Re: [PATCH v20 00/28] Intel SGX1 support

[Jethro Beekman <jethro@fortanix.com>]

On 2019-04-19 14:34, Thomas Gleixner wrote:
> And how so? You create writeable AND executable memory. That's a nono and
> you can argue in circles, that's not going to change with any of your
> proposed changes.

On 2019-04-19 14:38, Thomas Gleixner wrote:
> You are working around LSM nothing else and that's just not going to fly.

Based on your comments, I'm still unsure if we're on the same page with
regards to what I'm proposing.

Here's a regular non-SGX flow that LSM would likely prevent:

mmap(PROT_READ|PROT_WRITE)
memcpy()
mmap(PROT_READ|PROT_EXEC) <-- denied by LSM

Or just something based on regular PT permissions:

mmap(PROT_READ|PROT_EXEC)
memcpy() <-- SIGSEGV

Now, the equivalent for SGX:

mmap(PROT_READ|PROT_WRITE)
ioctl(EADD)
mmap(PROT_READ|PROT_EXEC) <-- denied by LSM

This works fine with v20 as-is. However, consider the equivalent of the
PT-based flow:

mmap(PROT_READ|PROT_EXEC)
ioctl(EADD) <-- no error!

It's not me that's working around the LSM, it's the SGX driver! It's
writing to memory that's not marked writable! The fundamental issue here
is that the SGX instruction set has several instructions that bypass the
page table permission bits, and this is (naturally) confusing to any
kind of reference monitor like the LSM framework. You can come up with
similar scenarios that involve PROT_READ|PROT_WRITE|PROT_EXEC or
ptrace(PTRACE_POKETEXT). So, clearly, the proper way to fix this failure
of complete mediation is by enforcing appropriate page-table permissions
even on the SGX instructions that don't do it themselves. Just make any
implicit memory access look like a regular memory access and now
everyone is on the same page (pun intended).

--
Jethro Beekman | Fortanix