Re: x86/sgx: v23-rc2

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Thu, Oct 10, 2019 at 04:37:53PM +0300, Jarkko Sakkinen wrote:
> On Thu, Oct 10, 2019 at 02:37:45PM +0300, Jarkko Sakkinen wrote:
> > tag v23-rc2
> > Tagger: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> > Date:   Thu Oct 10 14:33:07 2019 +0300
> > 
> > x86/sgx: v23-rc1 patch set
> > 
> > * Return -EIO instead of -ECANCELED when ptrace() fails to read a TCS page.
> > * In the reclaimer, pin page before ENCLS[EBLOCK] because pinning can fail
> >   (because of OOM) even in legit behaviour and after EBLOCK the reclaiming
> >   flow can be only reverted by killing the whole enclave.
> > * Fixed SGX_ATTR_RESERVED_MASK. Bit 7 was marked as reserved while in fact
> >   it should have been bit 6 (Table 37-3 in the SDM).
> > * Return -EPERM from SGX_IOC_ENCLAVE_INIT when ENCLS[EINIT] returns an SGX
> >   error code.
> > * In v22 __uaccess_begin() was used to pin the source page in
> >   __sgx_encl_add_page(). Switch to get_user_pages() in order to avoid
> >   deadlock (mmap_sem might get locked twice in the same thread).
> 
> __uaccess_begin() is also needed to performan access checks the legit

__uaccess_begin() doesn't check the address space, it temporarily disables
SMAP/SMEP so that the kernel can access a user mapping.  An explicit
access_ok() call should be added as well.

> user space address. What we can do is to use get_user_pages() just to
> make sure that the page is faulted while we perform ENCLS[EADD].


> I updated the master branch with the fix for this. Now the access
> pattern is:
> 
> 	ret = get_user_pages(src, 1, 0, &src_page, NULL);
> 	if (ret < 1)
> 		return ret;
> 
> 	__uaccess_begin();

This should be immediately before __eadd().  I also think it'd be a good
idea to disable page faults around __eadd() so that an unexpected #PF
manifests as an __eadd() failure and not a kernel hang.

> 	pginfo.secs = (unsigned long)sgx_epc_addr(encl->secs.epc_page);
> 	pginfo.addr = SGX_ENCL_PAGE_ADDR(encl_page);
> 	pginfo.metadata = (unsigned long)secinfo;
> 	pginfo.contents = (unsigned long)src;

> 	ret = __eadd(&pginfo, sgx_epc_addr(epc_page));
> 
> 	__uaccess_end();
> 	put_page(src_page);

Not shown here, but mmap_sem doesn't need to be held through EEXTEND. The
lock issue is that down_read() will block if there is a pending
down_write(), e.g. if userspace is doing mprotect() at the same time as
EADD, then deadlock will occur if EADD faults.  Holding encl->lock without
mmap_sem is perfectly ok.

I'll send a small series with the above changes.