linux-kernel - [RFC PATCH 1/2] lib/string: Disable instrumentation

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Message-Id: <20200905222323.1408968-2-nivedita@alum.mit.edu>
Date:   Sat,  5 Sep 2020 18:23:22 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     x86@...nel.org, kasan-dev@...glegroups.com
Cc:     Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: [RFC PATCH 1/2] lib/string: Disable instrumentation

String functions can be useful in early boot, but using instrumented
versions can be problematic: eg on x86, some of the early boot code is
executing out of an identity mapping rather than the kernel virtual
addresses. Accessing any global variables at this point will lead to a
crash.

Tracing and KCOV are already disabled, and CONFIG_AMD_MEM_ENCRYPT will
additionally disable KASAN and stack protector.

Additionally disable GCOV, UBSAN, KCSAN, STACKLEAK_PLUGIN and branch
profiling, and make it unconditional to allow safe use of string
functions.

Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
---
 lib/Makefile | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/lib/Makefile b/lib/Makefile
index a4a4c6864f51..5e421769bbc6 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -8,7 +8,6 @@ ccflags-remove-$(CONFIG_FUNCTION_TRACER) += $(CC_FLAGS_FTRACE)
 # These files are disabled because they produce lots of non-interesting and/or
 # flaky coverage that is not a function of syscall inputs. For example,
 # rbtree can be global and individual rotations don't correlate with inputs.
-KCOV_INSTRUMENT_string.o := n
 KCOV_INSTRUMENT_rbtree.o := n
 KCOV_INSTRUMENT_list_debug.o := n
 KCOV_INSTRUMENT_debugobjects.o := n
@@ -20,12 +19,16 @@ KCOV_INSTRUMENT_fault-inject.o := n
 # them into calls to themselves.
 CFLAGS_string.o := -ffreestanding
 
-# Early boot use of cmdline, don't instrument it
-ifdef CONFIG_AMD_MEM_ENCRYPT
+# Early boot use of string functions, disable instrumentation
+GCOV_PROFILE_string.o := n
+KCOV_INSTRUMENT_string.o := n
 KASAN_SANITIZE_string.o := n
+UBSAN_SANITIZE_string.o := n
+KCSAN_SANITIZE_string.o := n
 
 CFLAGS_string.o += -fno-stack-protector
-endif
+CFLAGS_string.o += $(DISABLE_STACKLEAK_PLUGIN)
+CFLAGS_string.o += -DDISABLE_BRANCH_PROFILING
 
 # Used by KCSAN while enabled, avoid recursion.
 KCSAN_SANITIZE_random32.o := n
-- 
2.26.2