Re: [RFC PATCH v4 05/26] x86/sgx: Introduce virtual EPC for use by KVM guests

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 10, 2021, Kai Huang wrote:
> On Tue, 2021-02-09 at 13:36 -0800, Dave Hansen wrote:
> > On 2/9/21 1:19 PM, Jarkko Sakkinen wrote:
> > > > Without that clearly documented, it would be unwise to merge this.
> > > E.g.
> > > 
> > > - Have ioctl() to turn opened fd as vEPC.
> > > - If FLC is disabled, you could only use the fd for creating vEPC.
> > > 
> > > Quite easy stuff to implement.

...

> What's your opinion? Did I miss anything?

Frankly, I think trying to smush them together would be a complete trainwreck.

The vast majority of flows would need to go down completely different paths, so
you'd end up with code like this:

diff --git a/arch/x86/kernel/cpu/sgx/driver.c b/arch/x86/kernel/cpu/sgx/driver.c
index f2eac41bb4ff..5128043c7871 100644
--- a/arch/x86/kernel/cpu/sgx/driver.c
+++ b/arch/x86/kernel/cpu/sgx/driver.c
@@ -46,6 +46,9 @@ static int sgx_release(struct inode *inode, struct file *file)
        struct sgx_encl *encl = file->private_data;
        struct sgx_encl_mm *encl_mm;
 
+       if (encl->not_an_enclave)
+               return sgx_virt_epc_release(encl);
+
        /*
         * Drain the remaining mm_list entries. At this point the list contains
         * entries for processes, which have closed the enclave file but have
@@ -83,6 +86,9 @@ static int sgx_mmap(struct file *file, struct vm_area_struct *vma)
        struct sgx_encl *encl = file->private_data;
        int ret;
 
+       if (encl->not_an_enclave)
+               return sgx_virt_epc_mmap(encl, vma);
+
        ret = sgx_encl_may_map(encl, vma->vm_start, vma->vm_end, vma->vm_flags);
        if (ret)
                return ret;
@@ -104,6 +110,11 @@ static unsigned long sgx_get_unmapped_area(struct file *file,
                                           unsigned long pgoff,
                                           unsigned long flags)
 {
+       struct sgx_encl *encl = file->private_data;
+
+       if (encl->not_an_enclave)
+               return sgx_virt_epc_mmap(encl, addr, len, pgoff, flags);
+
        if ((flags & MAP_TYPE) == MAP_PRIVATE)
                return -EINVAL;

I suspect it would also be tricky to avoid introducing races, since anything that
is different for virtual EPC would have a dependency on the ioctl() being called.

This would also prevent making /dev/sgx_enclave root-only while allowing users
access to /dev/sgx_vepc.  Forcing admins to use LSMs to do the same is silly.

For the few flows that can share code, just split out the common bits to helpers.