[kernel-hardening] [PATCH v3 2/3] arm64: usercopy: Implement stack frame object validation

kpark3469
2017-02-05T12:14:09Z

From: Sahara <keun-o.park@darkmatter.ae>

This implements arch_within_stack_frames() for arm64 that should
validate if a given object is contained by a kernel stack frame.

Signed-off-by: Sahara <keun-o.park@darkmatter.ae>
Reviewed-by: James Morse <james.morse@arm.com>
---
arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/thread_info.h | 64 ++++++++++++++++++++++++++++++++++++
2 files changed, 65 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 1117421..8bf70b4 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -97,6 +97,7 @@ config ARM64
select HAVE_SYSCALL_TRACEPOINTS
select HAVE_KPROBES
select HAVE_KRETPROBES if HAVE_KPROBES
+ select HAVE_ARCH_WITHIN_STACK_FRAMES
select IOMMU_DMA if IOMMU_SUPPORT
select IRQ_DOMAIN
select IRQ_FORCED_THREADING
diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
index 46c3b93..70baad3 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -68,7 +68,71 @@ struct thread_info {
#define thread_saved_fp(tsk) \
((unsigned long)(tsk->thread.cpu_context.fp))

+#define get_stack_start(fp) (fp + 2 * sizeof(void *))
+
+/*
+ * Walks up the stack frames to make sure that the specified object is
+ * entirely contained by a single stack frame.
+ *
+ * Returns:
+ * GOOD_FRAME if within a frame
+ * BAD_STACK if placed across a frame boundary (or outside stack)
+ * NOT_STACK unable to determine (no frame pointers, etc)
+ */
+
+static inline enum stack_type arch_within_stack_frames(const void * const stack,
+ const void * const stackend,
+ const void *obj, unsigned long len)
+{
+#ifdef CONFIG_FRAME_POINTER
+ const void *callee_fp = NULL;
+ const void *caller_fp = NULL;
+
+ callee_fp = __builtin_frame_address(1);
+ if (callee_fp)
+ caller_fp = *(const void * const *)callee_fp;
+ /*
+ * Case #1:
+ * low ----------------------------------------------> high
+ * [callee_fp][lr][args][local vars][caller_fp'][lr']
+ * ^----------------^
+ * allow copies only within here
+ *
+ * Case #2:
+ * low ----------------------------------------------> high
+ * [check_object_size_fp][lr][args][local vars][callee_fp][lr]
+ * ^----------------^
+ * dynamically allocated stack variable of
+ * callee frame copies are allowed within here
+ *
+ * < example code snippet for Case#2 >
+ * array_size = get_random_int() & 0x0f;
+ * if (to_user) {
+ * unsigned char array[array_size];
+ * if (copy_to_user((void __user *)user_addr, array,
+ * unconst + sizeof(array))) {
+ */
+ while (stack <= callee_fp && callee_fp < stackend &&
+ !((unsigned long)caller_fp & 0xf)) {
+ /*
+ * If obj + len extends past the caller frame, this
+ * check won't pass and the next frame will be 0,
+ * causing us to bail out and correctly report
+ * the copy as invalid.
+ */
+ if (!caller_fp || (obj + len <= caller_fp))
+ return (obj >= get_stack_start(callee_fp)) ?
+ GOOD_FRAME : BAD_STACK;
+ callee_fp = caller_fp;
+ caller_fp = *(const void * const *)caller_fp;
+ }
+ return BAD_STACK;
+#else
+ return NOT_STACK;
#endif
+}
+
+#endif /* !__ASSEMBLY__ */

/*
* thread information flags:
--
2.7.4