Re: [PATCH v19 19/27] x86/sgx: ptrace() support for the SGX driver

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Tue, Mar 19, 2019 at 03:22:01PM -0700, Sean Christopherson wrote:
> On Sun, Mar 17, 2019 at 11:14:48PM +0200, Jarkko Sakkinen wrote:
> > Add VMA callbacks for ptrace() that can be used with debug enclaves.
> > With debug enclaves data can be read and write the memory word at a time
> > by using ENCLS(EDBGRD) and ENCLS(EDBGWR) leaf instructions.
> > 
> > Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> > ---
> >  arch/x86/kernel/cpu/sgx/encl.c | 97 ++++++++++++++++++++++++++++++++++
> >  1 file changed, 97 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> > index 1b8874699dd3..5b5fc933ee19 100644
> > --- a/arch/x86/kernel/cpu/sgx/encl.c
> > +++ b/arch/x86/kernel/cpu/sgx/encl.c
> > @@ -256,10 +256,107 @@ static unsigned int sgx_vma_fault(struct vm_fault *vmf)
> >  	return ret;
> >  }
> >  
> > +static int sgx_edbgrd(struct sgx_encl *encl, struct sgx_encl_page *page,
> > +		      unsigned long addr, void *data)
> > +{
> > +	unsigned long offset;
> > +	int ret;
> > +
> > +	offset = addr & ~PAGE_MASK;
> > +
> > +	if ((page->desc & SGX_ENCL_PAGE_TCS) &&
> > +	    offset > offsetof(struct sgx_tcs, gs_limit))
> > +		return -ECANCELED;
> > +
> > +	ret = __edbgrd(sgx_epc_addr(page->epc_page) + offset, data);
> > +	if (ret)
> > +		return -EIO;
> > +
> > +	return 0;
> > +}
> > +
> > +static int sgx_edbgwr(struct sgx_encl *encl, struct sgx_encl_page *page,
> > +		      unsigned long addr, void *data)
> > +{
> > +	unsigned long offset;
> > +	int ret;
> > +
> > +	offset = addr & ~PAGE_MASK;
> > +
> > +	/* Writing anything else than flags will cause #GP */
> > +	if ((page->desc & SGX_ENCL_PAGE_TCS) &&
> > +	    offset != offsetof(struct sgx_tcs, flags))
> > +		return -ECANCELED;
> > +
> > +	ret = __edbgwr(sgx_epc_addr(page->epc_page) + offset, data);
> > +	if (ret)
> > +		return -EIO;
> > +
> > +	return 0;
> > +}
> > +
> > +static int sgx_vma_access(struct vm_area_struct *vma, unsigned long addr,
> > +			  void *buf, int len, int write)
> > +{
> > +	struct sgx_encl *encl = vma->vm_private_data;
> > +	struct sgx_encl_page *entry = NULL;
> > +	unsigned long align;
> > +	char data[sizeof(unsigned long)];
> > +	int offset;
> > +	int cnt;
> > +	int ret = 0;
> > +	int i;
> > +
> > +	/* If process was forked, VMA is still there but vm_private_data is set
> > +	 * to NULL.
> > +	 */
> > +	if (!encl)
> > +		return -EFAULT;
> > +
> > +	if (!(encl->flags & SGX_ENCL_DEBUG) ||
> > +	    !(encl->flags & SGX_ENCL_INITIALIZED) ||
> > +	    (encl->flags & SGX_ENCL_DEAD))
> > +		return -EFAULT;
> > +
> > +	for (i = 0; i < len; i += cnt) {
> > +		entry = sgx_encl_reserve_page(encl, (addr + i) & PAGE_MASK);
> 
> Taking a lock indirectly via sgx_encl_reserve_page() and releasing it
> directly via mutex_unlock() is a bit difficult to follow, having a helper
> to pair up with sgx_encl_reserve_page() would be more readable.  Open
> coding sgx_encl_reserve_page() would probably be even better since this
> is the only user AFAICT.
> 
> All that being said, I prefer the old approach of marking the page RESERVED
> instead of holding the enclave's lock, even though it was slightly more
> complex.

For me the big rationale is to minimize the amount of state in the
context of this patch set. And this approach should work just fine.

/Jarkko