lkdtm: tests for FORTIFY_SOURCE

Add code to test both:

- runtime detection of the overrun of a structure. This covers the
__builtin_object_size(x, 0) case. This test is called FORTIFY_OBJECT.

- runtime detection of the overrun of a char array within a structure.
This covers the __builtin_object_size(x, 1) case which can be used
for some string functions. This test is called FORTIFY_SUBOBJECT.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Daniel Axtens <[e-mail]>
Signed-off-by: Francis Laniel <[e-mail]>
Suggested-by: Kees Cook <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Cc: Daniel Micay <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/misc/lkdtm/bugs.c

···
 	pr_err("XFAIL: this test is arm64-only\n");
 #endif
 }
+
+void lkdtm_FORTIFY_OBJECT(void)
+{
+	struct target {
+		char a[10];
+	} target[2] = {};
+	int result;
+
+	/*
+	 * Using volatile prevents the compiler from determining the value of
+	 * 'size' at compile time. Without that, we would get a compile error
+	 * rather than a runtime error.
+	 */
+	volatile int size = 11;
+
+	pr_info("trying to read past the end of a struct\n");
+
+	result = memcmp(&target[0], &target[1], size);
+
+	/* Print result to prevent the code from being eliminated */
+	pr_err("FAIL: fortify did not catch an object overread!\n"
+	       "\"%d\" was the memcmp result.\n", result);
+}
+
+void lkdtm_FORTIFY_SUBOBJECT(void)
+{
+	struct target {
+		char a[10];
+		char b[10];
+	} target;
+	char *src;
+
+	src = kmalloc(20, GFP_KERNEL);
+	strscpy(src, "over ten bytes", 20);
+
+	pr_info("trying to strcpy past the end of a member of a struct\n");
+
+	/*
+	 * strncpy(target.a, src, 20); will hit a compile error because the
+	 * compiler knows at build time that target.a < 20 bytes. Use strcpy()
+	 * to force a runtime error.
+	 */
+	strcpy(target.a, src);
+
+	/* Use target.a to prevent the code from being eliminated */
+	pr_err("FAIL: fortify did not catch an sub-object overrun!\n"
+	       "\"%s\" was copied.\n", target.a);
+
+	kfree(src);
+}

drivers/misc/lkdtm/core.c

···
 	CRASHTYPE(UNSET_SMEP),
 	CRASHTYPE(CORRUPT_PAC),
 	CRASHTYPE(UNALIGNED_LOAD_STORE_WRITE),
+	CRASHTYPE(FORTIFY_OBJECT),
+	CRASHTYPE(FORTIFY_SUBOBJECT),
 	CRASHTYPE(OVERWRITE_ALLOCATION),
 	CRASHTYPE(WRITE_AFTER_FREE),
 	CRASHTYPE(READ_AFTER_FREE),

drivers/misc/lkdtm/lkdtm.h

···
 void lkdtm_UNSET_SMEP(void);
 void lkdtm_DOUBLE_FAULT(void);
 void lkdtm_CORRUPT_PAC(void);
+void lkdtm_FORTIFY_OBJECT(void);
+void lkdtm_FORTIFY_SUBOBJECT(void);
 
 /* lkdtm_heap.c */
 void __init lkdtm_heap_init(void);