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 <keesc...@chromium.org>
---
drivers/misc/lkdtm/bugs.c | 61 ++++++++++++++++++++++++++++++++++++++
drivers/misc/lkdtm/core.c | 1 +
drivers/misc/lkdtm/lkdtm.h | 1 +
3 files changed, 63 insertions(+)
diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
index 7eebbdfbcacd..6176384b4f85 100644
--- a/drivers/misc/lkdtm/bugs.c
+++ b/drivers/misc/lkdtm/bugs.c
@@ -255,3 +255,64 @@ 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 the middle of native_write_cr4() where the
+ * cr4 write happens, skipping the 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;
+ }
+ 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
+}
diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
index 2837dc77478e..fd668776414b 100644
--- a/drivers/misc/lkdtm/core.c
+++ b/drivers/misc/lkdtm/core.c
@@ -132,6 +132,7 @@ static const struct crashtype crashtypes[] = {
CRASHTYPE(CORRUPT_LIST_ADD),
CRASHTYPE(CORRUPT_LIST_DEL),
CRASHTYPE(CORRUPT_USER_DS),
+ CRASHTYPE(UNSET_SMEP),
CRASHTYPE(CORRUPT_STACK),
CRASHTYPE(CORRUPT_STACK_STRONG),
CRASHTYPE(STACK_GUARD_PAGE_LEADING),
diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
index 3c6fd327e166..9c78d7e21c13 100644
--- a/drivers/misc/lkdtm/lkdtm.h
+++ b/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);
--
2.17.1
