[PATCH v7 19/41] x86/mm: Check shadow stack page fault errors

Rick Edgecombe
2023-02-27T22:32:34Z

From: Yu-cheng Yu <yu-cheng.yu@intel.com>

The CPU performs "shadow stack accesses" when it expects to encounter
shadow stack mappings. These accesses can be implicit (via CALL/RET
instructions) or explicit (instructions like WRSS).

Shadow stack accesses to shadow-stack mappings can result in faults in
normal, valid operation just like regular accesses to regular mappings.
Shadow stacks need some of the same features like delayed allocation, swap
and copy-on-write. The kernel needs to use faults to implement those
features.

The architecture has concepts of both shadow stack reads and shadow stack
writes. Any shadow stack access to non-shadow stack memory will generate
a fault with the shadow stack error code bit set.

This means that, unlike normal write protection, the fault handler needs
to create a type of memory that can be written to (with instructions that
generate shadow stack writes), even to fulfill a read access. So in the
case of COW memory, the COW needs to take place even with a shadow stack
read. Otherwise the page will be left (shadow stack) writable in
userspace. So to trigger the appropriate behavior, set FAULT_FLAG_WRITE
for shadow stack accesses, even if the access was a shadow stack read.

For the purpose of making this clearer, consider the following example.
If a process has a shadow stack, and forks, the shadow stack PTEs will
become read-only due to COW. If the CPU in one process performs a shadow
stack read access to the shadow stack, for example executing a RET and
causing the CPU to read the shadow stack copy of the return address, then
in order for the fault to be resolved the PTE will need to be set with
shadow stack permissions. But then the memory would be changeable from
userspace (from CALL, RET, WRSS, etc). So this scenario needs to trigger
COW, otherwise the shared page would be changeable from both processes.

Shadow stack accesses can also result in errors, such as when a shadow
stack overflows, or if a shadow stack access occurs to a non-shadow-stack
mapping. Also, generate the errors for invalid shadow stack accesses.

Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

---
v7:
- Update comment in fault handler (David Hildenbrand)

v6:
- Update comment due to rename of Cow bit to SavedDirty

v5:
- Add description of COW example (Boris)
- Replace "permissioned" (Boris)
- Remove capitalization of shadow stack (Boris)

v4:
- Further improve comment talking about FAULT_FLAG_WRITE (Peterz)

v3:
- Improve comment talking about using FAULT_FLAG_WRITE (Peterz)
---
arch/x86/include/asm/trap_pf.h | 2 ++
arch/x86/mm/fault.c | 31 +++++++++++++++++++++++++++++++
2 files changed, 33 insertions(+)

diff --git a/arch/x86/include/asm/trap_pf.h b/arch/x86/include/asm/trap_pf.h
index 10b1de500ab1..afa524325e55 100644
--- a/arch/x86/include/asm/trap_pf.h
+++ b/arch/x86/include/asm/trap_pf.h
@@ -11,6 +11,7 @@
* bit 3 == 1: use of reserved bit detected
* bit 4 == 1: fault was an instruction fetch
* bit 5 == 1: protection keys block access
+ * bit 6 == 1: shadow stack access fault
* bit 15 == 1: SGX MMU page-fault
*/
enum x86_pf_error_code {
@@ -20,6 +21,7 @@ enum x86_pf_error_code {
X86_PF_RSVD = 1 << 3,
X86_PF_INSTR = 1 << 4,
X86_PF_PK = 1 << 5,
+ X86_PF_SHSTK = 1 << 6,
X86_PF_SGX = 1 << 15,
};

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index a498ae1fbe66..776b92339cfe 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1117,8 +1117,22 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
(error_code & X86_PF_INSTR), foreign))
return 1;

+ /*
+ * Shadow stack accesses (PF_SHSTK=1) are only permitted to
+ * shadow stack VMAs. All other accesses result in an error.
+ */
+ if (error_code & X86_PF_SHSTK) {
+ if (unlikely(!(vma->vm_flags & VM_SHADOW_STACK)))
+ return 1;
+ if (unlikely(!(vma->vm_flags & VM_WRITE)))
+ return 1;
+ return 0;
+ }
+
if (error_code & X86_PF_WRITE) {
/* write, present and write, not present: */
+ if (unlikely(vma->vm_flags & VM_SHADOW_STACK))
+ return 1;
if (unlikely(!(vma->vm_flags & VM_WRITE)))
return 1;
return 0;
@@ -1310,6 +1324,23 @@ void do_user_addr_fault(struct pt_regs *regs,

perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);

+ /*
+ * For conventionally writable pages, a read can be serviced with a
+ * read only PTE. But for shadow stack, there isn't a concept of
+ * read-only shadow stack memory. If it a PTE has the shadow stack
+ * permission, it can be modified via CALL and RET instructions. So
+ * core MM needs to fault in a writable PTE and do things it already
+ * does for write faults.
+ *
+ * Shadow stack accesses (read or write) need to be serviced with
+ * shadow stack permission memory, which always include write
+ * permissions. So in the case of a shadow stack read access, treat it
+ * as a WRITE fault. This will make sure that MM will prepare
+ * everything (e.g., break COW) such that maybe_mkwrite() can create a
+ * proper shadow stack PTE.
+ */
+ if (error_code & X86_PF_SHSTK)
+ flags |= FAULT_FLAG_WRITE;
if (error_code & X86_PF_WRITE)
flags |= FAULT_FLAG_WRITE;
if (error_code & X86_PF_INSTR)
--
2.17.1