[PATCH v4 16/39] x86/mm: Check Shadow Stack page fault errors

[Rick Edgecombe]

From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

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 stacks accesses to shadow-stack mappings can see 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.

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@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
---

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

v3:
 - Improve comment talking about using FAULT_FLAG_WRITE (Peterz)

v2:
 - Update commit log with verbiage/feedback from Dave Hansen
 - Clarify reasoning for FAULT_FLAG_WRITE for all shadow stack accesses
 - Update comments with some verbiage from Dave Hansen

Yu-cheng v30:
 - Update Subject line and add a verb

 arch/x86/include/asm/trap_pf.h |  2 ++
 arch/x86/mm/fault.c            | 38 ++++++++++++++++++++++++++++++++++
 2 files changed, 40 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 7b0d4ab894c8..3004ad044e9b 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1138,8 +1138,22 @@ 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;
@@ -1331,6 +1345,30 @@ void do_user_addr_fault(struct pt_regs *regs,
 
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
 
+	/*
+	 * When a page becomes COW it changes from a shadow stack permissioned
+	 * page (Write=0,Dirty=1) to (Write=0,Dirty=0,CoW=1), which is simply
+	 * read-only to the CPU. When shadow stack is enabled, a RET would
+	 * normally pop the shadow stack by reading it with a "shadow stack
+	 * read" access. However, in the COW case the shadow stack memory does
+	 * not have shadow stack permissions, it is read-only. So it will
+	 * generate a fault.
+	 *
+	 * For conventionally writable pages, a read can be serviced with a
+	 * read only PTE, and COW would not have to happen. But for shadow
+	 * stack, there isn't the concept of read-only shadow stack memory.
+	 * If it is shadow stack permissioned, it can be modified via CALL and
+	 * RET instructions. So COW needs to happen before any memory can be
+	 * mapped with shadow stack permissions.
+	 *
+	 * Shadow stack accesses (read or write) need to be serviced with
+	 * shadow stack permissioned memory, so in the case of a shadow stack
+	 * read access, treat it as a WRITE fault so both COW will happen and
+	 * the write fault path will tickle maybe_mkwrite() and map the memory
+	 * shadow stack.
+	 */
+	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