linux-hardening - [PATCH v4 07/21] lkdtm: Emit an indirect call for CFI tests

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Message-Id: <20220830233129.30610-8-samitolvanen@google.com>
Date:   Tue, 30 Aug 2022 16:31:15 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: [PATCH v4 07/21] 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@...gle.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Acked-by: Kees Cook <keescook@...omium.org>
---
 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);
-- 
2.37.2.672.g94769d06f0-goog