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
