Re: [PATCH v26 10/22] x86/sgx: Linux Enclave Driver

[Jordan Hand]

I ran our validation tests for the Open Enclave SDK against this patch
set and came across a potential issue.

On 2/9/20 1:25 PM, Jarkko Sakkinen wrote:
> +/**
> + * sgx_encl_may_map() - Check if a requested VMA mapping is allowed
> + * @encl:		an enclave
> + * @start:		lower bound of the address range, inclusive
> + * @end:		upper bound of the address range, exclusive
> + * @vm_prot_bits:	requested protections of the address range
> + *
> + * Iterate through the enclave pages contained within [@start, @end) to verify
> + * the permissions requested by @vm_prot_bits do not exceed that of any enclave
> + * page to be mapped.  Page addresses that do not have an associated enclave
> + * page are interpreted to zero permissions.
> + *
> + * Return:
> + *   0 on success,
> + *   -EACCES if VMA permissions exceed enclave page permissions
> + */
> +int sgx_encl_may_map(struct sgx_encl *encl, unsigned long start,
> +		     unsigned long end, unsigned long vm_prot_bits)
> +{
> +	unsigned long idx, idx_start, idx_end;
> +	struct sgx_encl_page *page;
> +
> +	/* PROT_NONE always succeeds. */
> +	if (!vm_prot_bits)
> +		return 0;
> +
> +	idx_start = PFN_DOWN(start);
> +	idx_end = PFN_DOWN(end - 1);
> +
> +	for (idx = idx_start; idx <= idx_end; ++idx) {
> +		mutex_lock(&encl->lock);
> +		page = radix_tree_lookup(&encl->page_tree, idx);
> +		mutex_unlock(&encl->lock);
> +
> +		if (!page || (~page->vm_max_prot_bits & vm_prot_bits))
> +			return -EACCES;
> +	}
> +
> +	return 0;
> +}
> +static struct sgx_encl_page *sgx_encl_page_alloc(struct sgx_encl *encl,
> +						 unsigned long offset,
> +						 u64 secinfo_flags)
> +{
> +	struct sgx_encl_page *encl_page;
> +	unsigned long prot;
> +
> +	encl_page = kzalloc(sizeof(*encl_page), GFP_KERNEL);
> +	if (!encl_page)
> +		return ERR_PTR(-ENOMEM);
> +
> +	encl_page->desc = encl->base + offset;
> +	encl_page->encl = encl;
> +
> +	prot = _calc_vm_trans(secinfo_flags, SGX_SECINFO_R, PROT_READ)  |
> +	       _calc_vm_trans(secinfo_flags, SGX_SECINFO_W, PROT_WRITE) |
> +	       _calc_vm_trans(secinfo_flags, SGX_SECINFO_X, PROT_EXEC);
> +
> +	/*
> +	 * TCS pages must always RW set for CPU access while the SECINFO
> +	 * permissions are *always* zero - the CPU ignores the user provided
> +	 * values and silently overwrites them with zero permissions.
> +	 */
> +	if ((secinfo_flags & SGX_SECINFO_PAGE_TYPE_MASK) == SGX_SECINFO_TCS)
> +		prot |= PROT_READ | PROT_WRITE;
> +
> +	/* Calculate maximum of the VM flags for the page. */
> +	encl_page->vm_max_prot_bits = calc_vm_prot_bits(prot, 0);

During mprotect (in mm/mprotect.c line 525) the following checks if
READ_IMPLIES_EXECUTE and a PROT_READ is being requested. If so and
VM_MAYEXEC is set, it also adds PROT_EXEC to the request.

	if (rier && (vma->vm_flags & VM_MAYEXEC))
		prot |= PROT_EXEC;

But if we look at sgx_encl_page_alloc(), we see vm_max_prot_bits is set
without taking VM_MAYEXEC into account:

	encl_page->vm_max_prot_bits = calc_vm_prot_bits(prot, 0);

sgx_encl_may_map() checks that the requested protection can be added with:

	if (!page || (~page->vm_max_prot_bits & vm_prot_bits))
		return -EACCESS

This means that for any process where READ_IMPLIES_EXECUTE is set and
page where (vma->vm_flags & VM_MAYEXEC) == true, mmap/mprotect calls to
that request PROT_READ on a page that was not added with PROT_EXEC will
fail.

- Jordan