non-malicious kernel > (and enclave), the EPCM permissions will never trigger a fault, i.e. > the kernel may make the permissions for an EPC page more restrictive, > e.g. mark it not-present to swap out the EPC page, but the kernel will > never make its permissions less restrictive. > > But, there is a legitimate scenario in which the kernel's page tables > can become less restrictive than the EPCM: on current hardware all > enclaves are destroyed (by hardware) on a transition to S3 or lower > sleep states, i.e. all EPCM entries are invalid (not-present) after > the system resumes from its sleep state. > > Unfortunately, on CPUs that support only SGX1, EPCM violations result > in a #GP. The upside of the #GP is that no kernel changes are needed > to deal with the EPCM being blasted away by hardware, e.g. userspace > gets a SIGSEGV, assumes the EPCM was lost and restarts its enclave > and everyone is happy. The downside is that userspace has to assume > the SIGSEGV was because the EPC was lost (or possibly do some leg work > to rule out other causes). > > In SGX2, the oddity of delivering a #GP due to what are inherently > paging related violations is remedied. CPUs that support SGX2 deliver > EPCM violations as #PFs with a new SGX error code bit set. So, now > that hardware provides us with a way to unequivocally determine that > a fault was due to a EPCM violation, define a signfo code for SIGSEGV > so that the information can be passed onto userspace. > > Cc: Dave Hansen <[email protected]> > Signed-off-by: Sean Christopherson <[email protected]> > Signed-off-by: Jarkko Sakkinen <[email protected]> > --- > include/uapi/asm-generic/siginfo.h | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/include/uapi/asm-generic/siginfo.h > b/include/uapi/asm-generic/siginfo.h > index 80e2a7227205..fdd898e2325b 100644 > --- a/include/uapi/asm-generic/siginfo.h > +++ b/include/uapi/asm-generic/siginfo.h > @@ -225,7 +225,11 @@ typedef struct siginfo { > #else > # define SEGV_PKUERR 4 /* failed protection key checks */ > #endif > +#ifdef __x86_64__
Argh, this needs to be "#if defined(__i386__) || defined(__x86_64__)" otherwise 32-bit builds break on later patches that use SEGV_SGXERR, e.g. the errors flagged by the 0-DAY bot. > +#define SEGV_SGXERR 5 /* SGX Enclave Page Cache Map fault */ > +#else > #define SEGV_ACCADI 5 /* ADI not enabled for mapped object */ > +#endif > #define SEGV_ADIDERR 6 /* Disrupting MCD error */ > #define SEGV_ADIPERR 7 /* Precise MCD exception */ > #define NSIGSEGV 7 > -- > 2.17.1 >
