lkdtm: Check for SMEP clearing protections

This adds an x86-specific test for pinned cr4 bits. A successful test
will validate pinning and check the ROP-style call-middle-of-function
defense, if needed. For example, in the case of native_write_cr4()
looking like this:

ffffffff8171bce0 <native_write_cr4>:
ffffffff8171bce0:       48 8b 35 79 46 f2 00    mov    0xf24679(%rip),%rsi
ffffffff8171bce7:       48 09 f7                or     %rsi,%rdi
ffffffff8171bcea:       0f 22 e7                mov    %rdi,%cr4
...
ffffffff8171bd5a:       c3                      retq

The UNSET_SMEP test will jump to ffffffff8171bcea (the mov to cr4)
instead of ffffffff8171bce0 (native_write_cr4() entry) to simulate a
direct-call bypass attempt.

Expected successful results:

  # echo UNSET_SMEP > /sys/kernel/debug/provoke-crash/DIRECT
  # dmesg
  [   79.594433] lkdtm: Performing direct entry UNSET_SMEP
  [   79.596459] lkdtm: trying to clear SMEP normally
  [   79.598406] lkdtm: ok: SMEP did not get cleared
  [   79.599981] lkdtm: trying to clear SMEP with call gadget
  [   79.601810] ------------[ cut here ]------------
  [   79.603421] Attempt to unpin cr4 bits: 100000; bypass attack?!
  ...
  [   79.650170] ---[ end trace 2452ca0f6126242e ]---
  [   79.650937] lkdtm: ok: SMEP removal was reverted

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/lkdtm/bugs.c

@@ -266,3 +266,69 @@ void lkdtm_STACK_GUARD_PAGE_TRAILING(void)
 
 	pr_err("FAIL: accessed page after stack!\n");
 }
+
+void lkdtm_UNSET_SMEP(void)
+{
+#ifdef CONFIG_X86_64
+#define MOV_CR4_DEPTH	64
+	void (*direct_write_cr4)(unsigned long val);
+	unsigned char *insn;
+	unsigned long cr4;
+	int i;
+
+	cr4 = native_read_cr4();
+
+	if ((cr4 & X86_CR4_SMEP) != X86_CR4_SMEP) {
+		pr_err("FAIL: SMEP not in use\n");
+		return;
+	}
+	cr4 &= ~(X86_CR4_SMEP);
+
+	pr_info("trying to clear SMEP normally\n");
+	native_write_cr4(cr4);
+	if (cr4 == native_read_cr4()) {
+		pr_err("FAIL: pinning SMEP failed!\n");
+		cr4 |= X86_CR4_SMEP;
+		pr_info("restoring SMEP\n");
+		native_write_cr4(cr4);
+		return;
+	}
+	pr_info("ok: SMEP did not get cleared\n");
+
+	/*
+	 * To test the post-write pinning verification we need to call
+	 * directly into the middle of native_write_cr4() where the
+	 * cr4 write happens, skipping any pinning. This searches for
+	 * the cr4 writing instruction.
+	 */
+	insn = (unsigned char *)native_write_cr4;
+	for (i = 0; i < MOV_CR4_DEPTH; i++) {
+		/* mov %rdi, %cr4 */
+		if (insn[i] == 0x0f && insn[i+1] == 0x22 && insn[i+2] == 0xe7)
+			break;
+		/* mov %rdi,%rax; mov %rax, %cr4 */
+		if (insn[i]   == 0x48 && insn[i+1] == 0x89 &&
+		    insn[i+2] == 0xf8 && insn[i+3] == 0x0f &&
+		    insn[i+4] == 0x22 && insn[i+5] == 0xe0)
+			break;
+	}
+	if (i >= MOV_CR4_DEPTH) {
+		pr_info("ok: cannot locate cr4 writing call gadget\n");
+		return;
+	}
+	direct_write_cr4 = (void *)(insn + i);
+
+	pr_info("trying to clear SMEP with call gadget\n");
+	direct_write_cr4(cr4);
+	if (native_read_cr4() & X86_CR4_SMEP) {
+		pr_info("ok: SMEP removal was reverted\n");
+	} else {
+		pr_err("FAIL: cleared SMEP not detected!\n");
+		cr4 |= X86_CR4_SMEP;
+		pr_info("restoring SMEP\n");
+		native_write_cr4(cr4);
+	}
+#else
+	pr_err("FAIL: this test is x86_64-only\n");
+#endif
+}

drivers/misc/lkdtm/core.c

@@ -114,6 +114,7 @@ static const struct crashtype crashtypes[] = {
 	CRASHTYPE(CORRUPT_USER_DS),
 	CRASHTYPE(STACK_GUARD_PAGE_LEADING),
 	CRASHTYPE(STACK_GUARD_PAGE_TRAILING),
+	CRASHTYPE(UNSET_SMEP),
 	CRASHTYPE(UNALIGNED_LOAD_STORE_WRITE),
 	CRASHTYPE(OVERWRITE_ALLOCATION),
 	CRASHTYPE(WRITE_AFTER_FREE),

drivers/misc/lkdtm/lkdtm.h

@@ -26,6 +26,7 @@ void lkdtm_CORRUPT_LIST_DEL(void);
 void lkdtm_CORRUPT_USER_DS(void);
 void lkdtm_STACK_GUARD_PAGE_LEADING(void);
 void lkdtm_STACK_GUARD_PAGE_TRAILING(void);
+void lkdtm_UNSET_SMEP(void);
 
 /* lkdtm_heap.c */
 void lkdtm_OVERWRITE_ALLOCATION(void);