Re: [PATCH v38 10/24] mm: Add vm_ops->mprotect()

Dave Hansen
2020-09-24T20:10:40Z

On 9/24/20 1:01 PM, Sean Christopherson wrote:
>> In pseudo-C, it's something logically like this for the "nice" case:
>>
>> ptr = mmap("/some/executable", PROT_EXEC);
>> ioctl(sgx_fd, ADD_ENCLAVE_PAGE, SGX_PROT_EXEC, ptr, size);
>> mmap(sgx_fd);
>> EENTER;
>>
>> And we're trying to thwart:
>>
>> ptr = mmap("/mnt/noexec/file", PROT_READ);
>> ioctl(sgx_fd, ADD_ENCLAVE_PAGE, SGX_PROT_EXEC, ptr, size);
>> mmap(sgx_fd);
>> EENTER;
>>
>> because that loads data into the enclave which is executable but which
>> was not executable normally. But, we're allowing this from anonymous
>> memory, so this would seem to work:
>>
>> ptr = mmap("/mnt/noexec/file", PROT_READ);
>> buffer = malloc(PAGE_SIZE);
>> memcpy(buffer, ptr, PAGE_SIZE);
>> // need mprotect(buf, PROT_EXEC)???
>> ioctl(sgx_fd, ADD_ENCLAVE_PAGE, SGX_PROT_EXEC, buffer, size);
>> mmap(sgx_fd);
>> EENTER;
>>
>> and give the same result. What am I missing?
> The last example, where the enclave is copied to a buffer, is out of scope
> for noexec. But, it is in scope for LSMs, e.g. for this last example, we
> could add an LSM upcall so that SELinux could require PROCESS_EXECMEM (or an
> SGX specific equivalent).

Why don't we just declare enclave memory as "out of scope for noexec" in
the same way that anonymous memory is, and just discard this patch?
That doesn't seem too much of a stretch.