[PATCH v5 07/22] lkdtm: Emit an indirect call for CFI tests

From: Sami Tolvanen
Date: Thu Sep 08 2022 - 17:56:03 EST


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@xxxxxxxxxx>
Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
drivers/misc/lkdtm/cfi.c | 15 +++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/drivers/misc/lkdtm/cfi.c b/drivers/misc/lkdtm/cfi.c
index 71483cb1e422..5245cf6013c9 100644
--- 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);
--
2.37.2.789.g6183377224-goog