[PATCH AUTOSEL 4.9 02/19] lkdtm/usercopy: Expand size of "out of frame" object

[Sasha Levin]

From: Kees Cook <keescook@xxxxxxxxxxxx>

[ Upstream commit f387e86d3a74407bdd9c5815820ac9d060962840 ]

To be sufficiently out of range for the usercopy test to see the lifetime
mismatch, expand the size of the "bad" buffer, which will let it be
beyond current_stack_pointer regardless of stack growth direction.
Paired with the recent addition of stack depth checking under
CONFIG_HARDENED_USERCOPY=y, this will correctly start tripping again.

Reported-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/lkml/762faf1b-0443-5ddf-4430-44a20cf2ec4d@xxxxxxxxxxxxx/
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/misc/lkdtm_usercopy.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/misc/lkdtm_usercopy.c b/drivers/misc/lkdtm_usercopy.c
index 1dd611423d8b..36438947244d 100644
--- a/drivers/misc/lkdtm_usercopy.c
+++ b/drivers/misc/lkdtm_usercopy.c
@@ -28,12 +28,12 @@ static const unsigned char test_text[] = "This is a test.\n";
  */
 static noinline unsigned char *trick_compiler(unsigned char *stack)
 {
-	return stack + 0;
+	return stack + unconst;
 }
 
 static noinline unsigned char *do_usercopy_stack_callee(int value)
 {
-	unsigned char buf[32];
+	unsigned char buf[128];
 	int i;
 
 	/* Exercise stack to avoid everything living in registers. */
@@ -41,7 +41,12 @@ static noinline unsigned char *do_usercopy_stack_callee(int value)
 		buf[i] = value & 0xff;
 	}
 
-	return trick_compiler(buf);
+	/*
+	 * Put the target buffer in the middle of stack allocation
+	 * so that we don't step on future stack users regardless
+	 * of stack growth direction.
+	 */
+	return trick_compiler(&buf[(128/2)-32]);
 }
 
 static noinline void do_usercopy_stack(bool to_user, bool bad_frame)
@@ -64,6 +69,12 @@ static noinline void do_usercopy_stack(bool to_user, bool bad_frame)
 		bad_stack -= sizeof(unsigned long);
 	}
 
+#ifdef ARCH_HAS_CURRENT_STACK_POINTER
+	pr_info("stack     : %px\n", (void *)current_stack_pointer);
+#endif
+	pr_info("good_stack: %px-%px\n", good_stack, good_stack + sizeof(good_stack));
+	pr_info("bad_stack : %px-%px\n", bad_stack, bad_stack + sizeof(good_stack));
+
 	user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
 			    PROT_READ | PROT_WRITE | PROT_EXEC,
 			    MAP_ANONYMOUS | MAP_PRIVATE, 0);
-- 
2.35.1