[PATCH v6 19/41] x86/mm: Check shadow stack page fault errors

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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 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

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@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>

 - Update comment due to rename of Cow bit to SavedDirty

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

 - Further improve comment talking about FAULT_FLAG_WRITE (Peterz)

 - Improve comment talking about using FAULT_FLAG_WRITE (Peterz)
 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..42885d8e2036 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 permission
+	 * page (Write=0,Dirty=1) to (Write=0,Dirty=0,SavedDirty=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 permission, 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 permission 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)

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