[PATCH 04/25] x86/sgx: Add pfn_mkwrite() handler for present PTEs

[Reinette Chatre]

By default a write page fault on a present PTE inherits the permissions
of the VMA. Enclave page permissions maintained in the hardware's
Enclave Page Cache Map (EPCM) may change after a VMA accessing the page
is created. A VMA's permissions may thus exceed the enclave page
permissions even though the VMA was originally created not to exceed
the enclave page permissions. Following the default behavior during
a page fault on a present PTE while the VMA permissions exceed the
enclave page permissions would result in the PTE for an enclave page
to be writable even though the page is not writable according to the
enclave's permissions.

Consider the following scenario:
* An enclave page exists with RW EPCM permissions.
* A RW VMA maps the range spanning the enclave page.
* The enclave page's EPCM permissions are changed to read-only.
* There is no page table entry for the enclave page.

Q.
 What will user space observe when an attempt is made to write to the
 enclave page from within the enclave?

A.
 Initially the page table entry is not present so the following is
 observed:
 1) Instruction writing to enclave page is run from within the enclave.
 2) A page fault with second and third bits set (0x6) is encountered
    and handled by the SGX handler sgx_vma_fault() that installs a
    read-only page table entry following previous patch that installs
    page table entry with permissions that VMA and enclave agree on
    (read-only in this case).
 3) Instruction writing to enclave page is re-attempted.
 4) A page fault with first three bits set (0x7) is encountered and
    transparently (from SGX and user space perspective) handled by the
    OS with the page table entry made writable because the VMA is
    writable.
 5) Instruction writing to enclave page is re-attempted.
 6) Since the EPCM permissions prevents writing to the page a new page
    fault is encountered, this time with the SGX flag set in the error
    code (0x8007). No action is taken by OS for this page fault and
    execution returns to user space.
 7) Typically such a fault will be passed on to an application with a
    signal but if the enclave is entered with the vDSO function provided
    by the kernel then user space does not receive a signal but instead
    the vDSO function returns successfully with exception information
    (vector=14, error code=0x8007, and address) within the exception
    fields within the vDSO function's struct sgx_enclave_run.

As can be observed it is not possible for user space to write to an
enclave page if that page's enclave page permissions do not allow so,
no matter what the VMA or PTE allows.

Even so, the OS should not allow writing to a page if that page is not
writable. Thus the page table entry should accurately reflect the
enclave page permissions.

Do not blindly accept VMA permissions on a page fault due to a write
attempt to a present PTE. Install a pfn_mkwrite() handler that ensures
that the VMA permissions agree with the enclave permissions in this
regard.

Considering the same scenario as above after this change results in
the following behavior change:

Q.
 What will user space observe when an attempt is made to write to the
 enclave page from within the enclave?

A.
 Initially the page table entry is not present so the following is
 observed:
 1) Instruction writing to enclave page is run from within the enclave.
 2) A page fault with second and third bits set (0x6) is encountered
    and handled by the SGX handler sgx_vma_fault() that installs a
    read-only page table entry following previous patch that installs
    page table entry with permissions that VMA and enclave agree on
    (read-only in this case).
 3) Instruction writing to enclave page is re-attempted.
 4) A page fault with first three bits set (0x7) is encountered and
    passed to the pfn_mkwrite() handler for consideration. The handler
    determines that the page should not be writable and returns SIGBUS.
 5) Typically such a fault will be passed on to an application with a
    signal but if the enclave is entered with the vDSO function provided
    by the kernel then user space does not receive a signal but instead
    the vDSO function returns successfully with exception information
    (vector=14, error code=0x7, and address) within the exception fields
    within the vDSO function's struct sgx_enclave_run.

The accurate exception information supports the SGX runtime, which is
virtually always implemented inside a shared library, by providing
accurate information in support of its management of the SGX enclave.

Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
---
 arch/x86/kernel/cpu/sgx/encl.c | 42 ++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index 20e97d3abdce..60afa8eaf979 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -184,6 +184,47 @@ static vm_fault_t sgx_vma_fault(struct vm_fault *vmf)
 	return VM_FAULT_NOPAGE;
 }
 
+/*
+ * A fault occurred while writing to a present enclave PTE. Since PTE is
+ * present this will not be handled by sgx_vma_fault(). VMA may allow
+ * writing to the page while enclave does not. Do not follow the default
+ * of inheriting VMA permissions in this regard, ensure enclave also allows
+ * writing to the page.
+ */
+static vm_fault_t sgx_vma_pfn_mkwrite(struct vm_fault *vmf)
+{
+	unsigned long addr = (unsigned long)vmf->address;
+	struct vm_area_struct *vma = vmf->vma;
+	struct sgx_encl_page *entry;
+	struct sgx_encl *encl;
+	vm_fault_t ret = 0;
+
+	encl = vma->vm_private_data;
+
+	/*
+	 * It's very unlikely but possible that allocating memory for the
+	 * mm_list entry of a forked process failed in sgx_vma_open(). When
+	 * this happens, vm_private_data is set to NULL.
+	 */
+	if (unlikely(!encl))
+		return VM_FAULT_SIGBUS;
+
+	mutex_lock(&encl->lock);
+
+	entry = xa_load(&encl->page_array, PFN_DOWN(addr));
+	if (!entry) {
+		ret = VM_FAULT_SIGBUS;
+		goto out;
+	}
+
+	if (!(entry->vm_max_prot_bits & VM_WRITE))
+		ret = VM_FAULT_SIGBUS;
+
+out:
+	mutex_unlock(&encl->lock);
+	return ret;
+}
+
 static void sgx_vma_open(struct vm_area_struct *vma)
 {
 	struct sgx_encl *encl = vma->vm_private_data;
@@ -381,6 +422,7 @@ const struct vm_operations_struct sgx_vm_ops = {
 	.mprotect = sgx_vma_mprotect,
 	.open = sgx_vma_open,
 	.access = sgx_vma_access,
+	.pfn_mkwrite = sgx_vma_pfn_mkwrite,
 };
 
 /**
-- 
2.25.1