lkdtm: Emit an indirect call for CFI tests

Clang can convert the indirect calls in lkdtm_CFI_FORWARD_PROTO into
direct calls. Move the call into a noinline function that accepts the
target address as an argument to ensure the compiler actually emits an
indirect call instead.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Kees Cook <keescook@chromium.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220908215504.3686827-8-samitolvanen@google.com

diff --git a/drivers/misc/lkdtm/cfi.c b/drivers/misc/lkdtm/cfi.c
index 71483cb..5245cf6 100644 (file)
--- a/drivers/misc/lkdtm/cfi.c
+++ b/drivers/misc/lkdtm/cfi.c
@@ -20,6 +20,13 @@ static noinline int lkdtm_increment_int(int *counter)
 
        return *counter;
 }
+
+/* Don't allow the compiler to inline the calls. */
+static noinline void lkdtm_indirect_call(void (*func)(int *))
+{
+       func(&called_count);
+}
+
 /*
  * This tries to call an indirect function with a mismatched prototype.
  */
@@ -29,15 +36,11 @@ static void lkdtm_CFI_FORWARD_PROTO(void)
         * Matches lkdtm_increment_void()'s prototype, but not
         * lkdtm_increment_int()'s prototype.
         */
-       void (*func)(int *);
-
        pr_info("Calling matched prototype ...\n");
-       func = lkdtm_increment_void;
-       func(&called_count);
+       lkdtm_indirect_call(lkdtm_increment_void);
 
        pr_info("Calling mismatched prototype ...\n");
-       func = (void *)lkdtm_increment_int;
-       func(&called_count);
+       lkdtm_indirect_call((void *)lkdtm_increment_int);
 
        pr_err("FAIL: survived mismatched prototype function call!\n");
        pr_expected_config(CONFIG_CFI_CLANG);