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SubjectRe: [PATCH v38 10/24] mm: Add vm_ops->mprotect()
On 9/28/20 9:05 PM, Jarkko Sakkinen wrote:
> On Mon, Sep 28, 2020 at 06:37:54PM -0700, Andy Lutomirski wrote:
>> I don’t personally care that much about EMODPE but, you could probably
>> get the point across with something like:
>> SGX’s EPCM permission bits do not obviate the need to enforce these
>> rules in the PTEs because enclaves can freely modify the EPCM
>> permissions using EMODPE.
>> IOW, EMODPE is not really special here; rather, EMODPE’s existence
>> demonstrates that EADD / EEXTEND are not special.
> So I did "disagree and commit" with this one. I'm not actually
> diagreeing on anything what Dave wrote, on the contrary it is an
> understandable high level description. I just thought that it would not
> hurt to remark that the ISA contains such peculiarities as EMODPE.
> I did only very rudimentary clean up for the text (e.g. fix the ioctl
> name, add shortt summary and not much else).
> Does not make sense to waste more time to this. I'll move on to
> implement the missing boot time patching for the vDSO so that we
> get the next version out.
> "
> mm: Add 'mprotect' hook to struct vm_operations_struct
> Background
> ==========
> 1. SGX enclave pages are populated with data by copying data to them
> from normal memory via ioctl(fd, SGX_IOC_ENCLAVE_ADD_PAGES).
> 2. We want to be able to restrict those normal memory data sources. For
> instance, before copying data to an executable enclave page, we might
> ensure that the source is executable.

I know I wrote that. I suck, and I wrote it in a changelog-unacceptable
way. Folks dislike the use of "we" in these things. Here's a better

2. It is desirable to be able to restrict those normal memory data
sources. For instance, the kernel can ensure that the source is
executable, before copying data to an executable enclave page.

> 3. Enclave page permissions are dynamic just like normal permissions and
> can be adjusted at runtime with mprotect() (along with a
> corresponding special instruction inside the enclave).
> 4. The original data source may have have long since vanished at the
> time when enclave page permission are established (mmap() or
> mprotect()).
> Solution
> ========
> The solution is to force enclaves creators to declare their intent up front
> to ioctl(fd, SGX_IOC_ENCLAVE_ADD_PAGES). This intent can me immediately
> compared to the source data mapping (and rejected if necessary). It is
> also stashed off and then later compared with enclave PTEs to ensure that
> any future mmap()/mprotect() operations performed by the enclave creator or
> the enclave itself are consistent with the earlier declared permissions.

Let's also say "... or *requested* by the enclave itself ...", since the
enclave itself can't directly make syscalls.

> Essentially, this means that whenever the kernel is asked to change an
> enclave PTE, it needs to ensure the change is consistent with that stashed
> intent. There is an existing vm_ops->mmap() hook which allows SGX to do
> that for mmap(). However, there is no ->mprotect() hook. Add a
> vm_ops->mprotect() hook so that mprotect() operations which are
> inconsistent with any page's stashed intent can be rejected by the driver.
> Implications
> ============
> However, there is currently no implementation of the intent checks at the
> time of ioctl(fd, SGX_IOC_ENCLAVE_ADD_PAGES). That means that the intent
> argument (SGX_PROT_*) is currently unused.

This was incorrect to say. Sean corrected me on this point. Could you
look through the thread and incorporate that?

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