Re: [PATCH] MIPS: usercopy: Implement stack frame object validation
From: Matt Redfearn
Date: Thu Aug 10 2017 - 04:24:50 EST
Hi Kees,
On 08/08/17 20:11, Kees Cook wrote:
On Tue, Aug 8, 2017 at 5:23 AM, Matt Redfearn <matt.redfearn@xxxxxxxxxx> wrote:
This implements arch_within_stack_frames() for MIPS that validates if an
object is wholly contained by a kernel stack frame.
With CONFIG_HARDENED_USERCOPY enabled, MIPS now passes the LKDTM tests
USERCOPY_STACK_FRAME_TO, USERCOPY_STACK_FRAME_FROM and
USERCOPY_STACK_BEYOND on a Creator Ci40.
Since the MIPS kernel does not use frame pointers, we re-use the MIPS
kernels stack frame unwinder which uses instruction inspection to deduce
the stack frame size. As such it introduces a larger performance penalty
than on arches which use the frame pointer.
Hmm, given x86's plans to drop the frame pointer, I wonder if the
inter-frame checking code should be gated by a CONFIG. This (3%) is a
rather high performance hit to take for a relatively small protection
(it's mainly about catching too-large-reads, since most
too-large-writes will be caught by the stack canary).
What do you think?
If x86 is going to move to a more expensive stack unwinding method than
the frame pointer then I guess it may end up seeing a similar
performance hit to what we see on MIPS. In that case it might make sense
to add a CONFIG for this such that only those who wish to make the trade
off of performance for the added protection need enable it.
Thanks,
Matt
-Kees
On qemu, before this patch, hackbench gives:
Running with 10*40 (== 400) tasks.
Time: 5.484
Running with 10*40 (== 400) tasks.
Time: 4.039
Running with 10*40 (== 400) tasks.
Time: 3.908
Running with 10*40 (== 400) tasks.
Time: 3.955
Running with 10*40 (== 400) tasks.
Time: 4.185
Running with 10*40 (== 400) tasks.
Time: 4.497
Running with 10*40 (== 400) tasks.
Time: 3.980
Running with 10*40 (== 400) tasks.
Time: 4.078
Running with 10*40 (== 400) tasks.
Time: 4.219
Running with 10*40 (== 400) tasks.
Time: 4.026
Giving an average of 4.2371
With this patch, hackbench gives:
Running with 10*40 (== 400) tasks.
Time: 5.671
Running with 10*40 (== 400) tasks.
Time: 4.282
Running with 10*40 (== 400) tasks.
Time: 4.101
Running with 10*40 (== 400) tasks.
Time: 4.040
Running with 10*40 (== 400) tasks.
Time: 4.683
Running with 10*40 (== 400) tasks.
Time: 4.387
Running with 10*40 (== 400) tasks.
Time: 4.289
Running with 10*40 (== 400) tasks.
Time: 4.027
Running with 10*40 (== 400) tasks.
Time: 4.048
Running with 10*40 (== 400) tasks.
Time: 4.079
Giving an average of 4.3607
This indicates an additional 3% overhead for inspecting the kernel stack
when CONFIG_HARDENED_USERCOPY is enabled.
This patch is based on Linux v4.13-rc4, and for correct operation on
microMIPS depends on my series "MIPS: Further microMIPS stack unwinding
fixes"
Signed-off-by: Matt Redfearn <matt.redfearn@xxxxxxxxxx>
Reviewed-by: James Hogan <james.hogan@xxxxxxxxxx>
---
arch/mips/Kconfig | 1 +
arch/mips/include/asm/thread_info.h | 74 +++++++++++++++++++++++++++++++++++++
2 files changed, 75 insertions(+)
diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 8dd20358464f..6cbf2d525c8d 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -35,6 +35,7 @@ config MIPS
select HAVE_ARCH_SECCOMP_FILTER
select HAVE_ARCH_TRACEHOOK
select HAVE_ARCH_TRANSPARENT_HUGEPAGE if CPU_SUPPORTS_HUGEPAGES && 64BIT
+ select HAVE_ARCH_WITHIN_STACK_FRAMES if KALLSYMS
select HAVE_CBPF_JIT if (!64BIT && !CPU_MICROMIPS)
select HAVE_EBPF_JIT if (64BIT && !CPU_MICROMIPS)
select HAVE_CC_STACKPROTECTOR
diff --git a/arch/mips/include/asm/thread_info.h b/arch/mips/include/asm/thread_info.h
index b439e512792b..931652460393 100644
--- a/arch/mips/include/asm/thread_info.h
+++ b/arch/mips/include/asm/thread_info.h
@@ -14,6 +14,80 @@
#include <asm/processor.h>
+#ifdef CONFIG_HAVE_ARCH_WITHIN_STACK_FRAMES
+
+/*
+ * 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
+ */
+static inline int arch_within_stack_frames(const void *const stack,
+ const void *const stackend,
+ const void *obj, unsigned long len)
+{
+ /* Avoid header recursion by just declaring this here */
+ extern unsigned long unwind_stack_by_address(
+ unsigned long stack_page,
+ unsigned long *sp,
+ unsigned long pc,
+ unsigned long *ra);
+ unsigned long sp, lastsp, ra, pc;
+ int skip_frames;
+
+ /* Get this frame's details */
+ sp = (unsigned long)__builtin_frame_address(0);
+ pc = (unsigned long)current_text_addr();
+
+ /*
+ * Skip initial frames to get back the function requesting the copy.
+ * Unwind the frames of:
+ * arch_within_stack_frames (inlined into check_stack_object)
+ * __check_object_size
+ * This leaves sp & pc in the frame associated with
+ * copy_{to,from}_user() (inlined into do_usercopy_stack)
+ */
+ for (skip_frames = 0; skip_frames < 2; skip_frames++) {
+ pc = unwind_stack_by_address((unsigned long)stack, &sp, pc, &ra);
+ if (!pc)
+ return BAD_STACK;
+ }
+
+ if ((unsigned long)obj < sp) {
+ /* obj is not in the frame of the requestor or it's callers */
+ return BAD_STACK;
+ }
+
+ /*
+ * low ---------------------------------------> high
+ * [local vars][saved regs][ra][local vars']
+ * ^ ^
+ * lastsp sp
+ * ^----------------------^
+ * allow copies only within here
+ */
+ do {
+ lastsp = sp;
+ pc = unwind_stack_by_address((unsigned long)stack, &sp, pc, &ra);
+ if ((((unsigned long)obj) >= lastsp) &&
+ (((unsigned long)obj + len) <= (sp - sizeof(void *)))) {
+ /* obj is entirely within this stack frame */
+ return GOOD_FRAME;
+ }
+ } while (pc);
+
+ /*
+ * We can't unwind any further. If we haven't found the object entirely
+ * within one of our callers frames, it must be a bad object.
+ */
+ return BAD_STACK;
+}
+
+#endif /* CONFIG_HAVE_ARCH_WITHIN_STACK_FRAMES */
+
/*
* low level task data that entry.S needs immediate access to
* - this struct should fit entirely inside of one cache line
--
2.7.4